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Section Five - Geodetic Datums: Combining Reference Ellipsoids and Geoids

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  • Home
  • Introduction to GIS
  • Chapter Two
  • Section Five - Geodetic Datums: Combining Reference Ellipsoids and Geoids

In Section Three, we looked at the idea that the Earth is not best represented by a sphere (or even a spheroid a sphere-like 3D object where the radius in one direction is longer than the radius in a direction at a right angle to the first ), but a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." , which is a model of the variation in gravitational pull over the surface of the Earth. This variation allows for a modeling of true mean sea level at any single location on the Earth's surface, since an assumption could be made between the strength of the pull of gravity and where water would pool (higher gravitational pull, more water pooling).

We also noted the best way to simplify the shape is to match it to a 3D reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , one which either was best-fit for the whole world (global reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid ) or one which was best-fit for a local area (local reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid - which could be regional, continental, state-wide, county wide, or city wide). We also learned that the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid was great for laying out the 2D portion of the map and we mentioned that the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." is the base for the Z value or elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface portion for making maps. In order to make useful maps, we need combine the 2D, XY half of the map and the 3D, Z half of the map. This is accomplished with the creation of a geodetic datum the result of attaching a "free-floating" reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid to a specifically measured geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." via control points and benchmarks. , or really just "datum" for short.

2.5.1: Geodetic Datums

So far, we understand 2D maps, consisting of the shapes of continents, states, counties, or city limits (among a whole boatload of other features), drawn on the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , while the elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point on the Earth's surface values are stored with the geodetic datums, or - what most people say - just "datums".

Reference ellipsoids come in two varieties - global, containing a map of all of the continents and oceans, and local, which consist of any are smaller than the whole globe. Geoids are almost always, and in our studies here will always, consists of the entire globe. When we create datums, the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is the half of the equation that decides the extent of the datum. If the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid only covers North America, it most likely fits inside the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." pretty darn good in North America but doesn't touch anywhere else in the world (or if it does, by chance, it is ignored). When the datum is created, the extent - the boundaries of the datum - is limited to North America, regardless of the fact that the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." maps the elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point on the Earth's surface of the entire planet. If the extent of the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is the entire globe, it will fit mostly okay everywhere on Earth (the extent of the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." ) - some place will be very close, some will be quite a bit off, and some will be average. But that is nature of global datums - kinda good everywhere, but not really great anywhere.

As we will see in the next section, the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is not blank to start, but actually covered with an evenly spaced Cartesian Coordinate System specifically called a geographic grid the result of using an established angular unit of measure to label the intersections of north-south and east-west lines on the surface of the Earth starting the labels at a principal meridian the north-south line from which the labeling begins.  East-west lines have a very obvious start point: the equator.  North-south lines must start somewhere, so when it is established for a particular geographic grid, it can be considered the principal meridian. . This grid is used to start the process of labeling exact locations on the Earth's surface, but in this section, we will just remember the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is not blank like a piece of printer paper, but covered in a grid like engineer's paper.

We haven't really talked much about the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." - the surface upon which we walk around and observe the Earth's landforms the descriptive words for individual features on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." , such as "hills", "valley", and "ridgelines" , but we need to remember that all of these mathematical models are created and used with the goal of labeling and navigating to locations on the Earth's surface. Since that is our goal, we need something to connect the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." to the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." to the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , and that real-world object is a benchmark. Benchmarks are, as we said, real world objects that are placed and maintained at specific and known locations of the Earth's surface. Some benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . are labeled monuments, some are chunks of rebar sunk into concrete, and some are just chunks of concrete, but all of them are scattered about world-wide and are used to connect the three surfaces together.

As, I'm sure you've noticed, there are not benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . covering the entire surface of the Earth, and most likely, you've never actually noticed a benchmark in real life ever. Since they are not covering the Earth at every location, we use the ones which do exist, then mathematically infer the rest.

Every benchmark is at a known location upon an existing Cartesian Coordinate System, we can use them as a connection point between the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." (a model of the difference between GMSL and LMSL via a model of gravitational acceleration) and the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid (a gridded 3D shape with the oceans and land areas drawn upon it). If, for example, a benchmark existing on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." in the real world has an "address" (the exact known location) of -114.03, 34.42 which has been carefully surveyed and recorded, the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." is created with GPS (an electronic means of finding Earth "addresses"), and the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid has a point marked on it's grid found by counting 34.42 intersections east of the 0,0 origin and 114.03 intersections south, we can simply connect the same location between the three different surfaces, creating beginning of a geodetic datum the result of attaching a "free-floating" reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid to a specifically measured geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." via control points and benchmarks. , or just datum. If we connect several benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . between the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." with the matching, known coordinates located on a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and some specific reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , we have a pretty good start to the datum. If we know the "address" of each connected benchmark/ geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." coordinates/ reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid 's Cartesian Coordinate System coordinates, using simple counting we can label all the remaining intersections on the datum and call those mathematically derived connections control points. And you thought you'd never use Cartesian Coordinate Systems again.....

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    • NOTE: Benchmarks are used for the basis of all datums. Once a benchmark is used to make the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." - geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." - reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid connection, it is "converted" into a control point aka: tie point  mathematically derived points that connect a two spatial objects together, such as a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and a reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid . The difference between the two is a benchmark an actual object which resides in the real world and is used over and over to create lots of datums and a control point aka: tie point  mathematically derived points that connect a two spatial objects together, such as a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and a reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is a mathematically determined connection between a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and a reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid which "resides" in the datum. So, within a datum, all benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . become control points, but not all control points started out as benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . .
FIGURE 2.17: An Example of a Benchmark
An example of a benchmark. While there is no single standard benchmark, they all include what they are, a height above sea level, and a cross or point in the middle for a surveyor to place the point of their survey equipment.
The Main Point...
Datums are: reference ellipsoids that have been linked to a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." via control points, which connect real-world points called benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . with mathematically derived points on the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid .

List of Supported Datums

View as a Separate Page
Datum NameWKIDSpheroid Name
1_Ceres_20151069721_Ceres_2015
4_Vesta_20151069734_Vesta_2015
D_Abidjan_19876143Clarke_1880_RGS
AbInvA96_2020_Intermediate_Reference_Frame1273GRS_1980
D_Accra6168War_Office
D_Aden_19251135Clarke_1880_RGS
D_Adindan6201Clarke_1880_RGS
D_Adrastea_2000106909Adrastea_2000_IAU_IAG
D_Afgooye6205Krasovsky_1940
D_Agadez6206Clarke_1880_IGN
D_Ain_el_Abd_19706204International_1924
D_Airy_18306001Airy_1830
D_Airy_Modified6002Airy_Modified
D_Alaskan_Islands106260Clarke_1866
D_Albanian_19876191Krasovsky_1940
D_Amalthea_2000106910Amalthea_2000_IAU_IAG
D_American_Samoa_19626169Clarke_1866
D_Amersfoort6289Bessel_1841
D_Ammassalik_19586196International_1924
D_Ananke_2000106911Ananke_2000_IAU_IAG
D_Anguilla_19576600Clarke_1880_RGS
D_Anna_1_19656708Australian
D_Antigua_19436601Clarke_1880_RGS
D_Aratu6208International_1924
D_Arc_19506209Clarke_1880_Arc
D_Arc_19606210Clarke_1880_RGS
D_Ariel_2000106945Ariel_2000_IAU_IAG
D_Ascension_Island_19586712International_1924
D_Astro_19526711International_1924
D_ATF6901Plessis_1817
D_Atlas_2000106926Atlas_2000_IAU_IAG
D_ATS_19776122ATS_1977
D_Australian6003Australian
D_Australian_19666202Australian
D_Australian_19846203Australian
D_Australian_Antarctic_19986176GRS_1980
Australian_Terrestrial_Reference_Frame_20141291GRS_1980
D_Ayabelle6713Clarke_1880_RGS
D_Azores_Central_Islands_19486183International_1924
D_Azores_Central_Islands_19956665International_1924
D_Azores_Occidental_Islands_19396182International_1924
D_Azores_Oriental_Islands_19406184International_1924
D_Azores_Oriental_Islands_19956664International_1924
D_Bab_South106269Clarke_1866
D_Barbados_19386212Clarke_1880_RGS
D_Batavia6211Bessel_1841
D_Beacon_E_19456709International_1924
D_Beduaram6213Clarke_1880_IGN
D_Beijing_19546214Krasovsky_1940
D_Bekaa_Valley_19201137Clarke_1880_RGS
D_Belge_19506215International_1924
D_Belge_19726313International_1924
D_Belinda_2000106946Belinda_2000_IAU_IAG
D_Bellevue_IGN6714International_1924
D_Bermuda_19576216Clarke_1866
D_Bermuda_20006762WGS_1984
D_Bern_18986217Bessel_1841
D_Bern_19386306Bessel_1841
D_Bessel_18416004Bessel_1841
D_Bessel_Modified6005Bessel_Modified
D_Bessel_Namibia6006Bessel_Namibia
D_Bhutan_National_Geodetic_Datum1058GRS_1980
D_Bianca_2000106947Bianca_2000_IAU_IAG
D_Bioko1136International_1924
D_Bissau6165International_1924
D_Bogota6218International_1924
D_Bukit_Rimpah6219Bessel_1841
Bulgaria_Geodetic_System_20051167GRS_1980
D_Cadastre_19971037International_1924
California_SRS_Epoch_2017.50_(NAD83)106012GRS_1980
D_Callisto_2000106912Callisto_2000_IAU_IAG
D_Calypso_2000106927Calypso_2000_IAU_IAG
D_Camacupa6220Clarke_1880_RGS
Camacupa_20151217Clarke_1880_RGS
D_Camp_Area6715International_1924
D_Campo_Inchauspe6221International_1924
D_Canton_19666716International_1924
D_Cape6222Clarke_1880_Arc
D_Cape_Canaveral6717Clarke_1866
D_Carme_2000106913Carme_2000_IAU_IAG
D_Carthage6223Clarke_1880_IGN
D_Cayman_Islands_Geodetic_Datum_20111100GRS_1980
D_CH19036149Bessel_1841
D_CH1903+6150Bessel_1841
D_Charon_2000106970Charon_2000_IAU_IAG
D_Chatham_Island_19716672International_1924
D_Chatham_Islands_19796673International_1924
D_China_20001043CGCS2000
D_Chos_Malal_19146160International_1924
D_Chua6224International_1924
D_Clarke_18586007Clarke_1858
D_Clarke_18666008Clarke_1866
D_Clarke_1866_Michigan6009Clarke_1866_Michigan
D_Clarke_18806034Clarke_1880
D_Clarke_1880_Arc6013Clarke_1880_Arc
D_Clarke_1880_Benoit6010Clarke_1880_Benoit
D_Clarke_1880_IGN6011Clarke_1880_IGN
D_Clarke_1880_RGS6012Clarke_1880_RGS
D_Clarke_1880_SGA6014Clarke_1880_SGA
D_Combani_19506632International_1924
D_Conakry_19056315Clarke_1880_IGN
D_Cordelia_2000106948Cordelia_2000_IAU_IAG
D_Corrego_Alegre6225International_1924
D_Corrego_Alegre_19611074International_1924
D_Costa_Rica_20051065WGS_1984
D_Cote_d_Ivoire6226Clarke_1880_IGN
D_Cressida_2000106949Cressida_2000_IAU_IAG
D_Croatian_Terrestrial_Reference_System6761GRS_1980
CR-SIRGAS1225GRS_1980
D_CSG_19676623International_1924
D_Cyprus_Geodetic_Reference_System_19931112WGS_1984
D_D48106278Bessel_1841
D_Dabola_19816155Clarke_1880_IGN
D_Datum_736274International_1924
D_Datum_Geodesi_Nasional_19956755WGS_1984
D_Datum_Lisboa_Bessel106262Bessel_1841
D_Datum_Lisboa_Hayford106263International_1924
D_Dealul_Piscului_19336316International_1924
D_Dealul_Piscului_19706317Krasovsky_1940
D_Deception_Island6736Clarke_1880_RGS
D_Deimos_2000106906Deimos_2000_IAU_IAG
D_Deir_ez_Zor6227Clarke_1880_IGN
D_Desdemona_2000106950Desdemona_2000_IAU_IAG
D_Despina_2000106961Despina_2000_IAU_IAG
D_Deutsche_Bahn_Reference_System1081Bessel_1841
D_Deutsches_Hauptdreiecksnetz6314Bessel_1841
D_Dione_2000106928Dione_2000_IAU_IAG
D_Dominica_19456602Clarke_1880_RGS
D_DOS_1968106218International_1924
D_DOS_71_46710International_1924
D_Douala6228Clarke_1880_IGN
D_Douala_19486192International_1924
D_Easter_Island_19676719International_1924
ECML14_NB_Intermediate_Reference_Frame1310GRS_1980
D_Egypt_19076229Helmert_1906
D_Egypt_19306199International_1924
D_Egypt_Gulf_of_Suez_S-650_TL6706Helmert_1906
D_Elara_2000106914Elara_2000_IAU_IAG
D_Enceladus_2000106929Enceladus_2000_IAU_IAG
EOS21_Intermediate_Reference_Frame1308GRS_1980
D_Epimetheus_2000106930Epimetheus_2000_IAU_IAG
D_Estonia_1937106101Bessel_1841
D_Estonia_19926133GRS_1980
D_Estonia_19976180GRS_1980
D_ETRF_19891178WGS_1984
ETRF2000_Poland1305GRS_1980
D_ETRS_19896258GRS_1980
D_Europa_2000106915Europa_2000_IAU_IAG
D_European_19506230International_1924
D_European_1950_ED776154International_1924
D_European_19796668International_1924
D_European_19876231International_1924
D_European_Libyan_19796159International_1924
European_Terrestrial_Reference_Frame_19901179GRS_1980
European_Terrestrial_Reference_Frame_19911180GRS_1980
European_Terrestrial_Reference_Frame_19921181GRS_1980
European_Terrestrial_Reference_Frame_19931182GRS_1980
European_Terrestrial_Reference_Frame_19941183GRS_1980
European_Terrestrial_Reference_Frame_19961184GRS_1980
European_Terrestrial_Reference_Frame_19971185GRS_1980
European_Terrestrial_Reference_Frame_20001186GRS_1980
European_Terrestrial_Reference_Frame_20051204GRS_1980
European_Terrestrial_Reference_Frame_20141206GRS_1980
D_Everest_18306042Everest_1830
D_Everest_Adj_19376015Everest_Adjustment_1937
D_Everest_Bangladesh106202Everest_Adjustment_1937
D_Everest_Def_19626044Everest_Definition_1962
D_Everest_Def_19676016Everest_Definition_1967
D_Everest_Def_19756045Everest_Definition_1975
D_Everest_India_Nepal106203Everest_Definition_1962
D_Everest_Modified6018Everest_1830_Modified
D_Everest_Modified_1969106006Everest_Modified_1969
EWR2_Intermediate_Reference_Frame1311GRS_1980
D_Fahud6232Clarke_1880_RGS
D_Faroe_Datum_19546741International_1924
D_Fatu_Iva_19726688International_1924
D_FD_19586132Clarke_1880_RGS
D_Fehmarnbelt_Datum_20101078GRS_1980
D_Fiji_19566721International_1924
D_Fiji_19866720WGS_1972
D_Fischer_1960106002Fischer_1960
D_Fischer_1968106003Fischer_1968
D_Fischer_Modified106004Fischer_Modified
D_fk896753International_1924
D_Fort_Desaix6625International_1924
D_Fort_Marigot6621International_1924
D_Fort_Thomas_1955106240Clarke_1880_RGS
D_Galatea_2000106962Galatea_2000_IAU_IAG
D_Gambia1139Clarke_1880_RGS
D_Gan_19706684International_1924
D_Ganymede_2000106916Ganymede_2000_IAU_IAG
D_Garoua6197Clarke_1880_RGS
GBK19_Intermediate_Reference_Frame1289GRS_1980
D_GDA_19946283GRS_1980
GDA20201168GRS_1980
D_GDBD20091056GRS_1980
D_GDM_20006742GRS_1980
D_GEM_10C6031GEM_10C
Geocentric_Datum_of_Mauritius_2008106028GRS_1980
Geodezicheskaya_Sistema_Koordinat_20111159GSK-2011
Georgia_Geodetic_Datum106010GRS_1980
D_GGRS_19876121GRS_1980
D_Graciosa_Base_SW_1948106241International_1924
D_Grand_Cayman_19596723Clarke_1866
D_Grand_Comoros6646International_1924
D_Greek6120Bessel_1841
D_Greenland_19966747GRS_1980
D_Grenada_19536603Clarke_1880_RGS
D_GRS_19676036GRS_1967
D_GRS_19806019GRS_1980
D_Guam_19636675Clarke_1866
D_Gulshan_3036682Everest_Adjustment_1937
D_Gunung_Segara6613Bessel_1841
Gusterberg_(Ferro)1188Zach_1812
D_GUX_1106221International_1924
D_Guyane_Francaise6235International_1924
D_Hanoi_19726147Krasovsky_1940
D_Hartebeesthoek_19946148WGS_1984
D_Helene_2000106931Helene_2000_IAU_IAG
D_Helle_19546660International_1924
D_Helmert_19066020Helmert_1906
D_Herat_North6255International_1924
D_Hermannskogel106102Bessel_1841
D_Himalia_2000106917Himalia_2000_IAU_IAG
D_Hito_XVIII_19636254International_1924
D_Hjorsey_19556658International_1924
D_Hong_Kong_19636738Clarke_1858
D_Hong_Kong_1963_676739International_1924
D_Hong_Kong_19806611International_1924
Hong_Kong_Geodetic1209GRS_1980
D_Hough_1960106005Hough_1960
HS2_Intermediate_Reference_Frame1264GRS_1980
D_Hughes_19806054Hughes_1980
D_Hungarian_19726237GRS_1967
D_Hungarian_Datum_19091024Bessel_1841
D_Hu_Tzu_Shan6236International_1924
D_Hyperion_2000106932Hyperion_2000_IAU_IAG
D_Iapetus_2000106933Iapetus_2000_IAU_IAG
IG05(2012)_Intermediate_Datum1144GRS_1980
IG05_Intermediate_Datum1142GRS_1980
IGb001246GRS_1980
IGb081248GRS_1980
IGb141272GRS_1980
D_IGC_1962_Arc_of_the_6th_Parallel_South6697Clarke_1880_RGS
D_IGM_19956670WGS_1984
D_IGN53_Mare6641International_1924
D_IGN56_Lifou6633International_1924
D_IGN63_Hiva_Oa6689International_1924
D_IGN72_Grande_Terre6634International_1924
D_IGN72_Nuku_Hiva6630International_1924
D_IGN_Astro_19606700Clarke_1880_RGS
IGS001245GRS_1980
IGS051247GRS_1980
IGS081141GRS_1980
IGS141191GRS_1980
IGS971244GRS_1980
D_Indian_19546239Everest_Adjustment_1937
D_Indian_19606131Everest_Adjustment_1937
D_Indian_19756240Everest_Adjustment_1937
D_Indonesian6021Indonesian
D_Indonesian_19746238Indonesian
D_Institut_Geographique_du_Congo_Belge_19556701Clarke_1880_RGS
D_International_19246022International_1924
D_International_19676023International_1967
International_Terrestrial_Reference_Frame_20141165GRS_1980
D_Io_2000106918Io_2000_IAU_IAG
D_Iraqi_Geospatial_Reference_System1029GRS_1980
D_Iraq_Kuwait_Boundary_19926667WGS_1984
D_IRENET956173GRS_1980
D_Islands_Network_19936659GRS_1980
D_Islands_Network_20041060GRS_1980
Islands_Net_20161187GRS_1980
D_Israel6141GRS_1980
Israel_Geodetic_Datum_20051114GRS_1980
Israeli_Geodetic_Datum_2005(2012)1115GRS_1980
D_ISTS_061_19686722International_1924
D_ISTS_073_19696724International_1924
D_ITRF_19886647GRS_1980
D_ITRF_19896648GRS_1980
D_ITRF_19906649GRS_1980
D_ITRF_19916650GRS_1980
D_ITRF_19926651GRS_1980
D_ITRF_19936652GRS_1980
D_ITRF_19946653GRS_1980
D_ITRF_19966654GRS_1980
D_ITRF_19976655GRS_1980
D_ITRF_20006656GRS_1980
D_ITRF_20056896GRS_1980
D_ITRF_20081061GRS_1980
D_Jamaica_18756241Clarke_1880
D_Jamaica_19696242Clarke_1866
D_Jamaica_20016758WGS_1984
D_Janus_2000106934Janus_2000_IAU_IAG
D_JGD_20006612GRS_1980
D_JGD_20111128GRS_1980
D_Johnston_Island_19616725International_1924
D_Jordan106277International_1924
D_Jouik_19616679Clarke_1880_RGS
D_Juliet_2000106951Juliet_2000_IAU_IAG
D_Jupiter_2000106908Jupiter_2000_IAU_IAG
D_Kalianpur_18806243Everest_1830
D_Kalianpur_19376144Everest_Adjustment_1937
D_Kalianpur_19626145Everest_Definition_1962
D_Kalianpur_19756146Everest_Definition_1975
D_Kandawala6244Everest_Adjustment_1937
D_Karbala_1979_Polservice6743Clarke_1880_RGS
D_Kasai_19536696Clarke_1880_RGS
D_Katanga_19556695Clarke_1866
D_Kerguelen_Island_19496698International_1924
D_Kertau6245Everest_1830_Modified
D_Kertau_RSO6751Everest_Modified_1969
Kingdom_of_Saudi_Arabia_Geodetic_Reference_
Frame_2017
1268GRS_1980
D_KKJ6123International_1924
D_Korea_20006737GRS_1980
D_Korean_Datum_19856162Bessel_1841
D_Korean_Datum_19956166WGS_1984
Kosovo_Reference_System_20011251GRS_1980
D_Kousseri6198Clarke_1880_RGS
D_Krasovsky_19406024Krasovsky_1940
D_Kusaie_19516735International_1924
D_Kuwait_Oil_Company6246Clarke_1880_RGS
D_Kuwait_Utility6319GRS_1980
D_Kyrgyz_Republic_2006106009GRS_1980
D_La_Canoa6247International_1924
D_Lake6249International_1924
D_Lao_19936677Krasovsky_1940
D_Lao_National_Datum_19976678Krasovsky_1940
D_Larissa_2000106963Larissa_2000_IAU_IAG
D_Latvia_19926661GRS_1980
D_LC5_1961106243Clarke_1866
D_Leda_2000106919Leda_2000_IAU_IAG
D_Leigon6250Clarke_1880_RGS
D_Le_Pouce_19346699Clarke_1880_RGS
D_Liberia_19646251Clarke_1880_RGS
D_Libyan_Geodetic_Datum_20066754International_1924
D_Lisbon6207International_1924
D_Lisbon_18906666Bessel_1841
D_Lithuania_19946126GRS_1980
D_Little_Cayman_19616726Clarke_1866
D_Locodjo_19656142Clarke_1880_RGS
D_Loma_Quintana6288International_1924
D_Lome6252Clarke_1880_IGN
D_Luxembourg_19306181International_1924
D_Luzon_19116253Clarke_1866
Lyon_Turin_Ferroviaire_20041295GRS_1980
D_Lysithea_2000106920Lysithea_2000_IAU_IAG
Macao_19201207International_1924
D_MACAO_20081208International_1924
D_Madeira_19366185International_1924
D_Madrid_18706903Struve_1860
D_Madzansua6128Clarke_1866
D_MAGNA6686GRS_1980
D_Mahe_19716256Clarke_1880_RGS
D_Majuro106270Clarke_1866
D_Makassar6257Bessel_1841
D_Malongo_19876259International_1924
D_Manoca6260Clarke_1880_RGS
D_Manoca_19626193Clarke_1880_IGN
D_Marco_Geodesico_Nacional1063GRS_1980
D_Mars_1979106904Mars_1979_IAU_IAG
D_Mars_2000106905Mars_2000_IAU_IAG
Mars_2000_(Sphere)106971Mars_2000_(Sphere)
D_Massawa6262Bessel_1841
D_Maupiti_19836692International_1924
D_Mauritania_19996702GRS_1980
D_Merchich6261Clarke_1880_IGN
D_Mercury_2000106900Mercury_2000_IAU_IAG
Mercury_2015106974Mercury_2015
D_Metis_2000106921Metis_2000_IAU_IAG
D_Mexican_Datum_of_19931042GRS_1980
D_Mexico_ITRF20081120GRS_1980
D_MGI6312Bessel_1841
D_MGI_19011031Bessel_1841
D_Mhast_19516703Clarke_1880_RGS
D_Mhast_Offshore6705International_1924
D_Mhast_Onshore6704International_1924
D_Midway_19616727International_1924
D_Mimas_2000106935Mimas_2000_IAU_IAG
Ministerio_de_Marina_Norte1258International_1924
Ministerio_de_Marina_Sur1259International_1924
D_Minna6263Clarke_1880_RGS
D_Miranda_2000106952Miranda_2000_IAU_IAG
MML07_Intermediate_Reference_Frame1271GRS_1980
D_MOLDREF991032GRS_1980
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D_Montserrat_19586604Clarke_1880_RGS
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D_MOP786639International_1924
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D_Moznet6130WGS_1984
D_Mporaloko6266Clarke_1880_IGN
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NAD_1983_(High_Accuracy_Reference_Network- Corrected)1212GRS_1980
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D_NAD_1983_HARN_Adj_MN_Beltrami_South106703S_GRS_1980_Adj_MN_Beltrami_South
D_NAD_1983_HARN_Adj_MN_Benton106704S_GRS_1980_Adj_MN_Benton
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D_NAD_1983_HARN_Adj_MN_Carlton106708S_GRS_1980_Adj_MN_Carlton
D_NAD_1983_HARN_Adj_MN_Carver106709S_GRS_1980_Adj_MN_Carver
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D_NAD_1983_HARN_Adj_MN_Chisago106713S_GRS_1980_Adj_MN_Chisago
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106737S_GRS_1980_Adj_MN_Lake_of_the_Woods
_North
D_NAD_1983_HARN_Adj_MN_Lake_of_the_Woods
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106738S_GRS_1980_Adj_MN_Lake_of_the_Woods
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D_Sphere6035Sphere
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St_Helena_Geodetic_Datum_20151174GRS_1980
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D_Struve_18606028Struve_1860
St._Stephen_(Ferro)1189Zach_1812
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Sun_2015106975Sun_2015
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D_Tahiti_19526628International_1924
D_Tahiti_19796690International_1924
D_Tananarive_19256297International_1924
Tapi_Aike1257International_1924
D_Telesto_2000106941Telesto_2000_IAU_IAG
D_Tern_Island_19616707International_1924
D_Tete6127Clarke_1866
D_Tethys_2000106942Tethys_2000_IAU_IAG
D_Thalassa_2000106967Thalassa_2000_IAU_IAG
D_Thebe_2000106924Thebe_2000_IAU_IAG
D_Timbalai_19486298Everest_Definition_1967
D_Titan_2000106943Titan_2000_IAU_IAG
D_Titania_2000106958Titania_2000_IAU_IAG
D_TM656299Airy_Modified
D_TM756300Airy_Modified
D_Tokyo6301Bessel_1841
D_Tonga_Geodetic_Datum_20051095GRS_1980
TPEN11_Intermediate_Reference_Frame1266GRS_1980
D_Trinidad_19036302Clarke_1858
D_Tristan_19686734International_1924
D_Triton_2000106968Triton_2000_IAU_IAG
D_Trucial_Coast_19486303Helmert_1906
D_Turkish_National_Reference_Frame1057GRS_1980
D_TWD_19671025GRS_1967_Truncated
D_TWD_19971026GRS_1980
D_Ukraine_20001077Krasovsky_1940
D_Umbriel_2000106959Umbriel_2000_IAU_IAG
D_Uranus_2000106944Uranus_2000_IAU_IAG
D_Vanua_Levu_19156748Clarke_1880_Intl_Ft
D_Venus_1985106901Venus_1985_IAU_IAG_COSPAR
D_Venus_2000106902Venus_2000_IAU_IAG
D_Vientiane_19826676Krasovsky_1940
D_Vietnam_20006756WGS_1984
D_Viti_Levu_19126752Clarke_1880_Intl_Ft
D_Viti_Levu_19166731Clarke_1880_RGS
D_Voirol_18756304Clarke_1880_IGN
D_Voirol_18796671Clarke_1880_IGN
D_Wake_Eniwetok_19606732Hough_1960
D_Wake_Island_19526733International_1924
D_Walbeck106007Walbeck
D_War_Office6029War_Office
D_WGS_19666760WGS_1966
D_WGS_19726322WGS_1972
D_WGS_1972_BE6324WGS_1972
D_WGS_19846326WGS_1984
World_Geodetic_System_1984_(G1150)1154WGS_1984
World_Geodetic_System_1984_(G1674)1155WGS_1984
World_Geodetic_System_1984_(G1762)1156WGS_1984
World_Geodetic_System_1984_(G730)1152WGS_1984
World_Geodetic_System_1984_(G873)1153WGS_1984
World_Geodetic_System_1984_(Transit)1166WGS_1984
D_Xian_19806610Xian_1980
D_Xrail84106050WGS_1984
D_Yacare6309International_1924
D_Yemen_NGN_19966163WGS_1984
D_Yoff6310Clarke_1880_IGN
D_Zanderij6311International_1924

Concept Quiz

2.5.2: Horizontal and Vertical Datums

We've explored the idea that the geodetic datum the result of attaching a "free-floating" reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid to a specifically measured geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." via control points and benchmarks. is the product of combining a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." with a selected reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , starting with the benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . , and establishing all the remaining control points utilizing the principles of labeling intersections with a Cartesian Coordinate System. We noted that the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid - covered with a grid and with the 2D land and ocean masses drawn upon it - is connected to a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." - a mathematical derivation of local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features at any point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface via the specific gravitational pull at said point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. , which stores our 3D elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface values.

Looking at the creation of the datum in this way makes it seem like there are only "2D datums" and "3D geoids", when in reality, the two work together to produce three kinds of 3D datums: three-dimensional datums. All three come from the initial product: combine a reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid with a geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." via control points (both benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . and mathematically derived intersections) and each serves a specific purpose in Geospatial Sciences. All three are actually a 3 dimensional product, regardless of the fact that only one is called "three dimensional". Everything we have looked at up to this point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. is really a 3D object. Reference ellipsoids, datums, geoids ... all three dimensional objects.

Horizontal datums contain only XY values upon a 3D Earth, vertical datums used to reference locations and distances above mean sea level; elevation. contain only Z values upon a 3D Earth, and three-dimensional datums contain XY and Z values upon a 3D Earth.

Horizontal datums assume that every point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface is at an equal zero elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface - including the tops of mountains, the bottoms of oceans, and everything in between. Horizontal datums assume there is no change in topography or relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , and that the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." is totally flat and level at every point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface.

Vertical datums come in three varieties: orthometric datums shows the changes in the Earth's gravitational pull from 0 - any height referenced to the Earth's gravity field can be called as "geopotential heights" which shows the changes in the Earth's gravitational pull from 0 - any height referenced to the Earth's gravity field can be called as "geopotential heights"; tidal datums show the changes in sea level due to tides and are based on local mean sea level which show the changes in sea level due to tides and are based on local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features ; and three-dimensional datums, which combine horizontal datums used to reference location on the Earth's surface, regardless of elevation with ellipsoidal height.

Three-dimensional datums are used by GPS units to determine a personal elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface at any given time. Introduction to GIS Geographic Information Systems the software used to create, store, and manage spatial data Data that deals with location, such as lists of addresses, the footprint of a building, the boundaries of cities and counties, etc. , analyze spatial problems, and display the data in cartographic layouts   Geographic Information Sciences doesn't work with vertical or three-dimensional datums at all, and GPS for GIS Geographic Information Systems the software used to create, store, and manage spatial data Data that deals with location, such as lists of addresses, the footprint of a building, the boundaries of cities and counties, etc. , analyze spatial problems, and display the data in cartographic layouts   Geographic Information Sciences only looks at the fact the if you stand at any given point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface, the elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface is known, so for now, just understand that there are a few products of the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid + geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." process.

Concept Quiz

2.5.3: Geodetic Height and Elevation

Elevation is defined as the measured distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . (as we've looked at while examining the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." creation method, local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features is specified because of the varied effects of the Earth’s gravitational pull; using a global mean sea level the average of the sea level as affected by the pull of gravity when there is a finite amount of water upon a model of the Earth. would create accurate measurements in some places and incorrect measurements in others.)

Altitude is defined as the height of an independent object, such as an aircraft, above ground level (AGL) or Above Sea Level (ASL). If an aircraft was flying at 2,000 feet AGL, it would be 2,000 feet above the ground right below it, no matter the elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface of that location. Aircraft can also record values Above Sea Level (ASL), where elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface and altitude the height of an independent object, such as an aircraft, above ground level (AGL) are combined. Regardless of how an aircraft is recording measurements, it is not possible for a person to hike to an altitude the height of an independent object, such as an aircraft, above ground level (AGL) , only an elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface .

Orthometric height is defined as measured distance between the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . Elevation measurements are created from and stored within vertical datums used to reference locations and distances above mean sea level; elevation. for accurate Z-values within a geographic coordinate system for accurate analysis.

Ellipsoid height is defined as the measured distance between the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . GPS receivers use ellipsoidal height since the calculation is easier to obtain on the fly. If the GPS knows where it is in relation to the XY positions on the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , it can easily calculate it's height above that established zero. For a GPS receiver, to calculate elevations based on the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." is much more labor intensive.

Geoid separation ( geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." height) is defined as the measured difference between the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid and the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . Worldwide, geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." separation varies from +278.87 feet (85 meters) to -351.05 feet (-107 meters), based on the WGS84 reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid and the Earth Gravitational Model 1996 (EGM96) geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . Geoid separation is used to increase the accuracy of GPS measurements in the post-processing phase of data collection since the GPS receiver uses elevation the vertical distance between local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features and a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface established between the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." , but accurate measurements are between the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." (or the orthometric height the measured distance between the geoid and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . ). In order to convert from ellipsoidal height to orthometric height the measured distance between the geoid and the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . , the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." separation needs to be a known value. Notice how these height determination definitions can be related back to the kinds of vertical datums used to reference locations and distances above mean sea level; elevation. .

Figure 2.18: A Graphical Explanation of Ellipsoid Height, Orthometric Height, and Geoid Height
Figure 2.19: Deviation of the Geoid and Reference Ellipsoid in For the World Geodetic Datum, 1984 (WGS84)
This image shows the areas where the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid and geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." deviate in the WGS84 Geographic Coordinate System. We see that there are really only a few places where the two meet exactly; just the places where the pink and the blue meet in a single line. Most areas of the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." fall either above or below the surface of the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , where the reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid is a smooth, even shape and the geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." is the undulating weirdo shape./td>
The Main Point...
Representing the Earth in three ways, geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." , reference ellipsoid an ellipsoid that is drawn to best-fit an area. World reference ellipsoids are drawn to best-fit the entire geoid; local ellipsoids are best fit on one side to a single place of the geoid , and topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." , leads us to a need to know the measured distances between each of them. Each surface serves a different purpose in geodesy the science of measuring and monitoring the size and shape of the Earth and the location of points on its surface , so eliminating one is impossible.

Concept Quiz

2.5.4: Datum Shifts

Over time, data collection technology improves, while at the same time land and ocean masses are in constant flux - moving, shifting, and boogieing down. As a response to these changes, datum measurements regarding the precision of control points must also be changed to keep up with the shifting world. While it might seem rather obvious, it is a gazillion times easier to move the mathematical points on the datum then to go out, dig up, and physically replace the benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . . And since we know that benchmarks Benchmarks are real-world locations which have been carefully surveyed with locations to match a specific geoid a model of the variation between global mean (average) sea level and local mean sea level the measurement above or below the global average at a single point A GIS vector data in any sort of digital science or art, is simply denoting a type of graphical representation using straight lines to construct the outlines of objects geometry type which is made up of just one vertex pl. vertices One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. , marking a single XY location in any given geographic or projected coordinate system. on the Earth's surface used for recording the elevation of topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." 's relief the difference between the highest and lowest point within a particular area while landforms are the descriptive words for individual features , which is used to measure precise elevations on the topographic surface a detailed map of the surface features of land. It includes the mountains, hills, creeks, and other bumps and lumps on a particular hunk of earth. The word is a Greek-rooted combo of topos meaning "place" and graphein "to write." . are used for several different datums, moving them to repair the accuracy of one datum would be catastrophic to all of the others.

A datum shift when control points are adjusted via better mathematical calculations or real-world surveying. Benchmarks cannot move, but control points can change via datum shifts.  ''Major'' Large effort; many points change; expensive and time-consuming. Noted with a two-digit year (ie NAD83)  ''Minor'' Just a few points change. Less expensive; less involved. Noted with a four-digit year (ie. NAD83(1985)) is when the coordinate associated with a benchmark (and the resulting control points) is adjusted or changed based upon either better surveying techniques, better mathematical calculations, or adjustments for continental shift. Datum shifts can be major, noted with a two-digit year following the datum name — i.e. NAD27 to NAD83 — or minor, noted with a four-digit year in parenthesis following the datum name (i.e. WGS84(1988)). Major datum shifts are extremely involved, and usually include a major survey project and mathematical calculations, while minor datum shifts are usually completed when just a few control points are deemed incorrect.

An Example of a Datum Shift

ATLAS OBSCURA: WWW.ATLASOBSCRUA.COM

Australia’s Entire GPS Navigation is Off By 5 Feet And now they’re going to fix it.

BY JESSIE GUY-RYAN JULY 31, 2016

A control marker set by the National Geodetic Survey to determine latitude and longitude. (Photo:Lee Cannon / CC BY-SA-2.0)

If you’re a regular user of satellite navigation services, you’ve likely noticed that the coordinates aren’t always pin-point accurate. If you’re in Australia, you might have noticed a strange consistency in this imprecision — specifically, that everything is about 1.5 meters ( just under 5 feet) off the mark.

Now, the Australian government has launched a project to update the datum underpinning its satellite navigation coordinates, compensating for tectonic shifts that have pushed everything on the continent about five feet over from where it’s “supposed” to be. Earth’s landmasses aren’t fixed in place; rather, Earth’s lithosphere (mantle and crust) is made up of numerous tectonic plates, which move over time. This movement explains the separation of Pangaea into the continents that exist today, earthquakes, island formation, and numerous other phenomena.

But satellite navigation systems aren’t somehow tracking the movement of Earth’s tectonic plates and updating their maps accordingly. In 2013, George Musser explained how sat-nav really works (and isn’t quite accurate) in a blog post for Scientific American. As he explains it, sat-nav relies on latitude and longitude grids—called datum—just like every other map. But not every system—again, just like maps—relies on the same datum. For example, the United States’ NAVSTAR Global Positioning System (one of only two operational global satellite navigation systems), uses WGS 84, which is tied to the Earth’s core rather than one tectonic plate, but most US. maps use NAD 83, which is tied to the North American tectonic plate. That’s why Google Maps is sometimes a little off, in case you were wondering.

As Dru Smith of NGS told Scientific American, the plate-based datum system is basically a compromise to keep surveyors from pulling their hair out. “Most surveyors and mapmakers would be happy to live in a world where the plates don’t move,” Smith explains. “We can’t fix that, but we can fix the datum so that the effect is not felt by the predominant number of users.”

As for Australia, it currently relies on the Australia-specific GDA94 datum developed in 1994. Because the eastern part of the Indo-Australian tectonic plate is moving about 5.6 centimeters per year (2.2 inches), the data has become increasingly inaccurate over the past two decades.

Five feet isn’t a big deal when you’re getting directions to a nearby town, but as satellite navigation becomes increasingly used in systems that need pinpoint accuracy (think self-driving cars) that little offset becomes a much bigger problem. The increasing need for precision tracking is what motivated Geoscience Australia to establish the AuScope Global Navigation Satellite System (GNSS) network, which is underpinning the GDA2020 effort that will update Australia’s geocentric datum, bringing it in line with the International Terrestrial Reference Frame used in other regions.

The new datum will initially map to where the Australian continent will be located in the year 2020 (hence the name). According to The Register, that means when it first comes into use in 2017, it’ll be about 20 centimeters off. Then, in 2020, phase 2 of the project starts and the project gets really interesting. Phase 2 will synchronize the datum with GNSS, allowing “locations of points and their movement over time to be modeled,” meaning the datum should be continuously up-to-date.

So, what about every other country’s sat-nav? We’re all on tectonic plates, and they’re all moving, right? Right—and as we refine our understanding of the planet’s size and shape, our datum will need to be updated too, eventually. In the US., the National Geodetic Survey plans to update NAD 83 in 2022; when that happens, latitude and longitude points in North America will shift at least a meter.

“We’re fast approaching the day when people will expect accuracies of centimeters in real time out of their handhold devices and then we’ll see a lot of head scratching as things no longer line up,” Smith told Scientific American three years ago. It looks like that day has arrived.

Concept Quiz

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