When we look at the model for understanding data and spatial problems in GIS - Reality > Conception > Representation > Analysis > Documentation > Storage > Distribution - geoprocessing fits into the “Analysis” portion of the equation. After we have experienced the world (reality), set the goals and objectives of the project, determined the spatial “question” to be answered, conjured an idea in our minds about how our data should look (conception), and created that data to represent reality (representation), we need to solve for those spatial questions through the process of analysis. GIS, as we know, is a vehicle for rapid and repeatable analysis of spatial data, and the software provides us the tools we need for this process.
Geoprocessing is the fundamental set of tools used to “solve spatial problems” using GIS. These tools are used on a daily basis and almost no project is complete without using a tool or two (or ten or twenty or ....). Geoprocessing tools vary from simple, such as a buffer tool, to complex spatial statistics tools which analyze spatial patterns present in a map.
Almost all geoprocessing tools in the GIS utilize the same pattern: an input layer or layers for the tool, a series of options and parameters associated with the tool are defined for this particular geoprocessing session, an output layer is defined by designating a name for the new file and a place to save it, and the tool is allowed to process.
Geoprocessing tools can be broken into five categories:
- overlay analysis: examines spatial relationships between two layers regarding direct feature interaction, answering questions such as “Which features from one layer intersect another?” and “How do these two layers interact and what does that mean in a spatial way?”;
- proximity analysis: examines spatial relationships between two layers regarding distance between features, answering questions such as “What is near what?” and “How far is something from something else?”;
- extraction analysis: tools which create smaller datasets from larger ones;
- surface analysis: tools that create layers of continuous data such as deriving a slope layer or an aspect layer from a DEM; and
- statistical analysis: examine both spatial and non-spatial statistical relationships utilizing the basic geographic principal that object near each other are more likely to be similar then objects that are far apart and statistical analysis of table values.
In this Chapter, we will look at each category in depth as well as examine some of the more common tools in each category, not with a goal of memorizing how each tool functions, as that will come in time, but instead to understand how categories of tools operate to build a foundation of geoprocessing comprehension. This means you understand what the output of the tools should be and thus, an expected outcome for a tool run. One thing that is true about ArcGIS (and other GIS softwares) is the fact that it does exactly what you tell it to do every single time you tell it to do something. Introductory students tend not to belive this fact, as it always seems that it's the softwares fault, but honestly, it is usually user error (even when you get really good at the software, it will do things that seem weird, but it's just that pesky user error!). When a GIS technician understands what the tool does, that technician can input a couple of layers and can expect some outcome. It's not expected that the technician is a human GIS, where they can solve the geospatial problem in their head, but instead, if the tool they are running exports the vector geometry type of the first layer added, and that first layer is a polyline, if the tool exports a point layer, the technician did something wrong. Part of learning about the tools is reading about what the tool does, understand what layer goes where in the tool, and what the expected output should roughly look like.
By having a foundation of geoprocessing tool comprehension, your ability to understand a large variety of tools is possible, not limiting your learning to only those tools which are presented in class or lab. You'll be able to read the description of a new tool and think about what the tool does, combine what you know about other tools, and make an educated guess regarding if the tool is appropriate at that given time or not. Honestly, this chapter (and this class) is less about teaching you how to use specific tools and what they do as it's about teaching you how to understand what that tool does and predicting the output based on the output which is demonstrated in lab. The ability to build strong geoprocessing comprehension skills is the key to becoming a great GIS Analyst, not just a technician who can push the buttons as described in a particular project.