A geographic information system (GIS) can manipulate and analyze spatial datasets with the purpose of solving geographic problems. GIS analysts perform all kinds of operations on data to make it useful for solving a focused problem. This includes clipping, reprojecting, buffering, merging, mosaicking, extracting subsets of the data, and hundreds of other operations. In the ArcGIS software used in this course, these operations are known as geoprocessing and they are performed using tools.
Successful GIS analysis requires selecting the most appropriate tools to operate on your data. ArcGIS uses a toolbox metaphor to organize its suite of tools. You pick the tools you need and run them in the proper order to make your finished product.
Suppose you’re responsible for selecting sites for a chain restaurant. You might use one tool to select land parcels along a major thoroughfare, another tool to select parcels no smaller than 0.25 acres, and other tools for other selection criteria. If this selection process were limited to a small area, it would probably make sense to perform the work manually.
However, let’s suppose you’re responsible for carrying out the same analysis for several areas around the country. Because this scenario involves running the same sequence of tools for several areas, it is one that lends itself well to automation. There are several major benefits to automating tasks like this:
- Automation makes work easier. Once you automate a process, you don't have to put in as much effort remembering which tools to use or the proper sequence in which they should be run.
- Automation makes work faster. A computer can open and execute tools in sequence much faster than you can accomplish the same task by pointing and clicking.
- Automation makes work more accurate. Any time you perform a manual task on a computer, there is a chance for error. The chance multiplies with the number and complexity of the steps in your analysis, as well as the fatigue incurred by repeating the task many times. In contrast, once an automated task is configured, a computer can be trusted to perform the same sequence of steps every time for a potentially endless number of cycles.
ArcGIS provides three ways for users to automate their geoprocessing tasks. These three options differ in the amount of skill required to produce the automated solution and in the range of scenarios that each can address.
The first option is to construct a model using Model Builder. Model Builder is an interactive program that allows the user to “chain” tools together, using the output of one tool as input in another. Perhaps the most attractive feature of Model Builder is that users can automate rather complex GIS workflows without the need for programming. You will learn how to use Model Builder early in this course.
Some automation tasks require greater flexibility than is offered by Model Builder, and for these scenarios it's recommended that you write short computer programs, or scripts. The bulk of this course is concerned with script writing.
A script typically executes some sequential procedure of steps. Within a script, you can run GIS tools individually or chain them together. You can insert conditional logic in your script to handle cases where different tools should be run depending on the output of the previous operation. You can also include iteration, or loops, in a script to repeat a single action as many times as needed to accomplish a task.
There are special scripting languages for writing scripts, including Python, JScript, and Perl. Often these languages have more basic syntax and are easier to learn than other languages such as C, Java, or Visual Basic.
Although ArcGIS supports various scripting languages for working with its tools, Esri emphasizes Python in its documentation and includes Python with the ArcGIS installation. In this course we’ll be working strictly with Python for this reason, as well as the fact that Python can be used for many other file and data manipulation tasks outside of ArcGIS. You’ll learn the basics of the Python language, how to write a script, and how to manipulate and analyze GIS data using scripts. Finally, you’ll apply your new Python knowledge to a final project, where you write a script of your choosing that you may be able to apply directly to your work.
A third option available to ArcGIS users looking to automate geoprocessing is to build a solution using ArcObjects, the programming building blocks used by Esri’s own developers to produce the ArcGIS desktop products. With ArcObjects, it is possible to customize the user interface to include specific commands and tools that either go outside the abilities of the out-of-the-box ArcGIS tools or modify them to work in a more focused way. ArcObjects programming and interface customization are outside the scope of this course, but are covered in the GIS Application Development course, GEOG 489. GIS customization with ArcObjects can be an advanced endeavor, and learning a scripting language like Python is a good way to prepare yourself by learning basic programming concepts.
The tools that you run in ModelBuilder and Python actually use ArcObjects "under the hood" to run GIS functions; however, the advantage of Python scripting with ArcGIS is that you don't need to learn all the ArcObjects logic behind the tools. Your job is just to learn the tools and how to run them in the appropriate order to accomplish your task.
This first lesson will introduce you to concepts in both model building and script writing. We’ll start by just getting familiar with how tools run in ArcGIS and how you can use those tools in the ModelBuilder interface. Then, we’ll cover some of the basics of Python and see how the tools can be run within scripts.