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.
The ArcGIS platform provides several ways for users to automate their geoprocessing tasks. These options differ in the amount of skill required to produce the automated solution and in the range of scenarios that each can address. The text below touches briefly on these automation options, in order from requiring the least coding skill to the most.
The first option is to construct a model using ModelBuilder. ModelBuilder 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 ModelBuilder is that users can automate rather complex GIS workflows without the need for programming. You will learn how to use ModelBuilder early in this course.
Some automation tasks require greater flexibility than is offered by ModelBuilder, 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 Desktop and Pro installations. 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 more recently developed automation option available on the ArcGIS platform is the ArcGIS API (Application Programming Interface) for Python. This is an environment in which Python scripting is better integrated with Esri's cloud- and server-based technologies (ArcGIS Online, Portal for ArcGIS, ArcGIS Enterprise). Code written to interact with the Python API is often written in a "notebook" environment such as Jupyter Notebook. In a notebook environment, code can be executed in a stepwise fashion with intermediate results displayed in between the Python statements. The use of the Python API in a Jupyter Notebook environment is a topic in our Advanced Python class, GEOG 489.
For geoprocessing tasks that require support for user interaction with the map or other UI elements, the ArcGIS Pro SDK (Software Development Kit) offers the ability to add custom tools to the Pro interface. The Pro SDK requires programming in the .NET framework using a .NET language such as Visual Basic .NET or C#. Working with this SDK's object model provides greater flexibility in terms of what can be built as compared to writing Python scripts around their geoprocessing framework. The tradeoff is a higher level of complexity involved in the coding.
Finally, developers who want to create their own custom GIS applications, typically focused on delivering much narrower functionality than the one-size-fits-all ArcGIS Pro, can develop apps using the ArcGIS Runtime SDKs. The Runtime SDKs make it possible to author apps for Windows, Mac, or Linux desktop machines, as well as for iOS and Android mobile devices, again involving a greater level of effort than your typical Python geoprocessing script.
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.