Geoprocessing with Python scripts and Models in GIS

The two main focuses of this week's Lab assignment in GIS Programming was an introduction to Model Builder in ArcGIS Pro and coding a geoprocessing script from scratch. The lessons show that Geoprocessing Tools can be run solely with Python scripts and the process be automated using models. Both use the ArcPy package, which contains several modules and other elements that add functionality to Python.

Geoprocessing is a series of actions performed on geographic data where there is an input or multiple inputs, a process or task, and then an output of data. There are two general categories of geoprocessing tools in ArcGIS Pro. There are the system or built-in tools created by ESRI. Then there are custom tools, including models and scripts, created by a user or a third-party.

Model Builder is used to run a sequence of tools to obtain a desired result. The application uses four elements: data variables which reference data on disk, value variables provided in formats such as linear units, connectors and tools. Model Builder uses a GUI interface and layout with some similarities to the program flowcharts designed in previous modules. The model elements are color coded to aid in their classification.

The model I developed showing the automation of using three Geoprocessing tools

With sample data provided, the model created for Module 4 took a polygon layer of soils and clipped it to a polygon layer of a basin. The extracted section of the soils feature class was then filtered to select only parcels that were deemed unsuitable for farming. That subset of the soils data was then removed from previously created soils layer including only areas within the original basin.

Output layer of suitable soils for farming

If you had this kind of data over a large area, such as a county or state, rerunning this model for specific locations could save a significant amount of time. ArcGIS Pro can also export this model as a stand alone Python script. The only caveat is making sure the workspace environment, the default location for the files being processed, is set. This typically means using more explicit information such as full file paths.

What is an environment? Our textbook describes them as "essentially hidden parameters that influence how a tool runs." Environments are properties of the env class in ArcPy. Part of object-orientated programming (OOP), classes can be used to create objects, which in turn has properties and methods that can be used. Classes are often implemented to avoid using long and complicated strings in code. The OOP concept is still somewhat fuzzy to me, but  it is becoming more clear with continued use of Python .

Using Python code to perform geoprocessing tasks was not as difficult as anticipated. The three tasks to complete started with a point feature class of hospitals as the input data. Geoprocessing tools first add field values for X and Y based upon the coordinate system of the dataset. The second created a 1,000 meter buffer around each point while the last dissolved the individual buffers as a single feature in a completely separate feature class.

Flowchart showing the general behavior of the Python script

Approached writing this script by researching the syntax for the three geoprocessing tools. With a basic understanding of required parameters and optional parameters, coding was fairly straight forward. Trying out some of the syntax options, I hard-coded parameters such as the input feature layer name while assigning a variable for another function argument.

I also tried out separate nomenclature for calling the tools. Tools can be called by its function such as arcpy.<toolname_tollboxalias>(<parameters>) or the toolbox as a module followed by the tool as a function as arcpy.<toolboxalias>.<toolname>(<parameters>. The main difference is the use of an underscore or dot between the tool name and tool alias.

Used comments on most lines of the script so I can return to it for reference. With the final line of code compiled, I ran the script and encountered an error referencing the incorrect parameters for the Dissolve tool. I then implemented a try-except statement and quickly identified that I forgot to add a comma between the in_features and out_feature_class parameters. With that, the script ran successfully!