PrintHere’s another simple script that finds all cells over 3500 meters in an elevation raster and makes a new raster that codes all those cells as 1. Remaining values in the new raster are coded as 0. By now, you’re probably familiar with this type of “map algebra” operation which is common in site selection and other GIS scenarios.
Just in case you’ve forgotten, the expression Raster(inRaster) tells arcpy that it needs to treat your inRaster variable as a raster dataset so that you can perform map algebra on it. If you didn't do this, the script would treat inRaster as just a literal string of characters (the path) instead of a raster dataset.
# This script uses map algebra to find values in an
# elevation raster greater than 3500 (meters).
import arcpy
from arcpy.sa import *
# Specify the input raster
inRaster = "C:/Data/Elevation/foxlake"
cutoffElevation = 3500
# Check out the Spatial Analyst extension
arcpy.CheckOutExtension("Spatial")
# Make a map algebra expression and save the resulting raster
outRaster = Raster(inRaster) > cutoffElevation
outRaster.save("C:/Data/Elevation/foxlake_hi_10")
# Check in the Spatial Analyst extension now that you're done
arcpy.CheckInExtension("Spatial")
You can probably easily work out what this script is doing but, just in case, the main points to remember on this script are:
- Notice the lines of code that check out the Spatial Analyst extension before doing any map algebra and check it back in after finishing. Because each line of code takes some time to run, avoid putting unnecessary code between checkout and checkin. This allows others in your organization to use the extension if licenses are limited. The extension automatically gets checked back in when your script ends, thus some of the Esri code examples you will see do not check it in. However, it is a good practice to explicitly check it in, just in case you have some long code that needs to execute afterward, or in case your script crashes and against your intentions "hangs onto" the license.
- inRaster begins as a string, but is then used to create a Raster object once you run Raster(inRaster). A Raster object is a special object used for working with raster datasets in ArcGIS. It's not available in just any Python script: you can use it only if you import the arcpy module at the top of your script.
- cutoffElevation is a number variable that you declare early in your script and then use later on when you build the map algebra expression for your outRaster.
- The expression outRaster = Raster(inRaster) > cutoffElevation is saying, in plain terms, "Make a new raster and call it outRaster. Do this by taking all the cells of the raster dataset at the path of inRaster that are greater than the number assigned to the variable cutoffElevation."
- outRaster is also a Raster object, but you have to call the method outRaster.save() in order to make it permanent on disk. The save() method takes one argument, which is the path to which you want to save.
Copy the code above into a file called Lesson1A.py (or similar as long as it has a .py extension) in spyder or your favorite IDE or text editor and then save it.
We don’t need to do anything to this code to get it to work in Python 3, it will be fine just as it is. Feel free to check it against Analyze Tools for Pro if you like. Your results should say “Analyze Tools for Pro Completed Successfully” with the lack of warnings signifying that the code you supplied is compatible with Python 3.
Next, we'll convert the script to a Tool.