GIS Programming: Working with Rasters

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Rasters are one of the two primary data models used in GIS. While vectors use points and line segments to identify locations on Earth, rasters define space using a matrix of equally sized cells. Two modules included within the ArcPy package allow access to many geoprocessing tools designed to work with raster data and imagery, which is a specific type of raster data. These modules are the Spatial Analyst (arcpy.sa) and Image Analyst (arcpy.ia) modules. 

Our task for this week's lab was to write a script that creates an output raster layer identifying areas that fall within a given set of parameters regarding slope, aspect, and land cover class. More specifically, the output layer had to identify areas with the following characteristics: 

  • Forest land cover (classifications 41, 42, and 43)
  • Slope between 5°and 20°
  • Aspect between 150°and 270°
Below is a flowchart which outlines the various steps of the script.


One challenge I had while writing the script was trying to assign both the upper and lower limits of the slope and aspect calculations to a single variable (i.e., goodSlope and goodAspect) using the logical and operator in the following lines of code:

goodSlope = (slope >= 5) and (slope <= 20)
goodAspect = (aspect >= 150) and (aspect <= 270)

 
This resulted in the following error: 

ValueError: The truth value of a raster is ambiguous. Invalid use of raster with Boolean operator or function. Check the use of parentheses where applicable.


To resolve this issue, I ended up having to assign the maximum and minimum slope and elevation to two separate variables as explained in the lab instructions, resulting in four total variables as follows: 

goodSlopeLow = slope >= 5
goodSlopeHigh = slope <= 20
goodAspectLow = aspect >= 150
goodAspectHigh = aspect <= 270


Below is a screenshot of the output raster result from my successful script. The areas meeting the slope, aspect, and land cover class criteria are shown in red. 




The following is a screenshot of the results of my script shown in the IPython Console within Spyder.





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