Project 6: Inverse Distance Weighted Interpolation (2)

Creating a map of interpolation errors

The Spatial Analyst IDW tool doesn't create a map of errors by default (why?) but, in this case, we have the correct data, so it is instructive to compile an error map to see where your interpolation output fits well and where it doesn't.

1. Use the Spatial Analyst tool - Map Algebra - Raster Calculator... tool to bring up the Raster Calculator dialog (Figure 6.7).

   

   Figure 6.7: The Raster Calculator user interface.

   Credit: Kessler.
   
   Here, you can define operations on raster map layers, such as calculating the error in your interpolation output.
2. The error at each location in your interpolated map is (interpolated elevation - actual elevation). It is a simple matter to enter this equation in the expression editor section of the dialog, as shown [note: the name of your interpolated layer may be different]. When you click Run, you should get a new raster layer showing the errors, both positive and negative, in your interpolation output.
3. Using the Spatial Analyst Tools - Surface - Contour tool, you can draw error contours and examine how these relate to both your interpolated and actual elevation maps. When designing your map, think about how you would choose colors to appropriately symbolize the positive and negative errors. Some ideas to think about when examining the error contours...
   • Where are the errors largest?
   • What are the errors at or near control points in the random spot heights layer?
   • What feature of these data does inverse distance weighted interpolation not capture well?