Terrain Category Overlays

Select this option from the Overlays menu with a map open.  There must be a DEM associated with the map.

With this window you select the values that define a category and the color to describe it, and then the program will mark all terrain that matches the selected values.

  The panels shown will depend on which options you have checked at the top of the form.
  • Elevations: Select the minimum and maximum elevations in meters, with indication below of the equivalent values in feet, and the percentiles within the DEM.
  • Slopes: Select slopes in percentages, and the program will display the corresponding slope angle in degrees to assist in your selection.
  • Relief: relief is the difference in elevation between the highest and lowest point within a specified radius of the point. The radius actually defines a square centered on the point.
  • Aspect: aspect is the downhill direction. You can select any combination of aspects. (aspect maps)
  • Color: Select colors for the categories that contrast with the map display. This will probably show up best on reflectance contour maps, or single band imagery.
  • IHS color merge: to let the terrain peek through.  Pick a pastel color for this. This selection affects basin flooding.


  • PLOT button draws the map.
  • Add category
  • Export grid: in the new grid, all points that in the category will have a single value.  This is designed to only work with a single category.
  • Export DB: for elevation maps only, every point in the category will be a record in the DB. This is designed to only work with a single category, and the resulting database could be very large.
  • Masking by category.  This allows edits of the DEM based on category membership. This will be in memory; to save, you must save the DEM.  Reload the DEM to undo the mask.
    • Mask cat in: in the DEM, all points that meet the single category will be retained, and all others removed. 
    • Mask cat out: in the DEM, all points that meet the single category will be removed. 
  • OK: exit the form, and save the category to be redrawn when the map redraws. 
  • Cancel: exit the form, and discard the category. 


If you only want to define the category based on a single characteristic, leave the others unchecked.  The default values should select the entire region.

The legend for the terrain categories will give the percentage of the map area that fits each criteria.

Last revision 11/14/2017