DEM/Grid Difference Maps

Select from the Raster GIS menu, Multiple grid arithmetic.

DEM difference maps will be drawn in one of two ways:

  1. If the DEMs/grid are compatible the difference will be computed for each grid location.
  2. If the DEMs vary and are not compatible, the new grid will match the first of the two DEMs in extent and point spacing. At each location in that grid, the elevation in the other grid will be interpolated. Reasons for the grids varying could be different extents, or one DEM being UTM and the other Lat/Long.

Think about do you want a positive difference to mean.  Along the coast or a river, or a landslide, this could be erosion or deposition, or it could be additon of a new building or tearing down an old building, or growth of trees, or loss of trees due to a fire.

The order of selecting the two DEM will determine the sign of the result.  The second DEM will be subtracted from the first.  The title of the map will show the order of the subtraction.  If you want to control the resolution of the grid, you can change the sign of the results using Single grid arithmetic on the Edit menu and multiply by -1.

  You can get:
  • XY scatter plot for the two grids.
  • Histograms for the two grids and the difference.
  • A difference map, which will have have the difference map highlight changes from a specified level
    • Points with greater positive change will be green
    • Points with greater negative change will be red
    • Other points will be gray
    • This is a hillshade/reflectance map, to show changes if you need to make adjustments.  Insure that you pick an appropriate highlight level.  Differences in NDVI will probably be less than 1 at the largest , and most will be much smaller  Changes in lidar DSM will be up to 10 or more meters and many will be more than 1 m. 
    • If the highlight level is too large, everything will be gray (with maybe a couple of colors areas).
    • If the highlight level is too small, everything will be graeen or gray.
  • The altenative to the difference map will a colored Elevation/grid value map, with the colors scaled from the largest to the smallest.


If you take the first grid as the later one, and subtract the earlier:

 

References:

 

Last revision 2/15/2023