DEM/Grid Difference Maps
Select from the Raster GIS menu, Multiple grid arithmetic.
DEM difference maps will be drawn in one of two ways:
- If the DEMs/grid are compatible the difference will be computed for each grid
- 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
- 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,
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
If you take the first grid as the later one, and subtract the earlier:
- Positive will the uplift, deposition, or building contstruction
- Negative will the subsidence erosion, or building removal
Last revision 2/15/2023