Aspect Difference Maps

Raster GIS,  Multiple grid arithmetic, Difference elevation, slope, aspect maps

Computes 4 new grids with the difference between two DEMs:

• Elevation
• Slope
• Aspect.  The downhill direction.  Map differences in the surface in flat areas lead to big differences in the aspect.
• Normals.  The normals to the surface are perpendicular to the aspect.

Trentino Italy, with: Hillshade from COP DEM Slope from COP DEM Aspect difference between COP and ALOS (signed, based on the shorter angular distance) Normal vector difference (unsigned, the dot product of the two normals) While the aspect and normal vectors are perpendicular, one is almost horizontal and the other almost vertical, and they behave very differently in flat terrain. In the valley running north0south in the center of the map, note that the slope and surface normal differences are very small, but the aspect differences are very large.

Aspect difference is strongly influenced by slope, and is particularly affected in flat areas, where integer DEM precision will further affect results.  While aspect is very important, it might not be the best metric to use for DEM comparisons and must be used with caution.  In particular it must use circular statistics.

This is related to SSO diagrams, and the suggestion by Jordan (2003) that the plots should use the downhill direction instead of the normals.

Last revision 10/22/2022