Geographic versus UTM DEMs

A number of free DEMs (e.g. SRTM, ASTER GDEM, USGS NED, and Canadian CDED) all use geographic coordinates.  A smart program would compensate, and correctly compute slopes and aspect, hillshades, and viewsheds by converted the decimal degrees into meters with an adjustment for earth curvature.  The default map would also account for the different spacings in X and Y.  With rare exceptions, this is not the case, and you must reproject the geographic DEMs to get reasonable results in most GIS programs.

100 m UTM grid
3" geographic graticule

Reprojection takes the measured elevations, at the intersections of the green geographic graticule, and interpolates new elevations at a different number of locations.  The original elevations are then not used.  This will tend to slightly smooth the DEM, as the highest and lowest values will be gone after interpolation.  There is no theoretical reason why reinterpolation should improve the quality or accuracy of a DEM; the algorithm can only attempt to minimize the changes.

 

 

Geographic SRTM data imported into ArcGIS without setting map projection.

  • 10 km UTM grid obviously distorted, with squares wider than they are tall
  • 1 x 1 cell is square in this projection

UTM SRTM data imported into ArcGIS without setting map projection.

  • 10 km UTM grid undistorted, with squares square
  • 1 x 1 cell is taller than it is wide in this projection

 

To the best of my knowledge, only MICRODEM and RiverTools can correctly handle geographic DEMs without reprojection to UTM.  There are two problem areas:

  1. Default map display, as shown above.
  2. Computation of slope, aspect, hillshade, and viewshed.  The option provided by many of the other programs, to use a single scaling factor from degrees to meters, does not account for the difference in data spacing with latitude.

last revision 9/27/2015