Comparing elevation arc second and UTM DTMs


Two versions of a DTM, created by aggregation from a 1 m UTM grid in Glacier National Park, Montana.  At this latitude, almost 49N, the arc second spacing is about 20.35x30.89 m.  Visually the two appear identical, so they do a good job capturing the terrain.


 The histogram shows they are indeed very similar.

The points in the grids are not at the same locations.  At places they coincide, and at worst they are offset by about 10 m in x and 15 m in y.  In this heavily glaciated area, there are slopes up to 800%.

To map the differences, one of the grids must be reinterpolated.  The maps above show both possible reinterpolations.  The maximum differences are about 80 m; 2.5 m captures 90% of the points from the 5th to the 95th percentiles.  The biggest differences are on the ridge tops and valleys, where the ridge crest and the steam location can be offset by up to 15 m depending on how the two grids line up at the location.

These two DTMs "should" be identical, but there are differences.  There are 900 elevations from the 1 m DTM for each pixel in the 30 m UTM aggregation, and about 600 elevations from the 1 m DTM for each pixel in the 1" grid.   Reinterpolating the 1" DTM to 30 m, or the 30 m DTM to 1", produces noticeable differences.

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