DEM elevation difference in GRASS GIS

[disco_pihtija]

Dear All,

I am doing topographic change analysis and I am comparing DEM created in PhotoScan and reference DEM. In GRASS GIS, I did a profile for these two DEMs and profile lines are almost the same. Only problem is that elevation difference is around 50m all ways down the profile.

Is there anybody who could know what is the problem of this "error"? I thought that maybe I was comparing DSM and DTM. But that is not the problem coz I already checked.

Best wishes,

Nastic

[RHenriques]

Hi

The most probable reason is related to the altimetric datum used. You are, probably, comparing a DEM with a national altimetric datum with one generated from material with WGS 84 coordinates. Even after normalise the x and y coordinates, the altimetry (z) is not the same. You must convert WGS84 altitudes to the altitude datum of your reference DEM. Your values seem to be related with differences in the WGS84 altimetry without geoid undulation discount. Get a geoid undulation model to your location and subtract it to your WGS84 DSM.
Cheers