I have downloaded a SRTM DEM with min -13 and max 91 meters elevation, the spatial reference of which is GSC-WGS-1984. When I changed its spatial reference to UTM, its altitude values changed.Is this normal? If not, how can I do this conversion without error?
Dear Mahdi,
If by "change" you mean min or max or the distribution of the values is changed, it is not ok.
But if the DEM value for a specific location is changed, it might be Ok or not.
The easiest way to chang SRS is to export you layer as the desired spatial reference system.
Good luck,
When you have data that already has a defined coordinate system, if you re-define the spatial reference, instead of running a reproject function, you are telling the GIS to treat the old coordinates as though they were coordinates in the new system (e.g. lat/long values will all be plotted within a few meters of each other near the equator on a UTM grid) . Since the different projections and coordinate systems may use different spheroids to define elevation, it's possible that this will result in changes in the distribution of elevation values. More importantly, since the data layer is now mis-projected, the elevation values measured at any location of interest will be different (and incorrect).
GIS will let you do this because it's sometimes important to be able to define the projection for data that are missing the information, or correct the projection if it was defined incorrectly. As others have already said, regardless of the GIS application you are using, there will be a function to re-project data (raster and vector data are usually handles by different functions) that applies mathematical transformations to the data to update the coordinates to the same location in the desired projection.
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