I'm having a hard time trying to use elevation data from EU-DEM.
This data is in ETRS89 (EPSG:4258), according to their site.
Now, when I extract the elevation for a shore, I get 0 meters of altitude from those GeoTIFF files. This must be elevation over sea level, as it is a beach. A GPS using EPSG:4326, however, gives around 50 meters for the same spot (which matches the Geoid height for that location).
I need to be able to compare elevation values from GPS devices with data from this DEM files, I thought this would be maybe because the geoids are different (one is GRS80 and the other is WGS84). It turns out they are practically equivalent, and I tried converting between the two and the z value is not changed:
echo "-8.790094 42.198734 0.0" | gdaltransform -s_srs EPSG:4258 -t_srs EPSG:4326 -8.790094 42.198734 0
So my questions are:
- What kind of elevation is provided by those EU-DEM files? It doesn't seem to be height over the ellipsoid, but rather height over mean sea level. I can't believe this is the case, as it would difficult using GPS 3D positions along with elevation data, and it seemed to me more straighforward that elevation survey satellites referenced elevation data to a standard ellipsoid.
- What is the correct way to convert between elevation values in these GeoTIFF files to WGS84 elevation coordinates?