I'm using ASTER GDEM V2 to pick heights at lon-lat coordinates obtained from Google Maps but I found that at some locations - particularly San Francisco: https://goo.gl/maps/w7p1K - the DEM seem to be horizontally shifted by about 100m in NNW direction which is more than declared ASTER GDEM precision.

I'm using QGis 2.0.1 under Ubuntu.

Reproduce: Open freshly downloaded/unmodified ASTGTM2_N37W123_dem in QGis. The tile should open in it's native GCS EPSG:4326. Note coordinates of outline of San Francisco Treasure Island and display those in Google Maps.

The question is: Am I missing something about coordinate systems and CS references involved? As far as I know Google's lon-lat is also EPSG:4326.

Or is that a bug in ASTER GDEM's georeferencing?

Further analysis:

The shift is not homogeneous. It is definitely smaller or negligible in Czech Republic (ASTGTM2_N50E014_dem) but the area lacks such strong features as coastlines to tell for sure.

In San Francisco I compared ASTER to SRTM and SRTM does not seem to exhibit described shift.

  • Can you add an Image which shows this issue? – Devdatta Tengshe Apr 30 '15 at 6:57

According to http://epsg.io/3857 , google & OSM use EPSG:3857.

From memory, http://epsg.io/4326 is more an old standard.

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