[Proj] Inconsistencies in projection definition (epsg 3857)

[Jon Olav Skoien]

Dear list,

First of all, I am sorry if this is not the right list for the following 
question.

We are working with some data that has been transformed from latlong via 
different software and have encountered some inconsistencies with the 
coordination system definitions of epsg:3857 (WGS 84 / Pseudo-Mercator, 
often referred to as web mercator as it is used in web services such as 
Google Maps). Some software (e.g. psql2shape) seems to use the 
definition from spatialreference.org:
http://spatialreference.org/ref/sr-org/6864/
which gives the proj4 definition as:
+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378137 +b=6378137 
+towgs84=0,0,0,0,0,0,0 +units=m +no_defs

ogr2ogr appears to use a proj4 definition that is quite similar (gives a 
dislocation of 10-20 meters in y-direction for some locations we have 
checked)
+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs 
+ellps=WGS84 +towgs84=0,0,0

However, the epsg-source of proj seems to use the following definition:
+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 
+k=1.0 +units=m +nadgrids=@null +wktext  +no_defs

The difference for a random location in Europe is about 30 km in 
y-direction between the two definitions (using e.g. cs2cs). Which 
definition should we trust?

Thanks,
Jon


-- 
Jon Olav Skøien
Joint Research Centre - European Commission
Institute for Environment and Sustainability (IES)
Land Resource Management Unit

Via Fermi 2749, TP 440,  I-21027 Ispra (VA), ITALY

jon.skoien at jrc.ec.europa.eu
Tel:  +39 0332 789206

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