[Proj] Inconsistencies in projection definition (epsg 3857)

[Mikael Rittri]

Trust the last one, with +nadgrids. 
See http://trac.osgeo.org/proj/wiki/FAQ (question 8).

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

4 maj 2012 kl. 16:25 skrev "Jon Olav Skoien" <jon.skoien at jrc.ec.europa.eu>:

> 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
> 
> Disclaimer: Views expressed in this email are those of the individual and do not necessarily represent official views of the European Commission.
> 
> 
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