[Proj] Lat-Lon values under different ellipsoides

Frank Warmerdam warmerdam at pobox.com
Tue Jan 27 10:41:20 EST 2009

Jan Hartmann wrote:
> Yes, this works, thanks Frank. I added the same datum definition to both 
> input and output proj definitions, and now the transformation is 
> correct. One very minor point: if I add the Datums in the form 
> +proj=longlat +datum=WGS84 +ellps= ...  +to +proj=longlat +datum=WGS84 
> +ellps=..., no ellipse transformation is computed. If I add the 
> definitions in the form of a +towgs84 string (the same for both 
> projections),  the ellipses are computed correctly. There is probably 
> some if-statement somewhere that skips the datum (and ellipse!) 
> computation when both datums are the same. Would it be possible to 
> remove that if-statement when ellipse-parameters are also given?


It is not meaningful to use +datum=WGS84 and an ellipse setting that is
not WGS84.  The +datum=WGS84 expands internally into
+towgs84=0,0,0 +ellps=WGS84 so your declaration will end up with two ellipse
settings and which one gets selected will depend on order - a rather unstable

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

More information about the Proj mailing list