[Proj] Difference between +nadgrids=@null and +towgs84=0,0,0

[Frank Warmerdam]

Aaron Friesen wrote:
> Everyone,
> 
>>From the FAQ "Changing Ellipsoid / Why can't I convert from WGS84
> to Google Earch /Virtual Globe Mercator?" it says to add
> 
> +nadgrids=@null to "trick proj.4 into applying no change to
> the lat/long values when going to WGS84".
> 
> How is that different than +towgs84=0,0,0?

Aaron,

When you use +towgs84=0,0,0 PROJ.4 still converts from the source
ellipsoid to geocentric coordinates, applies the offset and then
transforms from geocentric coordinates to the wgs84 ellipsoid.  That is,
a +towgs84=0,0,0 results in simple conversion of ellipsoid via geocentric
coordinates which still applies a large offset when the ellipsoids are
quite different.

+nadgrids=@null tricks PROJ.4 into skipping all that.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent