[Proj] How to use +towgs84

[Frank Warmerdam]

Jan Hartmann wrote:
> 
> Aaron Friesen wrote:
>> Is +towgs84 actually a misnomer?  Is it not in general the means to
>> apply a 3 or 7-parm transform?
>>
>>
>>   
> Although I work at a University, I hate to cite myself, but perhaps in 
> this case it's legitimate:
> 
> http://lists.maptools.org/pipermail/proj/2009-February/004381.html
> 
> I tried to give an answer to this question and a few others that have 
> been hotly debated on this list. I got no response at all to this (too?) 
> long posting. As this list is generally not very inhibited on speaking 
> its mind when it doesn't like something, I hope there are not too many 
> idiocies in it (my mailing, I mean). Even so, some casual comments would 
> be really appreciated.

Jan,

Perhaps there was no response because everything it said was sensible and
there was no obvious question?

In choosing WGS84 as a "pivot datum" PROJ.4 is replicating a relatively
common practice with other GIS software though this approach does have
some pitfalls.

It is possible to use +towgs84 to apply datum shifts that aren't really
to or from WGS84 (likewise with +nadgrids) but this must be done with some
care and caution since the library conceptually considers it is using
WGS84 as the pivot datum.  So trick the library carefully if you wish/need
to do this.  If someone wants to carefully consider how to do this and
document it, it would be a nice addition to the FAQ list in Trac.

Best regards,
-- 
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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