[Proj] How to use +towgs84

[Eric Miller]

>>> On 3/4/2009 at 3:00 PM, Jan Hartmann <j.l.h.hartmann at uva.nl> 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.

The assumption with +towgs84 is that the transformation parameters will convert the coordinates to or from WGS84.  If you have transformation parameters that go from one datum to another, neither of which are WGS84, it should work.  But, I'd be careful about defining coordinate systems in an +init file (like "epsg") that have datum transformation parameters that aren't for WGS84.  It could cause some serious problems.

Regarding your other message: The +datum parameter doesn't always expand to +ellps and +towgs84.  In the case of NAD27 it expands to +ellps and +nadgrids. (src/pj_datums.c)