[Proj] Problems using grid shift files

[Ziegler Stefan]

Hi Paul

just want to let you know that it works now. I had to change 4 lines in
the pj_transform.c file (some wgs84 hardcoded stuff for shift grids) and
then changed the definitions in the epsg file as suggested. And the best
thing is, that it works with MapServer too (vector and raster data). I'm
now able to do an on-the-fly equalization :)

Thanks
Stefan

> -----Ursprüngliche Nachricht-----
> Von: Paul Kelly [mailto:paul-grass at stjohnspoint.co.uk]
> Gesendet am: Freitag, 3. April 2009 14:58
> An: Ziegler Stefan
> Cc: proj
> Betreff: Re: AW: Re: AW: Re: [Proj] Problems using grid shift files
> 
> Hello,
> I think it would help if you give an example of one of the 
> transformations 
> that gave the incorrect results, with the exact cs2cs 
> command-line you 
> used and test points.
> 
> On Thu, 2 Apr 2009, Ziegler Stefan wrote:
> 
> > Mmmmh, I think this is beyond my mental skills ;-) So I 
> start with some
> > questions: what is a pivot datum? And how can I chain datum
> > transformation (without piping in the shell)? How does proj does a
> > re-projection? I assumed something like: projection A -> ellipsoid A
> > ----> here comes the datum transformation ----> ellipsoid B ->
> > projection B. So if I know the accurate relation between 
> ellipsoid A and
> > B (like the +towgs84 params) I'm able to reproject more or 
> less exact?
> 
> No, there is generally an extra stage:
> 
> Source Datum --> datum transformation A --> Pivot datum --> datum
> transformation B --> Target Datum
> 
> Only when either your source or target datum is the same as the pivot 
> datum is there only one datum transformation. For all other 
> cases you need 
> two sets of datum transformation parameters, for the source 
> and target 
> datum to some intermediate pivot datum. Parameters for use of 
> WGS84 as a 
> pivot datum are well defined for most datums. But in your 
> case when you 
> want to use CH1903+ as a pivot datum instead you need to 
> calculate a new 
> set of datum transformation parameters.
> 
> As of yet I'm not sure exactly how to calculate the new 
> parameters but if 
> you post some details of the conversion you're trying to do with some 
> sample points, maybe someone will be able to work it out for you.
> 
> Paul
> 

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