[Proj] Problems using grid shift files

[Ziegler Stefan]

Thanks Paul! It's getting clearer and clearer. I'll give it a try before
coming back and moaning :)

regards
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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