[Proj] Problems using grid shift files

[Ziegler Stefan]

> Remember that we are slightly abusing the PROJ.4 API here by 
> using CH1903+ 
> as a pivot datum instead of WGS84. So the +towgs84 parameters 
> for other 
> co-ordinate systems must be adjusted so they describe a 
> transformation to 
> CH1903+, rather than a transformation to WGS84, in exactly 
> the same way as 
> we have used parameters of +towgs84=0,0,0 for CH1903+. If you have an 
> accurate set of +towgs84 parameters for the CH1903+ --> WGS84 
> transformation, I suspect you might be able to determine the 
> parameters 
> for your target datum to CH1903+ through some matrix 
> multiplication (bear 
> in mind that a set of +towgs84 parameters consist of rotational, 
> translational and scaling components) but I'm not sure of the 
> exact nature 
> of the calculations you would need to do and I haven't tried it.
> 
> In other words, you should be able to avoid a chain of 
> re-projections by 
> chaining the datum transformations before doing the 
> re-projection. That's 
> how I understand it anyway but I could have missed something.
> Paul

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?

All our data is CH1903 and I'd like to be able either to apply the
equalization ('hidden' in the grid file) to the data with each
reprojection or at least that it's only applied in the reprojection from
CH1903 to CH1903+ but is ignored for other reprojections (e.g. CH1903
--> UTM).

Thanks a lot.
Stefan


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