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<div class="Normal"><span style='font-size: 10pt; '>> Remember that we are
slightly abusing the PROJ.4 API here by
</span><br><span style='font-size: 10pt; '>> using CH1903+
</span><br><span style='font-size: 10pt; '>> as a pivot datum instead of
WGS84. So the +towgs84 parameters
</span><br><span style='font-size: 10pt; '>> for other
</span><br><span style='font-size: 10pt; '>> co-ordinate systems must be
adjusted so they describe a </span><br><span style='font-size: 10pt; '>>
transformation to </span><br><span style='font-size: 10pt; '>> CH1903+,
rather than a transformation to WGS84, in exactly
</span><br><span style='font-size: 10pt; '>> the same way as
</span><br><span style='font-size: 10pt; '>> we have used parameters of
+towgs84=0,0,0 for CH1903+. If you have an
</span><br><span style='font-size: 10pt; '>> accurate set of +towgs84
parameters for the CH1903+ --> WGS84
</span><br><span style='font-size: 10pt; '>> transformation, I suspect you
might be able to determine the </span><br><span style='font-size: 10pt; '>>
parameters </span><br><span style='font-size: 10pt; '>> for your target datum
to CH1903+ through some matrix </span><br><span style='font-size: 10pt; '>>
multiplication (bear </span><br><span style='font-size: 10pt; '>> in mind
that a set of +towgs84 parameters consist of rotational,
</span><br><span style='font-size: 10pt; '>> translational and scaling
components) but I'm not sure of the
</span><br><span style='font-size: 10pt; '>> exact nature
</span><br><span style='font-size: 10pt; '>> of the calculations you would
need to do and I haven't tried
it.</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> In other words, you should be
able to avoid a chain of </span><br><span style='font-size: 10pt; '>>
re-projections by </span><br><span style='font-size: 10pt; '>> chaining the
datum transformations before doing the
</span><br><span style='font-size: 10pt; '>> re-projection. That's
</span><br><span style='font-size: 10pt; '>> how I understand it anyway but I
could have missed something.</span><br><span style='font-size: 10pt; '>>
Paul</span></div>
<br>
<div class="Normal"><span style='font-size: 10pt; '>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?</span></div>
<br>
<div class="Normal"><span style='font-size: 10pt; '>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).</span></div>
<br>
<div class="Normal"><span style='font-size: 10pt; '>Thanks a
lot.</span><br><span style='font-size: 10pt; '>Stefan</span></div>
<br>
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