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<div class="Normal"><span style='font-size: 10pt; '>Thanks Paul! It's getting
clearer and clearer. I'll give it a try before coming back and moaning
:)</span></div>
<br>
<div class="Normal"><span style='font-size: 10pt; '>regards</span><br><span style='font-size: 10pt; '>Stefan</span></div>
<br>
<div class="Normal"><span style='font-size: 10pt; '>> -----Ursprüngliche
Nachricht-----</span><br><span style='font-size: 10pt; '>> Von: Paul Kelly
[mailto:paul-grass@stjohnspoint.co.uk]</span><br><span style='font-size: 10pt; '>>
Gesendet am: Freitag, 3. April 2009
14:58</span><br><span style='font-size: 10pt; '>> An: Ziegler
Stefan</span><br><span style='font-size: 10pt; '>> Cc:
proj</span><br><span style='font-size: 10pt; '>> Betreff: Re: AW: Re: AW: Re:
[Proj] Problems using grid shift
files</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>>
Hello,</span><br><span style='font-size: 10pt; '>> I think it would help if
you give an example of one of the
</span><br><span style='font-size: 10pt; '>> transformations
</span><br><span style='font-size: 10pt; '>> that gave the incorrect results,
with the exact cs2cs </span><br><span style='font-size: 10pt; '>>
command-line you </span><br><span style='font-size: 10pt; '>> used and test
points.</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> On Thu, 2 Apr 2009, Ziegler
Stefan wrote:</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> > Mmmmh, I think this is
beyond my mental skills ;-) So I </span><br><span style='font-size: 10pt; '>>
start with some</span><br><span style='font-size: 10pt; '>> > questions:
what is a pivot datum? And how can I chain
datum</span><br><span style='font-size: 10pt; '>> > transformation
(without piping in the shell)? How does proj does
a</span><br><span style='font-size: 10pt; '>> > re-projection? I assumed
something like: projection A -> ellipsoid
A</span><br><span style='font-size: 10pt; '>> > ----> here comes the
datum transformation ----> ellipsoid B
-></span><br><span style='font-size: 10pt; '>> > projection B. So if I
know the accurate relation between
</span><br><span style='font-size: 10pt; '>> ellipsoid A
and</span><br><span style='font-size: 10pt; '>> > B (like the +towgs84
params) I'm able to reproject more or
</span><br><span style='font-size: 10pt; '>> less
exact?</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> No, there is generally an extra
stage:</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> Source Datum --> datum
transformation A --> Pivot datum -->
datum</span><br><span style='font-size: 10pt; '>> transformation B -->
Target Datum</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> Only when either your source or
target datum is the same as the pivot
</span><br><span style='font-size: 10pt; '>> datum is there only one datum
transformation. For all other </span><br><span style='font-size: 10pt; '>>
cases you need </span><br><span style='font-size: 10pt; '>> two sets of datum
transformation parameters, for the source
</span><br><span style='font-size: 10pt; '>> and target
</span><br><span style='font-size: 10pt; '>> datum to some intermediate pivot
datum. Parameters for use of </span><br><span style='font-size: 10pt; '>>
WGS84 as a </span><br><span style='font-size: 10pt; '>> pivot datum are well
defined for most datums. But in your
</span><br><span style='font-size: 10pt; '>> case when you
</span><br><span style='font-size: 10pt; '>> want to use CH1903+ as a pivot
datum instead you need to </span><br><span style='font-size: 10pt; '>>
calculate a new </span><br><span style='font-size: 10pt; '>> set of datum
transformation parameters.</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>> As of yet I'm not sure exactly
how to calculate the new </span><br><span style='font-size: 10pt; '>>
parameters but if </span><br><span style='font-size: 10pt; '>> you post some
details of the conversion you're trying to do with some
</span><br><span style='font-size: 10pt; '>> sample points, maybe someone
will be able to work it out for
you.</span><br><span style='font-size: 10pt; '>>
</span><br><span style='font-size: 10pt; '>>
Paul</span><br><span style='font-size: 10pt; '>> </span></div>
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