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<font face="Times New Roman, Times, serif">I forward this mail from
Hermann Peifer, about the differences between the official German
grid shift file, and the one computed by gdal utilities based on
31 test points. It's a matter of different interpolation methods.
Can anyone comment on this pretended accuracy of the official grid
shift files, and about the interpolation methods used, in Germany
or elsewhere?<br>
<br>
Jan</font><br>
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<div class="moz-cite-prefix"><font face="Times New Roman, Times,
serif">On 01/22/2013 11:30 AM, Hermann Peifer wrote:<br>
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<blockquote cite="mid:50FE6A35.50207@gmx.eu" type="cite"><font
face="Times New Roman, Times, serif"><br>
As far as I can see: the agreement between "myshift.gsb" and
BETA2007.gsb is only in the immediate neighbourhood of the 31
test points. For these points, the differences are below .5
millimeter. <br>
</font> <font face="Times New Roman, Times, serif"><br>
For the interpolated areas in between, there is no real
agreement between "myshift.gsb" and BETA2007.gsb. Which is no
surprise, as the official data might be interpolated using
different methods and to my knowledge, the interpolation has
also been done differently for the ex-GDR area compared to the
area of former West-Germany. <br>
</font> <font face="Times New Roman, Times, serif"><br>
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<font face="Times New Roman, Times, serif">This is important.What
were those methods? The official grid-shift file pretends to a
certain accuracy. How valid is that pretension, when different
interpolations result in such different results? And two
different interpolations are certainly inacceptable. Mind again:
the pretended accuracy is several decimals behind the point.
This doesn't make any sense, neither scientific nor in real
world geodesy, if the interpolation method is that instable.<br>
</font><font face="Times New Roman, Times, serif"><br>
Jan</font><br>
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