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Yes, this works, thanks Frank. I added the same datum definition to
both input and output proj definitions, and now the transformation is
correct. One very minor point: if I add the Datums in the form
+proj=longlat +datum=WGS84 +ellps= ... +to +proj=longlat +datum=WGS84
+ellps=..., no ellipse transformation is computed. If I add the
definitions in the form of a +towgs84 string (the same for both
projections), the ellipses are computed correctly. There is probably
some if-statement somewhere that skips the datum (and ellipse!)
computation when both datums are the same. Would it be possible to
remove that if-statement when ellipse-parameters are also given?<br>
<br>
Jan <br>
<br>
Frank Warmerdam wrote:
<blockquote cite="mid:497DE731.2050206@pobox.com" type="cite">
<pre wrap="">Jan Hartmann wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Hi,
This is something I have long been banging my head against. I think it
is a bug, but I am not sure. If I take a lat-lon value, computed on a
particular ellipsoid, and convert it to the lat-lon value on another
ellipsoid, I should get a different value, right? (e.g. cs2cs
+proj=longlat +ellps=bessel +to +proj=longlat +ellps=WGS84). PROJ4
always gives the same value, but I have an extensive list of coordinates
of church towers in the Netherlands with their lat-lon values in 1850,
based an a slightly smaller ellipsoid than we use nowadays, and the
lat-lon of the same towers derived from our modern RD-system, based on
the Bessel-ellipsoid, and without the WGS84 correction. There is a
systematic difference of about 50 meters. If I do the same computation
with the projected coordinates, I get the correct answer. Moreover, in
that case the transformation changes when I change the
ellipse-parameter, something that does not happen with lat-lon coordinates.
So, is this a bug in PROJ? If so, can someone with geodetic experience
here explain to me how people can get different lat-lon values for the
same point, based on astronomical measurements?
</pre>
</blockquote>
<pre wrap=""><!---->
Jan,
As of PROJ 4.6.0, the policy is to not attempt any conversion of lat/long
values between coordinate systems where only an ellipsoid is given. So, to
get a datum shift it is now necessary to provide some sort of datum
shift information for both the source and destination coordinate systems.
This is a deliberate change of policy to avoid lots of other complaints in
the past.
I would suggest using something appropriate like +datum=potsdam for your
Bessel data, and +datum=WGS84 instead of +ellps=WGS84.
Best regards,
</pre>
</blockquote>
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