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<font face="Times New Roman">In my opinion this is a bug. When
trying to convert a lonlat value from one ellipse to another,
cs2cs ignores the change of ellipsoid. Some time ago a decision
was made to only perform a datum transform when both input and
output projections had a full datum description, i.e. both
ellipsoid and towgs84 parameters. This is the right way of doing
datum transform, but unfortunately it also seems to prevent a
conversion from latlong to latlong on different ellipses where no
datum shift has to be applied (+nadgrids=@null doesn't work in
this case).<br>
<br>
Funny thing is that for projected coordinates a transform between
different ellipses *does* work. It took me a long time to figure
this out when trying to convert 19th century latlon values, as
computed on a slightly smaller ellipsoid, to contemporary latlon
values on the Bessel ellips, where no datum shift had taken place,
i.e. where the direction and alignment of the axes of the
ellipsoid were the same and only their extents were different. The
solution turned out to be to convert the original latlongs to an
intermediate projection on the original ellipsoid, convert those
projected coordinates to a modern projection based on the modern
ellipsoid, and convert those modern coordinates back to latlon
values on the target ellipsoid.<br>
<br>
I don't like this procedure, as the original 19th century
triangulation was computed with spheroidical coordinates that were
only afterwards converted to a planar projection. There were small
errors on those original tables (around 50 meters) that were
extremely hard to pinpoint, so every intermediate computational
step should be avoided. Especially so when the point of reference
for the ellipsoid computation is a place as outlandish as
Amsterdam. I think I solved the problem now, but IMHO it shouldn't
have existed in the first place. The same location on the world
globe has *different* latlong values when computed on the basis of
different ellipsoids, and that is not shown by cs2cs.<br>
<br>
Jan<br>
</font><br>
On 09/01/10 11:23, Heiko Klein wrote:
<blockquote cite="mid:4C7E1BAF.6000706@met.no" type="cite">
<pre wrap="">Hi,
I'm working with meteorological model-data. The models usually use a
spherical earth. Since the resolution of the models is now ~1km, I have
to think more about converting those data for people using GIS systems.
I have only a very rough understanding of datum, geocentric and
geodetic. Up to now I've only used the ellps-parameters of proj to
describe the earth.
If I understand it correctly, a spherical earth is completely described
by the major axis, and geodetic and geocentric coordinates are the same?
Using the following does not change the coordinates
./cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +to +proj=latlong
+ellps=WGS84
10 60
10dE 60dN 0.000
since it calculates completely in geocentric coordinates?
Giving a WGS84 datum on both input and output gives geodetic coordinates?
/cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +datum=WGS84 +to
+proj=latlong +ellps=WGS84 +datum=WGS84
10 60
10dE 60d9'58.075"N 8921.648
Why should the sphere-datum be WGS84? Using NAD27 gives me geocentric
coordinates?
./cs2cs +proj=latlong +ellps=sphere +a=6371000 +e=0 +datum=NAD27 +to
+proj=latlong +ellps=WGS84 +datum=WGS84
10 60
10dE 60dN 0.000
Does there exist a correct 'datum' for a sphere? Isn't it all the same
for a sphere? Why do I have to give it at all?
If I have a measurement station with GPS coordinates, latitude and
longitude, are they geocentric or geodetic?
And the final and most important question: What is the best description
of a spherical earth to convert from with cs2cs?
Best regards,
Heiko
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