[Proj] Datum shift for Dutch maps 1850-2000
Jan Hartmann
j.l.h.hartmann at uva.nl
Thu Nov 27 09:30:53 EST 2008
Yes, an automatic datum shift to WGS84 seems to be causing the trouble,
but I don't see the system in this madness:
Test point 9018/375648 (Dutch RD, EPSG:28992 without WGS84 datum shift)
The source PROJ string is:
+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889
+k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel +units=m +no_defs.
I tested this string both without and with +nadgrids=@null on four
target projections:
1) to latlon Bessel (+proj=longlat +ellps=bessel +no_defs)
--> (without nadgrids in source) 3.291612 51.352060
--> (with nadgrids null in source) 3.291612 51.351476
2) to latlon Bessel with nadgrids null (+proj=longlat +ellps=bessel
+no_defs +nadgrids=@null)
--> (without nadgrids in source) 3.291612 51.352643
--> (with nadgrids null in source) 3.291612 51.352060
3) to latlon Bessel on the WGS84 datum (+proj=longlat +ellps=bessel
+no_defs +datum=WGS84)
--> (without nadgrids in source) 3.291612 51.352060
--> (with nadgrids null in source) 3.291612 51.351476
4) to latlon on the WGS84 datum (+proj=longlat +no_defs +datum=WGS84)
--> (without nadgrids in source) 3.291612 51.352643
--> (with nadgrids null in source) 3.291612 51.352060
1) and 3) give the same results; so do 2) and 4), which seems to inicate
that an automatic datum shift to WGS84 takes place, even when not
requested. I'm not sure about the effects of +nadgrids=@null.
Specifying +ellips=bessel without adding +nadgrids=@null seems to cause
an automatic WGS84 datum shift, whether or no datum=WGS84 is specified
(1a = 3a). If +nadgrids=@null is specified in the the source projection,
but not in the target projection (1b = 3b), the latlon coordinates shift
about 65m to the south. When +nadgrids=@null is omitted in the source
projection, but added in the target projection, the resulting
coordinates are shifted of 65M to the *north* (2a). Finally, when
+nadgrids=@null is specified in both source and target projections, the
datum shift is applied nevertheless (2b is equal to 1a and 3a). Finally,
specifying a WGS84 datum shift for the target projection without the
Bessel ellipsoide gives the same values as 2. I don't understand this,
but filed it as a bug nevertheless (http://trac.osgeo.org/proj/ticket/22).
Finally I would like to quote Frank Warmerdam
(http://article.gmane.org/gmane.comp.gis.gdal.devel/17261)
<quote FrankW>
Grr. I'm so sick of coordinate system related issues!
</quote FrankW>
I agree :-)
Jan
Frank Warmerdam wrote:
> Jan Hartmann wrote:
>> This certainly makes a difference! Adding +nadgrids=@null to the
original definition gives a map with a 120m South shift, still too much
but nearing a solution. Isn't this a bug in PROJ? I did not request a
datum shift, and neither the from-projection, nor the to-projection had
a datum defined, only ellipsoids. I am using Proj 4.6.1.
>
> Jan,
>
> An ellipsoid/geocentric based datum shift should not be done if no
towgs84
> or nadgrids value is provided with 4.6.0 or later. If you believe
there is
> a bug please file a detailed ticket showing exactly what you were doing.
>
> Best regards,
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