[postgis-users] Error in SPATIAL_REF_SYS table

[Jan Hartmann]

Hi Barend,

The problem lies in PROJ4's  projection definition of EPSG:28992, the 
Dutch National Grid. This is "stere" (Stereographic), but can be better 
approximated by "sterea" (Oblique Stereographic Alternative). I don't 
think it has to do with the original EPSG database; it has to do with 
the way PROJ4 handles coordinate systems that use epsg code 9809. There 
has been a long discussion on the October PROJ4 mailing list on this 
projection. It starts at

http://lists.maptools.org/pipermail/proj/2005-October/001817.html

and see also all postings under the subject "Roussilhe"

A bugreport has been entered in PROJ4's Bugzilla:

http://bugzilla.remotesensing.org/show_bug.cgi?id=980

In the meantime, I have found (with the help of Kees Krikke, 
krikke at nlr.nl), that a more accurate definition for the Dutch system is:

+proj=sterea +lat_0=52.15616055555555 +lon_0=5.38763888888889 
+k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel  +units=m 
+towgs84=565.2369,50.0087,465.658,-0.406857330322398,0.350732676542563,-1.8703473836068,4.0812 
+no_defs +to +proj=latlong +datum=WGS84

The "towgs84" parameter is also missing in the PROJ4 epsg table 
(although it is in the original EPSG database). You need it for 
overlaying on WGS84-based maps, like Google Earth. This formula gives 
precision from 1 mm to about 25 cm, compared to the "official" 
transformation available at http://www.rdnap.nl. The remaining 25 cm are 
due to corrections needed for the slightly irregular Dutch grid, and are 
almost impossible to incorporate into PROJ (see 
http://lists.maptools.org/pipermail/proj/2005-October/001809.html).

You can put this definition into your local epsg file and 
spatial_ref_sys table until the official PROJ update.

HTH

Jan Hartmann
Department of Geography
University of Amsterdam



Barend Köbben wrote:
> Hi there,
> 
> I just started using PostGIS instead of the Spatial Extensions of MySQL, mainly because of the lack of SRID support in the latter. Everything went nicely until yesterday, when I found that there's an error in the SPATIAL_REF_SYS table for the Dutch National Grid. This, officialy called "RijksDriehoeksstelsel" is defined with Projection = Oblique Stereographic. But it should be Double Stereographic...
> 
> All the other coordinates seem right, but the result is still an offset of about 2 meters when combined with properly projected data....
> 
> Any ideas on how this could be remedied, or should I file a bug report?
> 
> ____________________________
> 
> Barend Köbben
> International Institute for Geo-information 
> Sciences and Earth Observation (ITC) 
> PO Box 6, 7500AA Enschede (The Netherlands)
> ph: +31 (0)53 4874253; fax: +31 (0)53 4874335
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