[Proj] Implementation(s) for Oblique Stereographic

Frank Warmerdam warmerdam at pobox.com
Wed Oct 26 15:09:42 EDT 2005

On 10/26/05, Kees Krikke <c.krikke at hccnet.nl> wrote:
> Thanks Oscar,
> I'am aware of the sterea and RDNAPTRANS options. Even if the RD projection
> is a odd one, it is well documented and the conversion between RD and other
> coordinate systems is not my problem.
> I just wanted to point out that there is a problem when a coordinate system
> uses "Oblique Stereographic projection". The problem is that there is more
> than one implementation of this projection. In the EPSG document
> (http://www.epsg.org/guides/docs/G7-2.pdf) a remark is made that the USGS
> implementation of Oblique Stereographic is considered a different
> projection.
> So if a coordinate system use Oblique Stereographic projection (epsg code
> 9809) you need to know if the epsg (Roussilhe) or usgs implementation should
> be used. (If I am wrong here than proj4 needs to use sterea or a more exact
> implementation of the EPSG Oblique Stereographic projection whenever it sees
> epsg code 9809 and the problem would be solved.)
> For example if you use the "proj +init=epsg:28992"command (28992 being the
> epsg code for the dutch RD coordinate system) you will  get the stere
> conversion because that is the implementation proj uses whenever it sees the
> 9809 code. For RD this would be wrong because the sterea conversion provides
> better results. (I understand that this is also true for the Polish Uklad
> 1965 system.)
> In proj4 you can easily change the epsg file so the sterea conversion is
> used, but other applications (even if they rely on proj4, such as gdal) just
> look at the projection codes (such as 9809) and use just the implementation
> that is linked to this code. (for gdal: I think this is the stere
> implementation)
> An improvement for proj4 would be to implement the epsg and usgs
> implementation and make sure the epsg file refers to the correct
> implementation.


I haven't been following the details of these discussions closely,
but this seems to relate to me.  Does anyone know if all the EPSG
coordinate systems using code 9809 ought to be using +proj=sterea
(or the Roussilhe?).  If that is the case, file a bug and I will modify
the OGR code used to generate the PROJ.4 "epsg" file to translate
that way.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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