[Proj] Implementation(s) for Oblique Stereographic

[Kees Krikke]

Frank,

It is my understanding that the epsg uses the code 9809 for the Roussilhe
implementation (http://www.epsg.org/guides/docs/G7-2.pdf).

I will file a bug on this issue...

The question still remains if anyone relies on the stere implementation for
this code.

Regards,

Kees Krikke


-----Original Message-----
From: fwarmerdam at gmail.com [mailto:fwarmerdam at gmail.com] On Behalf Of Frank
Warmerdam
Sent: 26 October 2005 21:10
To: kees.krikke at wanadoo.nl; PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Implementation(s) for Oblique Stereographic

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.

Kees,

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,
--
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