[Proj] Implementation(s) for Oblique Stereographic

[Gerald I. Evenden]

On Wednesday 26 October 2005 02:39 pm, Kees Krikke wrote:
> Thanks Oscar,
>
	...
> 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.

The Oblique Stereographic code in EPSG 7.2 IS NOT Roussilhe.  The EPSG code is 
the Double Projection method using conformal conversion of the ellipsoid to 
the sphere and then using the spherical stereographic projection.  Roussilhe 
is not mentioned in the EPSG 7.2 publication.

Libproj4 provides for three (3) different Oblique Stereographics projections: 
"+proj=stere" for the USGS version (Snyder), "+proj=sterea" for the Double 
Projection method and "+proj=rouss" based upon Roussilhe's 1922 paper.
The description of the +proj=sterea projection in the libproj4 manual looks 
different from the EPSG description because the process is organized so that 
the conformal conversion software is also used by the Krovak and Swiss 
projections.

The results of a shell script shows lproj's operation on the EPSG example:

Double Projection Stereographic --- libproj4 +proj=sterea
lproj script parameters:
+a=6377397.155 +rf=299.15281 +lat_0=52d9'22.178 +lon_0=5d23'15.5 
+k_0=0.9999079+x_0=155000 +y_0=463000
comparison data from EPSG 7.2 : 196105.283 557057.739
lproj forward projection
196105.283      557057.739 from EPSG 7.2 196105.283 557057.739
Forward->Inverse cycle-binary intermediate
6dE     53dN

Thus thread gets a bit confusing at times and I feel that misuse of 
nomenclature is causing some of the confusion.
...

-- 
Jerry and the low riders:Daisy May and Joshua
"The being cannot be termed rational or virtuous,
who obeys any authority, but that of reason."
---Mary Wollstonecraft 1792
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