[Proj] Implementation(s) for Oblique Stereographic

[Kees Krikke]

Hi all,

I've been following recent posts about the Roussilhe Projection.

As I understand from EPSG documentation and EPSG database Oblique
Stereographic (epsg code 9809) (with Roussilhe given as an alias) is not the
same as the USGS Oblique Stereographic projection.

I guess that stere is the USGS implementation and sterea is an approximation
of the EPSG implementation.

Anyway, you get into trouble if the wrong implementation is used. 

For example:

When using proj to convert from or to the Dutch RD system:

C:\Program Files\FWTools1.0.0a4\proj_lib>proj -v +init=epsg:28992
#Stereographic
#       Azi, Sph&Ell
#       lat_ts=
# +init=epsg:28992 +proj=stere +lat_0=52.15616055555555
# +lon_0=5.38763888888889 +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel
# +units=m +no_defs

The results are way off (up to 20m!)

Better results (within a couple of centimeters) are obtained when using
Oblique Stereographic Alternative. This can be fixed by changing the epsg
and esri files (sterea instead of stere). It might also be a good idea to
add the towgs84 parameters:

C:\Program Files\FWTools1.0.0a4\proj_lib>proj -v +init=epsg:28992
#Oblique Stereographic Alternative
#       Azimuthal, Sph&Ell
# +init=epsg:28992 +proj=sterea +lat_0=52.15616055555555
# +lon_0=5.38763888888889 +k=0.999908 +x_0=155000 +y_0=463000 +ellps=bessel
# +units=m
# +towgs84=565.04,49.91,465.84,-0.409394387,0.35970519561,-1.868491,4.0772
# +no_defs

But the best results are obtained when using the formulas specified in
http://www.epsg.org/guides/docs/G7-2.pdf. An RD conversion is used as an
example of Oblique Stereographic (epsg code 9809). Their results are spot
on.


I think that part of the confusion is that you don't always know which
implementation you need and that the same EPSG code (9809) is used for both
implementations (or isn't it?).

This is especially a problem if you need to reproject an geotiff image with
EPSG code 9809 in it. For example gdalwarp will use the "stere" formulas
when it sees code 9809. 

Cheers,

Kees Krikke