# [Proj] Implementation(s) for Oblique Stereographic

Oscar van Vlijmen ovv at hetnet.nl
Tue Oct 25 16:10:30 EDT 2005

```> From: "Kees Krikke"

> 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.
...
> Anyway, you get into trouble if the wrong implementation is used.
...
> When using proj to convert from or to the Dutch RD system:
...
> Better results (within a couple of centimeters) are obtained when using
> Oblique Stereographic Alternative.

* Use PROJ/libproj/etc sterea
* Or, use the official conversion program RDNAPTRANS(tm)2004. Windows only.

The Dutch RD grid uses its own version of the double stereographic
projection, by the Dutch cadastre also called the Schreiber double
stereographic projection.
I have found that there are only very small differences between (my version
of) PROJ/libproj's sterea and the 'double Dutch' (ermm...) projection. Think
of tens of microns in easting, northing, not centimeters.
Official sources (Het Kadaster) have published the exact formulae for this
projection, including a very good polynomial approximation.
There is an official program RDNAPTRANS available that also performs grid
corrections.
See http://www.rdnap.nl
I have found that the EPSG Guidance Note 7.2 description of this projection
produces good results indeed.

Just ask me if you want more references, mainly in Dutch and available
on-line. Much of it can be found on www.rdnap.nl

A test point, near Westkapelle, far away from datum Amersfoort, using my
stuff; no grid corrections:

units: degrees, meters
lat0 =52.1561605555556; lon0 =5.38763888888889;
x0 =155000.00; y0 =463000.00; k0 =0.9999079;
Bessel 1841 ellipsoid

lat=51d 31m; lon=3d 27m;
RD polynomial solution on the Bessel ellipsoid gives RDxy:
20534.0171854597, 393649.052747085
Analytical solution (Schreiber double projection):
20534.0171628404, 393649.052684693
sterea:
20534.0171705519, 393649.052733924
(not all decimals are significant)

```