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.

But at this time my problem is that I'm not able to use gdal to reproject

images from/to the RD coordinate system. (without modifying or making s/w)

A long term solution would be to identify multiple "Oblique Stereographic

projection" implementations and make sure that for each implementation a

unique epsg code is determined that can be used in geotiff images. (also

codes for other geo code systems would be needed)

Any thoughts on this subject?

Cheers,

Kees Krikke

-----Original Message-----

From:

[hidden email]
[mailto:

[hidden email]] On Behalf Of Oscar van Vlijmen

Sent: 25 October 2005 22:11

To: PROJ.4 and general Projections Discussions

Cc: Kees Krikke

Subject: Re: [Proj] Implementation(s) for Oblique Stereographic

> 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.

Short answer:

* Use PROJ/libproj/etc sterea

* Or, use the official conversion program RDNAPTRANS(tm)2004. Windows only.

Recent versions also perform grid correction.

Longer answer:

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.nlI 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)

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