[Proj] Custom projection wildly inaccurate

Thy, Kristian Kristian.Thy at atkinsglobal.com
Mon Nov 20 08:07:57 EST 2006

From: Oscar van Vlijmen
> There seem to be several approximations. Most straight (read: 
> what you can do per proj/cs2cs command line alone) projective 
> approximations are bad.
> There is really only one way to do it correctly: use the 
> official polynomial solution.

Yes, but that's not easily translatable for use in Proj-based
applications like GDAL and MapServer (as far I know, at least).
We will most likely have to convert the data manually, but in
the meantime it would be nice to have a bad-and-fast projection
for demonstration and testing purposes.

> An interesting approximation, using transverse mercator with 
> some pre- & postprocessing, can be read in:
> Kp2000 - en nødvendighed eller ikke?
> Dansk Vejtidsskrift, (6/7) 2001, p. 18-19
> Leif Kahl Kristensen, Institut for fysik og astronomi
> <http://asp.vejtid.dk/Artikler/2001/06-07%5C2930.pdf>
> He gives an approximation for two System34 zones: Sjælland 
> and Jylland/Fynn.
> Using his approximation I got for the coordinates you mentioned:
> lon =12d 20m 42.97906s; lat = 55d 52m 40.50251s; wgs84 datum.
> -> x = 86714.465; y = 162554.937 m
> The values you mentioned were:
> -86714.42, 162554.98
> Kristensen states that his procedure results in an average 
> difference in X of 5 cm, in Y of 9 cm, with a maximum difference
> of 18 cm.
> For his Jylland/Fynn approximation the differences are larger.

Interesting. For our application, that error would be acceptable.
My next question would then be if I can then somehow use the
formulae presented to warp an orthophoto using gdalwarp, but I
guess that's a question for another list :-)

> Algorithms and formulae are usually not copyrightable, but they are
> patentable. So, still publishable, more or less, but if 
> patented, commercial exploitation and sometimes other forms
> of exploitation are prohibited, at least in the countries
> where the patent is valid.
> Note: I'm no lawyer, so this is no legal advise!

Makes sense. But them again, IANAL (either) :-)

best regards,

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