[Proj] Roussilhe Projection
Oscar van Vlijmen
ovv at hetnet.nl
Thu Oct 20 11:45:39 EDT 2005
Following a lead provided by Maciek Sieczka, I stumbled upon testpoints and
formulae for the Polish quasi-stereographic Roussilhe projection (Uklad
Bugzilla Bug 818 report
Formulae (in Polish):
It appears that the formulae are derived from the official publication
"Wytyczne techniczne G-1.10 ".
As with every other publication with formulae, you will find errors and
vagueries in the text. I think I've found them plus the solutions.
* Seems to be correct, but it is not immediately clear what xG0 is. The text
describes it in Polish. I meditated on it a bit and found that xG0 is the
xG, computed for the central point B0,L0.
* x0 is the false northing, y0 is the false easting. There are more
countries (e.g. Switzerland) with the meaning of x and y interchanged.
* error 1: yG=yG-xq2*yQ/(4*R02)+... should be: yG=yQ-xq2*yQ/(4*R02)+...
* error 2: chsa=cos(h)*sin(alfa); fi=atan(chsa/(1-sqr(chsa)))
should be: fi=atan(chsa/sqrt(1-sqr(chsa)))
Testpoint for zone 1 (Strefa I) (units in degrees, meters):
B0=50d 37m 30s; L0=21d 5m;
x0=5467000; y0=4637000; m0=0.9998;
I computed the xG0 as: 5610467.57697895
x65 = 5621984.46761389, y65 = 4493939.53052073
The Transpol program mentioned in the bug report gave:
The inverse with my x65,y65 values gave:
lat = 51.9999999991681, lon =18.9999999227301
The text says (if I'm reading it correctly) that with a complete
forward-inverse-forward projection the error stays within 0.1 mm within the
Interestingly, my corrected libproj-derived rouss gave:
take y0=5467000; x0=4637000; m0=0.9998;
lat0=50d 37m 30s; lon0=21d 5m;
Note the x,y interchange.
Difference in easting with the Polish formulae: 4 mm, in northing: 7 mm.
Also interesting, the double stereographic projection (yes, sterea, at least
my translation of it) gives:
Difference in easting: 0.7 mm, in northing: 4 mm
Conclusion: now it should be possible to add - as soon as the developers and
maintainers can find the time of course - the (Polish) Roussilhe projection.
If you have difficulties understanding the article: I don't read Polish too
On reply: please try to delete some parts of this relatively long email!
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