[Proj] problem with +proj=omerc +no_rot

Alex Deckmyn alex.deckmyn at oma.be
Fri Jun 5 06:05:23 EST 2009


I am not an expert on using proj, so my appologies if this is a trivial question.

When projecting data to an oblique mercator (omerc) but without rotating the coordinates
back (+no_rot), I always get the X-component mirrored and I have to convert x -> -x 
manually afterwards to get my graphs correct. If the angle alpha is positive, it is the Y 
component that is inverted.

If "no_rot" only turns on or off a rotation alpha, I don't understand why the X or Y 
component is flipped in this way.

Looking at the code in PJ_omerc.c, the outputs with or without +no_rot appear not to be 
rotated versions of each other: either X or Y is inverted, depending on the sign of P->sinrot.

How can I get the data correct in the +no_rot case?


Dr. Alex Deckmyn                   e-mail: alex.deckmyn at oma.be
Royal Meteorological Institute     http://www.meteo.be
Ringlaan 3, 1180 Ukkel, Belgium    tel. (32)(2)3730646

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