[Proj] Rectified Skew Orthomorphic...

Oscar van Vlijmen ovv at hetnet.nl
Wed Feb 7 16:38:06 EST 2007

> From: "Michael Childs"
> Date: Wed, 7 Feb 2007 11:50:50 -0600
> Subject: [Proj] Rectified Skew Orthomorphic...

> I am trying to use Proj4 v4.4.9 to implement RSO support as described below
> from a 2004 thread, but it does not appear that the changes discussed in the
> thread ever made it into Proj4. Am I missing something or did these changes
> never get made? Does anyone have a copy of the code that implemented the
> 'gamma_c' parameter for the 'omerc' projection?

I am inclined to believe that all useful omerc RSOs can be done with
libproj/proj with the natural x_0, y_0 and the no_off/no_uoff flag set.
Libproj is more flexible (or transparent) with respect to the alpha and
gamma parameters, but proj does its best.
Point is, as mr. Evenden pointed out in a posting dated Feb 6, 2004, that
you don't need both alpha and gamma. Manuals and parameter files often
specify both, but for reasonable projections you only need one; the other is
calculated automatically.
It could be that proj is somewhat more difficult to handle because it
doesn't recognize a gamma. But the gamma is calculated from the specified
alpha. If you need to rotate the u,v coordinates over gamma instead of
alpha, you can use the rot_conv flag in proj. One might want to study Table
4 in the proj manual proj.4.3.pdf.

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