[Proj] How does scaling function?

Michael Speth SpethM at landcareresearch.co.nz
Thu Nov 22 23:24:41 EST 2012

   We have recently discovered a new scaling problem within the HEALPix projection separate from the previous scaling error.  We have identified the problem which can be identified in the code below.

FORWARD(e_healpix_forward); /* ellipsoidal */
    double beta = auth_lat(lp.phi, P->e, 0);
    lp.phi = beta;
    return healpix_sphere(lp, P);

Here the HEALPix projection is being called on an ellipsoidal model of the earth.  The following is necessary:

Convert from Ellipsoidal to Sphere
Convert from Sphere to Plane

We need proj.4 to scale based on the sphere model not the ellipsoidal model.  How does scaling work (I have seen in proj.c that there are pre and post scaling functions).  Is it possible to set a configuration option in P so that proj.4 scales according to a Sphere and not an Ellipsoidal model?



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