I've submitted a pull request (PR) https://github.com/OSGeo/PROJ/pull/3522
for PROJ to implement an ellipsoidal generalization of the gnomonic projection. The spherical gnomonic projection has the property that geodesics map to straight lines. There's no projection with the same property for the ellipsoid. However, the spherical gnomonic projection is also the limit of a double azimuthal projection, a projection which preserves the azimuths from two points, in the limit that the two points coalesce. This property carries over to the ellipsoidal version of the projection implemented by this PR. For earth ellipsoids, geodesics which pass within a few hundred km of the center of the projection are very nearly straight. For details and for a derivation of the projection, see Sec. 8 of my 2013 paper Algorithms for geodesics https://doi.org/10.1007/s00190-012-0578-z Merging this PR will result in a minor incompatibility: With the gnomonic projection in current version of PROJ, an ellipsoid is treated (inconsistently!) as a sphere whose radius matches the equatorial radius of the ellipsoid. After the merge, the ellipsoidal gnomonic projection will be used (and you will get somewhat different results). To retain the previous behavior you'll need to set the flattening of the ellipsoid to zero, +f=0. Comments are welcome... -- Charles Karney <[email protected]> 702 Prospect Ave Princeton, NJ 08540-4037 _______________________________________________ PROJ mailing list [email protected] https://lists.osgeo.org/mailman/listinfo/proj
