[PROJ] How does proj deal with ellipsoid with respect to reprojection

[Charles Karney]

Maybe one of these days I'll get around to implementing the ellipsoidal
version of the gnomonic projection in PROJ.  I derived this in
Algorithms for Geodesics (2013) and it's been in GeographicLib since
2010.  With this projection, geodesics on the ellipsoid and *nearly*
straight.  It's a good way of converting several problems involving
geodesics into equivalent 2d problems.

By the way, the documentation labels the gnomonic projection as
"pseudocylindrical".  This should be "azimuthal".

On 3/28/20 10:58 AM, DeTracey, Brendan wrote:
> Hi,
> 
> I am using a gnomonic projection for point in polygon testing. The transformation is:
> 
> projinfo -s EPSG:4326 -t ' +type=crs +ellps=WGS84  +datum=WGS84 +proj=gnom +lat_0=61.39107201212929 +lon_0=-58.56360634793518'
> 
> Will transformed great circles still be straight lines? Or does the fact that my source and target ellipse/datum are the same mean the gnomonic projection has the WGS4 coordinates passed directly to it? My confusion comes from gnomonic only being defined for a sphere, not an ellipsoid.
> 
> Any clarification would be appreciated.
> 
> Thanks,
> Brendan
>