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

[Pierre Abbat]

On Saturday, 28 March 2020 10:58:53 EDT 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.

In Bezitopo, I use a stereographic projection for point-in-polygon testing. 
This avoids the problem with the gnomonic projection that a polygon may cross 
the great circle which is projected to infinity. In stereographic, only one 
point is projected to infinity, and I chose it to be in the ocean, more than a 
megameter from any land, and with unround coordinates. Stereographic turns 
great circles into circles, so a spherical polygon turns into a polyarc in the 
plane, which I can handle easily.

Pierre
-- 
When a barnacle settles down, its brain disintegrates.
Já não percebe nada, já não percebe nada.