New answers tagged spherical-geometry
If you're happy with the characteristics of your projection, you should just use ST_Distance(geometry, geometry). In your example you seem to be in UTM, which is a pretty good projection, so why not? It's much much less CPU intensive than the geodetic functions ST_Distance(geography, geography). (Note that ST_Distance_Spheroid() just calls into the same ...
Deduce what is not visible. Suppose you stand at a vertex on the beach, looking at two remote vertices of a neighboring polygon. Then you can assume that any vertex in the whole sector behind these vertices is invisible from this vertex.
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