ST_DistanceSpheroid doesn't return 0 when one geometry contains the other

[cdestigter]

The docs say

Returns minimum distance in meters between two lon/lat geometries given a particular spheroid

Based on this description it seems that any intersection between the two geometries should cause the result to be 0 (the minimum distance between the geometries is 0 because they overlap)

However, it seems that actually compares the _edges_ of the geometries - if one geometry contains the other you get a positive number:

-- touching the edge of the polygon, the result is zero
=> select st_distancespheroid('SRID=4326;POLYGON((0 0,0 1,1 1,1 0,0 0))'::GEOMETRY, 'SRID=4326;POINT(0 0.5)'::geometry);

0


-- close to the edge, the result is nonzero
=> select st_distancespheroid('SRID=4326;POLYGON((0 0,0 1,1 1,1 0,0 0))'::GEOMETRY, 'SRID=4326;POINT(0.1 0.5)'::geometry);

11131.528045775543


-- the further you get from the edge, the larger the result
=> select st_distancespheroid('SRID=4326;POLYGON((0 0,0 1,1 1,1 0,0 0))'::GEOMETRY, 'SRID=4326;POINT(0.5 0.5)'::geometry);

55657.640177233385

Interestingly, ST_DistanceSphere gives the expected result:

=> select st_distancesphere('SRID=4326;POLYGON((0 0,0 1,1 1,1 0,0 0))'::GEOMETRY, 'SRID=4326;POINT(0.5 0.5)'::geometry);

0