[PostGIS] #5740: ST_DistanceSpheroid doesn't return 0 when one geometry contains the other

[PostGIS]

#5740: ST_DistanceSpheroid doesn't return 0 when one geometry contains the other
-------------------------+---------------------------
  Reporter:  cdestigter  |      Owner:  pramsey
      Type:  defect      |     Status:  new
  Priority:  medium      |  Milestone:  PostGIS 3.4.3
 Component:  postgis     |    Version:  3.4.x
Resolution:              |   Keywords:
-------------------------+---------------------------
Description changed by cdestigter:

Old description:

> 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 to cause the result to be 0 (the minimum distance between the
> geometries is 0 because they overlap)
>
> However, perhaps it actually compares the _edges_ of the geometries and
> so if one geometry contains the other you get a positive number:
>

> {{{
> #!sql
>
> -- 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
>
> }}}

New description:

 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:


 {{{
 #!sql

 -- 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

 }}}

--
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5740#comment:1>
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