[postgis-users] st_distance between a point and an edge using GeographicLib.

Hector Gonzalez hagonzal at gmail.com
Thu Mar 30 07:50:50 PDT 2017

Hi Paul,

Thank you very much for the answer. But don't curse me yet :) I care, but I
can probably work around it. I can modify my edge and add interior vertices
that are on the wgs84 edge (tessellate) and thus reduce the error when
projecting a point to the edge on the sphere.


> Yes, you've got it right. The projected point is done in sphere world for
> speed, and then the point-to-point distance is done on spheroid. Most
> people don't care. Curse you for caring :)
> P

>> Hello,
>> I am trying to verify the distance calculation (using geographies and
>> use_spheroid=true) between a line and a point.
>> I have used GeographicLib to generate a geodesic (using the wgs84
>> ellipsoid) between points 0 0 and 10 50 (lng lat). I generate a point
>> on the edge (at the mid point)  which is 3.91451185 25.11977359. When I
>> st_distance to compute the distance between the edge and the point I
>> the answer to be close to 0. But it is not.
>> select st_distance(st_geogfromtext('SRID=4326;point( 3.91451185
>> 25.11977359 )'),st_geogfromtext('SRID=4326;linestring(0.00000000
>> 0.00000000,   10 50 )'), true);
>> produces: 274.54332271 meters. If I use use_spheroid=false the answer
>> is 274.10911308.
>> Is this because the distance calculation between a point and a line is
>> done by projecting the point to the line assuming a perfect sphere? Or
am I
>> doing something wrong?
>> I am using Postgresql-9.5 and PostGIS-2.2 on Ubuntu 16.04.
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