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

[Paul Ramsey]

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

On Tue, Mar 28, 2017 at 4:36 PM, Hector Gonzalez <hagonzal at gmail.com> wrote:

> 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 lies
> on the edge (at the mid point)  which is 3.91451185 25.11977359. When I use
> st_distance to compute the distance between the edge and the point I expect
> 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.
>
>
> Thanks
> --
> Hector
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20170328/f84751be/attachment.html>