[postgis-users] voronoi fails on regular grid

[J.J. Green]

Hi all,

I have some point data in latlong which I project onto
some SRID, then perform the following query:

   CREATE TEMPORARY TABLE cell_poly_tmp ON COMMIT DROP AS
     SELECT
       Geometry(
         (
           ST_Dump(
             ST_CollectionExtract(
               ST_VoronoiPolygons(
                 ST_Collect(point),
                 0.0
               ),
               3
             )
           )
         ).geom
       ) AS poly
     FROM cell_point_tmp;

For most SRIDs this works OK, but for 3857, I get

   56966976: ERROR:  GEOSVoronoiDiagram: TopologyException:
   Input geom 1 is invalid: Self-intersection at or near point
   444332.38647522771 8548816.0569835864 at
   444332.38647522771 8548816.0569835864

Now this is a bit of a pain, since 3857 is SRID I want to use --
the target is an OpenLayers application and their 3857 tiles.

I observe that the (projected) points in this case are on a
uniform grid (clearly the source used a uniform grid in 3875
and transformed to latlong), and that the failed point is at
the dead-centre of one of the grid-cells at the edge of the
domain.

A plot of the geometry and the input data point are at

   https://gist.github.com/jjgreen/b0846716cfe076ae194217d1585783bb

I've tried sliding ST_MakeValid() in various places in the query,
but without success.

Any thoughts as to how to proceed?

My setup is Ubuntu Xenial with PostGIS from apt-postgresql.org,
and

   POSTGIS="2.3.3 r15473"
   GEOS="3.5.0-CAPI-1.9.0 r4084"
   PROJ="Rel. 4.9.2, 08 September 2015"
   GDAL="GDAL 1.11.3, released 2015/09/16"
   LIBXML="2.9.3"
   LIBJSON="0.11.99" RASTER

Cheers

Jim
--
J.J. Green
http://soliton.vm.bytemark.co.uk/pub/jjg/en/