[postgis-users] Overlap analysis / polygonize precision problem

[Martin Davis]

It seems odd that precision is being reduced that much.  In order to
isolate where it is happening, you should capture output from each stage of
the CTE (which is easy to do, by just truncating the CTE after each query
and doing a "SELECT * FROM cte_tbl" .  Then post the results here if you
need help analyzing them, and so we can investigate further.

Even better would be to provide the dataset which is causing the problem,
to aid in sorting this out.

On Mon, Mar 16, 2020 at 7:16 PM Alexandre Neto <senhor.neto at gmail.com>
wrote:

> Hi,
>
> I am running a Overlay analysis, and I am following what is proposed by
> Paul Ramsey in this article:
>
> http://blog.cleverelephant.ca/2019/07/postgis-overlays.html
>
> I am using this query on PostgreSQL 10 and PostGIS
>
> With isolated_polygons as
> (
> select st_exteriorring((st_dumprings((st_dump(geom)).geom)).geom) as geom
> from OUTPUTS_FINAIS.estabelecimentos_as_400
> )
> , boundaries as (
> SELECT st_union(geom) AS geom
> FROM isolated_polygons
> )
> , poligonos as (
> SELECT (st_dump(st_polygonize(geom))).geom as geom
> FROM boundaries
> )
> , n_poligonos as (
> SELECT row_number() over() as id, geom
> FROM poligonos
> )
> SELECT p.id, count(id) as n_sobreposicoes, p.geom::geometry(polygon,4326)
> into temp.sobreposicoes_400m
> FROM n_poligonos as p
> join outputs_finais.estabelecimentos_as_400 as t
> on st_contains(t.geom,st_pointonsurface(p.geom))
> GROUP BY id, p.geom;
>
> When I use this with a sample set of input multipolygons, The results are
> what we would expect, all the overlapping polygons split and the count of
> the number of overlapping polygons.
>
> When I run it with the full set (around 4800 polygons)
>
> The precison of the output polygons gets truncated and instead of detailed
> polygons I get a "pixelied" output:
>
> POLYGON ((-8.99 39.42, -8.99 39.43, -8.98 39.43, -8.97 39.43, -8.97 39.42,
> -8.98 39.42, -8.99 39.42))
> POLYGON ((-8.19 39.56, -8.2 39.56, -8.2 39.57, -8.19 39.57, -8.18 39.57,
> -8.19 39.56))
> POLYGON ((-8.54 41.02, -8.55 41.02, -8.55 41.03, -8.54 41.03, -8.54 41.02))
> POLYGON ((-8.39 40.17, -8.4 40.17, -8.4 40.18, -8.39 40.18, -8.39 40.17))
> POLYGON ((-8.57 38.83, -8.58 38.83, -8.59 38.84, -8.58 38.84, -8.57 38.84,
> -8.57 38.83))
>
> My gut feeling is that this is happening at the polygonize level, but I am
> not sure.
>
> I have also tried to use st_node + st_collect getting the same kind of
> results
>
> Any idea of what can this be?
>
> Since not all polygons are overlapping each other, I wanted to try to
> separate the analysis by chunks, but I fail to see how can I union or
> collect only not disjoint line groups.
>
> I am using
>
> PostgreSQL 10.12 (Ubuntu 10.12-0ubuntu0.18.04.1) on x86_64-pc-linux-gnu,
> compiled by gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0, 64-bit
>
> POSTGIS="2.5.3 r17699" [EXTENSION] PGSQL="100" GEOS="3.7.1-CAPI-1.11.1
> 27a5e771" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.2.3, released
> 2017/11/20" LIBXML="2.9.4" LIBJSON="0.12.1" LIBPROTOBUF="1.2.1" RASTER
>
> Thanks,
>
> Alexandre Neto
>
>
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