[postgis-users] Overlap analysis / polygonize precision problem

[Alexandre Neto]

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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