[postgis-users] Overlap analysis / polygonize precision problem

[Alexandre Neto]

I will, thank you!

On Tue, Mar 17, 2020 at 4:10 PM Martin Davis <mtnclimb at gmail.com> wrote:

> 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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