Sorry for the late answer. This problem is probably related to ST_Union erroneous behavior in some cases. It was described on this list last month (with multilinestring example). You can try ST_MemUnion - it's slower but should yield correct results. Greetings, Marcin
sob., 13 cze 2020 o 03:54 Jonathan Schultz <[email protected]> napisał(a): > Hello, > > I think I've reached the limit of my PostGIS skills so hoping someone can > help me solve a problem. > > What I'm trying to do is create overlays from a collection of overlapping > polygons as described here: > http://blog.cleverelephant.ca/2019/07/postgis-overlays.html > > The data I am working from is a publicly available dataset of the fire > history of Western Australia: > https://catalogue.data.wa.gov.au/dataset/dbca-fire-history I downloaded > the data as CSV and loaded it into a PostgreSQL table. That much seems fine. > > I then select a region and dump the MULTIPOLYGONs into POLYGONs using the > following query: > > > CREATE TABLE dbca_fire_history_dump AS > > SELECT fih_perimeter, fih_author, fih_season1, fih_capt_meth, > fih_year1, fih_district, fih_burn_purp, fih_detect_fdi, fih_poly_type, > fih_fire_type, fih_master_key, fih_number, fih_date1, fih_cause, > fih_ignit_type, fih_comment, fih_hectares, fih_hist_distr, fih_fire_seaso, > fih_name, (st_dump(geometry)).geom AS geometry FROM dbca_fire_history > > WHERE st_intersects(geometry, st_makeenvelope(<longmin>, <latmin>, > <longmax>, <latmax>, 4326)); > > and from there create the boundary polygon using this query: > > > CREATE TABLE dbca_fire_history_boundaries AS > > SELECT ST_Union(ST_ExteriorRing(geometry)) AS geometry > > FROM dbca_fire_history_dump; > > This is where things get weird. If I select a small enough region (eg 36S > 116E to 35S 117E) the result looks fine. But if I select a slightly larger > region (eg 36S 116E to 34S 117E) then the result is very strange - the > geometry seems to have snapped to a much larger grid to the extent that it > bears no obvious resemblance to the data. > > I've pasted images that demonstrate the result here: > https://pasteboard.co/JcPeDUk.png and here: > https://pasteboard.co/JcPfju0H.png > > Thanks in advance for any help or suggestions. > > Cheers, > Jonathan > _______________________________________________ > postgis-users mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/postgis-users
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