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
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