Hi,
I have found what seems to be an extremely bizarre bug. I am not sure if
it is a postgis issue or postgresql issue, but it leads
ST_Distance(geography(Polygon,4326),geography(MultiPolygon,4326)) to
return 0 (zero) when the true answer is not zero under certain peculiar
situations.
The table data for a set of polygons that generate the error can be
downloaded here:
http://www.davidmkaplan.fr/bad_geography_dist_calcs.csv
Once you have that data, the code to generate the error is:
CREATE TEMP TABLE tt (gid int PRIMARY KEY, geom geometry);
\copy tt FROM 'bad_geography_dist_calcs.csv' WITH (FORMAT 'csv',HEADER TRUE)
-- Incorrect dist_geography for t1.gid=1 and t2.gid=3
SELECT t1.gid AS gid1, t2.gid AS gid2,
ST_Distance(t1.geom,t2.geom) AS dist_lonlat,
ST_Distance(ST_Transform(t1.geom,26918),
ST_Transform(t2.geom,26918)) AS dist_utm,
ST_Distance(t1.geom::geography,t2.geom::geography) AS
dist_geography,
ST_Distance((ST_Dump(t1.geom)).geom::geography,(ST_Dump(t2.geom)).geom::geography)
AS dist_geography_dump
FROM tt t1 JOIN tt t2 ON t1.gid<t2.gid
WHERE t1.gid=1
;
-- No error just by specifying t2.gid=3 in the WHERE clause
SELECT t1.gid AS gid1, t2.gid AS gid2,
ST_Distance(t1.geom,t2.geom) AS dist_lonlat,
ST_Distance(ST_Transform(t1.geom,26918),
ST_Transform(t2.geom,26918)) AS dist_utm,
ST_Distance(t1.geom::geography,t2.geom::geography) AS
dist_geography,
ST_Distance((ST_Dump(t1.geom)).geom::geography,(ST_Dump(t2.geom)).geom::geography)
AS dist_geography_dump
FROM tt t1 JOIN tt t2 ON t1.gid<t2.gid
WHERE t1.gid=1 AND t2.gid=3
;
Hopefully, if I am not crazy, you will have a 0 value of dist_geography
in the first query for t2.gid=3, but not in the second.
Does anyone have any idea what could be causing this? To be honest, I am
not really certain that I know where to start do diagnose the problem...
The specifics of my installation are:
POSTGIS="2.4.3 r16312" PGSQL="100" GEOS="3.7.0-CAPI-1.11.0 673b9939"
PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 2.3.2, released 2018/09/21"
LIBXML="2.9.4" LIBJSON="0.12.1" LIBPROTOBUF="1.2.1" RASTER
psql 10.6 (Ubuntu 10.6-0ubuntu0.18.04.1))
Ubuntu 18.04.1 LTS
I have also tested this with postgis 2.5.0 r16836 and there the
incorrect 0 is in both queries!?!?!?
Thanks,
David
--
**********************************
David M. Kaplan
Charge de Recherche 1
Institut de Recherche pour le Developpement (IRD)
UMR MARBEC (IRD/Ifremer/CNRS/UMII)
av. Jean Monnet
CS 30171
34203 Sete cedex
France
Email: david.kap...@ird.fr
Phone: +33 (0)4 99 57 32 25
Fax: +33 (0)4 99 57 32 95
http://www.umr-marbec.fr/kaplan-david.html
http://www.davidmkaplan.fr/
**********************************
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