Should be really straight forward, but I can't understand what I'm doing wrong. I'm trying to measure the distance between the centre of a polygon (suburb) and an arbitrary point.

    ST_AsText(ST_Centroid(geom)) as poly_centroid
    , ST_AsText(ST_SetSrid(ST_MakePoint(151.182995, -33.890395), 4283)) as pin
    , ST_Distance(
        ST_Transform(ST_SetSrid(ST_MakePoint(151.182995, -33.890395), 4283), 3112),
        ST_Transform(ST_Centroid(geom), 3112)
    ) as dist
FROM public.suburbs

I'm using 3112, Australia Lambert projection (which is in metres) - and entering the arbitrary pin point in long/lat (EPSG 4283) - but the resulting distance is clearly wrong:

poly_centroid pin dist
POINT(150.9300421895883 -33.72537351810487) POINT(151.182995 -33.890395) 4229563.228668396
POINT(151.03083602374133 -33.75769279240827) POINT(151.182995 -33.890395) 4229563.161116713
POINT(151.15847466229258 -33.75004174996872) POINT(151.182995 -33.890395) 4229563.120652905
POINT(150.93371519943338 -33.73946132331832) POINT(151.182995 -33.890395) 4229563.214226549
POINT(150.97498052582384 -34.12744863603382) POINT(151.182995 -33.890395) 4229562.838806998

These should be 10s of Km apart, not 1000's.

Note that the polygons are all loaded in EPSG 3112 projection, so the ST_Transform isn't necessary.

  • 1
    If SELECT ST_AsText(ST_Centroid(geom)) as poly_centroid returns POINT(150.9300421895883 -33.72537351810487) then geom obviously is not in EPSG:3112. Your SQL should still work but check what you get for the second term with select ST_AsText(ST_Transform(ST_Centroid(geom), 3112)).
    – user30184
    Dec 23, 2020 at 8:09
  • I believe that you have loaded some long-lat data into PostGIS and just assigned SRID EPSG:3112 without reprojecting the data. select st_distance( st_geomfromtext('POINT(150.9300421895883 -33.72537351810487)'), st_geomfromtext('POINT(1555654.93979939 -3903769.92078054)')) gives 4202231 meters, that is close to your numbers.
    – user30184
    Dec 23, 2020 at 12:01

2 Answers 2


I get different values for the distances in my PostGIS. postgis_full_version string is:

POSTGIS="3.0.0 r17983" [EXTENSION] PGSQL="120" GEOS="3.8.0-CAPI-1.13.1 " PROJ="6.3.1" LIBXML="2.9.4" LIBJSON="0.13.1" LIBPROTOBUF="1.3.3" WAGYU="0.4.3 (Internal)"

I created a simple point table from your centroids:

    id integer PRIMARY KEY,
    geom geometry(Point, 4283)
INSERT INTO centr VALUES (1, ST_GeomFromText('POINT(150.9300421895883 -33.72537351810487)',4283));  
INSERT INTO centr VALUES (2, ST_GeomFromText('POINT(151.03083602374133 -33.75769279240827)',4283));
INSERT INTO centr VALUES (3, ST_GeomFromText('POINT(151.15847466229258 -33.75004174996872)',4283));     
INSERT INTO centr VALUES (4, ST_GeomFromText('POINT(150.93371519943338 -33.73946132331832)',4283));     
INSERT INTO centr VALUES (5, ST_GeomFromText('POINT(150.97498052582384 -34.12744863603382)',4283));     

and selected the distances:

SELECT id, ST_Distance(ST_Transform(ST_SetSrid(ST_MakePoint(151.182995, -33.890395), 4326), 3112),
                       ST_Transform(geom, 3112))
FROM centr;

and I got the following values:

 id |    st_distance     
  1 |  29565.26990848302
  2 | 20264.268485336703
  3 |   15648.1409013042
  4 | 28358.850986011512
  5 | 32405.833529177606

I checked the first one and looks correct. There is probably problem with your SRS definitions in spatial_ref_sys table. The definitions for EPSG 4283 and 3112 in my table are:

 EPSG      |      4283 | +proj=longlat +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +no_defs 
 EPSG      |      3112 | +proj=lcc +lat_1=-18 +lat_2=-36 +lat_0=0 +lon_0=134 +x_0=0 +y_0=0 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs 
  • 1
    You are correct I loaded the .shp file using the wrong SRID.
    – QuinRiva
    Dec 23, 2020 at 23:30

Looks like I missed an important step. The .shp file was encoded in EPSG 4283, but I loaded it in as EPSG 3112.

I had thought that the PostGIS Shapefile Import Manager transformed the shapefiles to the desired SRID, and auto identified the encoded SRID from the .prj file (which is how I use the command line tool). This is not how the import manager works, it requires the SRID to be manually entered - as far as I can tell, it doesn't appear to use the .prj file at all?

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.