Yah I agree we should improve the docs. That particular FAQ you linked to was mostly focused on Geometry type usage where you are not working with long/lat coordinates and working on a measurement preserving spatial ref sys. That should have been made clearer.
I think for geography doing a segmentize before you apply ST_Subdivide should mostly do the trick. So something like SELECT sd.geom::geography AS geog FROM your_geog_table AS g CROSS JOIN LATERAL ST_SubDivide(ST_Segmentize(geog, 10000)::geometry ) AS sd(geom); -- you will need to tweak that 10000 a bit that would force a point every 10km Paul, Would it be possible to do a ST_Subdivide specifically for geography - https://postgis.net/docs/ST_Subdivide.html Like we have for ST_Segmentize https://postgis.net/docs/ST_Segmentize.html . I see a ST_Subdivide specifically geared for geography being a very useful thing since you are more likely to run into fat geographies than you are fat geometries. > -----Original Message----- > From: nikolai.berkoff <nikolai.berk...@pm.me> > Sent: Tuesday, September 9, 2025 3:01 PM > To: Regina Obe <l...@pcorp.us> > Cc: postgis-users@lists.osgeo.org > Subject: RE: Geometry / geography woes > > Thank you very much, > > I guess my toy example was too much of a toy but I still think the problem > exists with the suggestion to use ST_Subdivide as it will chop up a geometries > long "straight lines" into smaller "straight lines". > > https://postgis.net/documentation/faq/big-objects-performance/ > seems to suggest to use ST_Subdivide to work with large geometries but > https://postgis.net/documentation/faq/geography-inside/ > and the responses here make it clear that it doesn't work if I want to then > use > any geographical calculation. > > A better toy example is getting the distance of the geom to a point: > > SELECT ST_Distance(geography('POINT(-77 48)'), geometry('POLYGON((-71.37 > 55.8,-63.96 54.61,-72.49 51.77,-81.32 46.47,-81.89 46.09,-81.95 46.09,-81.97 > 46.09,-82 46.09,-82.01 46.09,-82.02 46.1,-82.03 46.1,-82.04 46.1,-82.05 > 46.1,-82.07 46.11,-82.09 46.11,-82.12 46.11,-82.12 46.1,-82.13 46.1,-82.14 > 46.11,-82.14 46.1,-82.15 46.1,-82.15 46.11,-82.16 46.11,-82.19 46.12,-82.19 > 46.13,-82.2 46.13,-82.21 46.14,-82.21 46.13,-82.22 46.13,-82.22 46.14,-82.23 > 46.14,-82.24 46.14,-82.26 46.14,-82.28 46.14,-82.3 46.15,-82.3 46.14,-82.33 > 46.15,-82.34 46.15,-82.35 46.15,-82.35 46.16,-82.36 46.16,-82.36 46.17,- > 82.35 46.17,-82.32 46.16,-82.32 46.17,-82.32 46.18,-82.34 46.18,-82.37 > 46.18,-82.38 46.18,-82.39 46.18,-82.4 46.18,-82.42 46.18,-82.43 46.18,-82.44 > 46.17,-82.45 46.17,-82.45 46.19,-82.46 46.2,-82.47 46.19,-82.48 46.18,-82.49 > 46.18,-82.5 46.18,-82.52 46.17,-82.53 46.17,-82.54 46.16,-82.55 46.17,-82.56 > 46.17,-82.58 46.17,-82.59 46.16,-82.6 46.16,-82.62 46.16,-82.62 46.15,-82.63 > 46.15,-82.64 46.15,-82.67 46.15,-82.68 46.16,-82.67 46.17,-82.68 46.17,- > 82.68 46.18,-82.67 46.19,-82.66 46.19,-82.64 46.19,-82.62 46.2,-82.65 46.2,- > 82.67 46.2,-82.71 46.19,-82.74 46.19,-82.76 46.18,-82.78 46.18,-82.79 > 46.18,-82.8 46.17,-82.81 46.17,-82.82 46.17,-82.84 46.17,-82.86 46.17,-82.87 > 46.17,-82.88 46.17,-82.91 46.18,-82.92 46.18,-82.93 46.17,-82.94 46.18,- > 82.96 46.17,-83 46.16,-83.01 46.17,-83.03 46.16,-83.05 46.16,-83.06 46.16,- > 83.09 46.16,-83.1 46.16,-83.1 46.17,-83.06 46.17,-83.05 46.18,-83.07 46.18,- > 83.08 46.19,-83.09 46.19,-83.1 46.19,-83.12 46.18,-83.13 46.18,-83.14 > 46.18,-83.15 46.18,-83.15 46.19,-83.17 46.2,-83.18 46.2,-83.19 46.19,-83.2 > 46.19,-83.22 46.2,-83.25 46.2,-83.26 46.21,-83.27 46.21,-83.28 46.21,-83.29 > 46.21,-83.3 46.21,-83.31 46.21,-83.32 46.22,-83.33 46.22,-83.34 46.22,-83.35 > 46.23,-83.38 46.23,-83.41 46.24,-83.43 46.24,-83.45 46.24,-83.47 46.24,- > 84.51 46.82,-84.5 46.82,-84.49 46.84,-84.48 46.84,-84.47 46.84,-84.46 > 46.84,-84.44 46.84,-84.43 46.83,-84.42 46.82,-84.42 46.81,-84.4 46.81,-84.4 > 46.82,-84.39 46.83,-84.38 46.83,-84.38 46.84,-84.37 46.84,-84.37 46.85,- > 84.37 46.86,-84.36 46.87,-84.36 46.88,-84.36 46.89,-84.38 46.89,-84.39 > 46.9,-84.4 46.91,-84.42 46.91,-84.43 46.91,-84.43 46.92,-84.44 46.92,-84.47 > 46.93,-84.48 46.93,-84.49 46.93,-84.51 46.92,-84.52 46.92,-84.53 46.93,- > 84.54 46.93,-84.55 46.94,-84.57 46.94,-84.58 46.93,-84.59 46.93,-84.6 > 46.93,-84.6 46.92,-84.6 46.91,-84.6 46.9,-84.61 46.89,-84.65 46.91,-84.65 > 46.92,-84.66 46.95,-84.67 46.96,-84.69 46.96,-84.7 46.96,-84.72 46.95,-84.73 > 46.95,-84.75 46.96,-84.76 46.97,-84.78 46.97,-84.78 46.98,-84.79 46.99,- > 84.79 47,-84.78 47,-84.78 47.01,-84.77 47.01,-84.78 47.02,-84.78 47.03,- > 84.78 47.04,-84.77 47.04,-84.77 47.05,-84.76 47.05,-84.75 47.06,-84.74 > 47.07,-84.73 47.07,-84.72 47.09,-84.71 47.1,-84.71 47.11,-84.72 47.11,-84.73 > 47.12,-84.73 47.13,-84.74 47.13,-84.74 47.14,-84.73 47.14,-84.72 47.14,- > 84.71 47.15,-84.7 47.15,-84.69 47.17,-84.71 47.17,-84.71 47.18,-84.7 47.18,- > 84.69 47.19,-84.68 47.19,-84.68 47.2,-84.67 47.2,-84.67 47.21,-84.66 47.21,- > 84.66 47.22,-84.65 47.22,-84.65 47.23,-84.65 47.24,-84.64 47.24,-84.64 > 47.25,-84.63 47.25,-84.62 47.26,-84.61 47.26,-84.6 47.27,-84.6 47.28,-84.59 > 47.28,-84.59 47.29,-84.58 47.29,-84.58 47.3,-84.59 47.3,-84.6 47.3,-84.61 > 47.34,-84.62 47.34,-84.63 47.34,-84.63 47.35,-84.64 47.35,-84.64 47.36,- > 84.65 47.36,-84.66 47.36,-84.67 47.35,-84.68 47.36,-84.69 47.36,-84.7 > 47.37,-84.7 47.38,-84.7 47.39,-84.7 47.4,-84.71 47.41,-84.71 47.42,-84.72 > 47.42,-84.73 47.43,-84.74 47.43,-84.75 47.44,-84.76 47.44,-84.76 47.45,- > 84.78 47.46,-84.79 47.47,-84.82 47.48,-84.83 47.48,-84.83 47.49,-84.87 > 47.49,-84.87 47.5,-84.88 47.5,-84.89 47.5,-84.9 47.51,-80.86 49.7,-79.08 > 51.52,-79.06 51.51,-79.04 51.51,-79.03 51.49,-79 51.47,-78.99 51.47,-78.98 > 51.46,-78.98 51.45,-78.98 51.43,-78.98 51.42,-78.98 51.41,-78.97 51.4,-78.97 > 51.38,-78.96 51.35,-78.95 51.34,-78.95 51.33,-78.96 51.32,-78.96 51.31,- > 78.95 51.3,-78.95 51.29,-78.94 51.28,-78.94 51.27,-78.95 51.26,-78.96 > 51.25,-78.96 51.24,-78.96 51.23,-78.95 51.21,-78.95 51.2,-78.94 51.19,-78.93 > 51.18,-78.92 51.18,-78.91 51.18,-78.9 51.17,-78.88 51.17,-78.87 51.17,-78.86 > 51.17,-78.86 51.18,-78.87 51.18,-78.87 51.19,-78.87 51.2,-78.89 51.2,-78.89 > 51.21,-78.9 51.21,-78.91 51.21,-78.92 51.22,-78.92 51.23,-78.93 51.25,-78.92 > 51.26,-78.91 51.28,-78.91 51.3,-78.91 51.31,-78.9 51.31,-78.89 51.32,-78.88 > 51.32,-78.88 51.33,-78.87 51.34,-78.87 51.35,-78.86 51.35,-78.85 51.35,- > 78.84 51.35,-78.84 51.36,-78.85 51.36,-78.86 51.36,-78.87 51.36,-78.89 > 51.37,-78.9 51.37,-78.9 51.38,-78.9 51.39,-78.9 51.4,-78.89 51.4,-78.87 > 51.41,-78.86 51.42,-78.84 51.43,-78.83 51.44,-78.82 51.45,-78.81 51.45,- > 78.81 51.46,-78.8 51.46,-78.79 51.48,-78.77 51.49,-78.76 51.49,-78.73 > 51.49,-78.72 51.49,-78.71 51.49,-78.7 51.49,-78.69 51.48,-78.68 51.49,-78.69 > 51.5,-78.7 51.5,-78.71 51.51,-78.72 51.51,-78.73 51.5,-78.75 51.5,-78.76 > 51.5,-78.77 51.5,-78.78 51.5,-78.79 51.5,-78.81 51.5,-78.83 51.51,-78.83 > 51.52,-78.82 51.53,-78.82 51.54,-78.82 51.55,-78.83 51.57,-78.83 51.58,- > 78.82 51.59,-78.82 51.6,-78.82 51.61,-78.83 51.61,-78.84 51.61,-78.85 > 51.61,-78.85 51.62,-78.86 51.63,-78.87 51.64,-78.89 51.64,-78.9 51.65,-78.91 > 51.65,-78.93 51.65,-78.94 51.66,-77.83 52.8,-71.37 55.8))')); > > st_distance > ----------------- > 105664.41899567 > (1 row) > > > and then getting the distance after having pre-conditioning my data via > ST_Subdivide gives a shorter result. Thanks to your answers I now understand > why but it feels like this is a large caveat in using ST_Subdivide. > > SELECT ST_Distance(geography('POINT(-77 48)'), > ST_Subdivide(geometry('POLYGON((-71.37 55.8,-63.96 54.61,-72.49 51.77,- > 81.32 46.47,-81.89 46.09,-81.95 46.09,-81.97 46.09,-82 46.09,-82.01 46.09,- > 82.02 46.1,-82.03 46.1,-82.04 46.1,-82.05 46.1,-82.07 46.11,-82.09 46.11,- > 82.12 46.11,-82.12 46.1,-82.13 46.1,-82.14 46.11,-82.14 46.1,-82.15 46.1,- > 82.15 46.11,-82.16 46.11,-82.19 46.12,-82.19 46.13,-82.2 46.13,-82.21 > 46.14,-82.21 46.13,-82.22 46.13,-82.22 46.14,-82.23 46.14,-82.24 46.14,- > 82.26 46.14,-82.28 46.14,-82.3 46.15,-82.3 46.14,-82.33 46.15,-82.34 46.15,- > 82.35 46.15,-82.35 46.16,-82.36 46.16,-82.36 46.17,-82.35 46.17,-82.32 > 46.16,-82.32 46.17,-82.32 46.18,-82.34 46.18,-82.37 46.18,-82.38 46.18,- > 82.39 46.18,-82.4 46.18,-82.42 46.18,-82.43 46.18,-82.44 46.17,-82.45 > 46.17,-82.45 46.19,-82.46 46.2,-82.47 46.19,-82.48 46.18,-82.49 46.18,-82.5 > 46.18,-82.52 46.17,-82.53 46.17,-82.54 46.16,-82.55 46.17,-82.56 46.17,- > 82.58 46.17,-82.59 46.16,-82.6 46.16,-82.62 46.16,-82.62 46.15,-82.63 > 46.15,-82.64 46.15,-82.67 46.15,-82.68 46.16,-82.67 46.17,-82.68 46.17,- > 82.68 46.18,-82.67 46.19,-82.66 46.19,-82.64 46.19,-82.62 46.2,-82.65 46.2,- > 82.67 46.2,-82.71 46.19,-82.74 46.19,-82.76 46.18,-82.78 46.18,-82.79 > 46.18,-82.8 46.17,-82.81 46.17,-82.82 46.17,-82.84 46.17,-82.86 46.17,-82.87 > 46.17,-82.88 46.17,-82.91 46.18,-82.92 46.18,-82.93 46.17,-82.94 46.18,- > 82.96 46.17,-83 46.16,-83.01 46.17,-83.03 46.16,-83.05 46.16,-83.06 46.16,- > 83.09 46.16,-83.1 46.16,-83.1 46.17,-83.06 46.17,-83.05 46.18,-83.07 46.18,- > 83.08 46.19,-83.09 46.19,-83.1 46.19,-83.12 46.18,-83.13 46.18,-83.14 > 46.18,-83.15 46.18,-83.15 46.19,-83.17 46.2,-83.18 46.2,-83.19 46.19,-83.2 > 46.19,-83.22 46.2,-83.25 46.2,-83.26 46.21,-83.27 46.21,-83.28 46.21,-83.29 > 46.21,-83.3 46.21,-83.31 46.21,-83.32 46.22,-83.33 46.22,-83.34 46.22,-83.35 > 46.23,-83.38 46.23,-83.41 46.24,-83.43 46.24,-83.45 46.24,-83.47 46.24,- > 84.51 46.82,-84.5 46.82,-84.49 46.84,-84.48 46.84,-84.47 46.84,-84.46 > 46.84,-84.44 46.84,-84.43 46.83,-84.42 46.82,-84.42 46.81,-84.4 46.81,-84.4 > 46.82,-84.39 46.83,-84.38 46.83,-84.38 46.84,-84.37 46.84,-84.37 46.85,- > 84.37 46.86,-84.36 46.87,-84.36 46.88,-84.36 46.89,-84.38 46.89,-84.39 > 46.9,-84.4 46.91,-84.42 46.91,-84.43 46.91,-84.43 46.92,-84.44 46.92,-84.47 > 46.93,-84.48 46.93,-84.49 46.93,-84.51 46.92,-84.52 46.92,-84.53 46.93,- > 84.54 46.93,-84.55 46.94,-84.57 46.94,-84.58 46.93,-84.59 46.93,-84.6 > 46.93,-84.6 46.92,-84.6 46.91,-84.6 46.9,-84.61 46.89,-84.65 46.91,-84.65 > 46.92,-84.66 46.95,-84.67 46.96,-84.69 46.96,-84.7 46.96,-84.72 46.95,-84.73 > 46.95,-84.75 46.96,-84.76 46.97,-84.78 46.97,-84.78 46.98,-84.79 46.99,- > 84.79 47,-84.78 47,-84.78 47.01,-84.77 47.01,-84.78 47.02,-84.78 47.03,- > 84.78 47.04,-84.77 47.04,-84.77 47.05,-84.76 47.05,-84.75 47.06,-84.74 > 47.07,-84.73 47.07,-84.72 47.09,-84.71 47.1,-84.71 47.11,-84.72 47.11,-84.73 > 47.12,-84.73 47.13,-84.74 47.13,-84.74 47.14,-84.73 47.14,-84.72 47.14,- > 84.71 47.15,-84.7 47.15,-84.69 47.17,-84.71 47.17,-84.71 47.18,-84.7 47.18,- > 84.69 47.19,-84.68 47.19,-84.68 47.2,-84.67 47.2,-84.67 47.21,-84.66 47.21,- > 84.66 47.22,-84.65 47.22,-84.65 47.23,-84.65 47.24,-84.64 47.24,-84.64 > 47.25,-84.63 47.25,-84.62 47.26,-84.61 47.26,-84.6 47.27,-84.6 47.28,-84.59 > 47.28,-84.59 47.29,-84.58 47.29,-84.58 47.3,-84.59 47.3,-84.6 47.3,-84.61 > 47.34,-84.62 47.34,-84.63 47.34,-84.63 47.35,-84.64 47.35,-84.64 47.36,- > 84.65 47.36,-84.66 47.36,-84.67 47.35,-84.68 47.36,-84.69 47.36,-84.7 > 47.37,-84.7 47.38,-84.7 47.39,-84.7 47.4,-84.71 47.41,-84.71 47.42,-84.72 > 47.42,-84.73 47.43,-84.74 47.43,-84.75 47.44,-84.76 47.44,-84.76 47.45,- > 84.78 47.46,-84.79 47.47,-84.82 47.48,-84.83 47.48,-84.83 47.49,-84.87 > 47.49,-84.87 47.5,-84.88 47.5,-84.89 47.5,-84.9 47.51,-80.86 49.7,-79.08 > 51.52,-79.06 51.51,-79.04 51.51,-79.03 51.49,-79 51.47,-78.99 51.47,-78.98 > 51.46,-78.98 51.45,-78.98 51.43,-78.98 51.42,-78.98 51.41,-78.97 51.4,-78.97 > 51.38,-78.96 51.35,-78.95 51.34,-78.95 51.33,-78.96 51.32,-78.96 51.31,- > 78.95 51.3,-78.95 51.29,-78.94 51.28,-78.94 51.27,-78.95 51.26,-78.96 > 51.25,-78.96 51.24,-78.96 51.23,-78.95 51.21,-78.95 51.2,-78.94 51.19,-78.93 > 51.18,-78.92 51.18,-78.91 51.18,-78.9 51.17,-78.88 51.17,-78.87 51.17,-78.86 > 51.17,-78.86 51.18,-78.87 51.18,-78.87 51.19,-78.87 51.2,-78.89 51.2,-78.89 > 51.21,-78.9 51.21,-78.91 51.21,-78.92 51.22,-78.92 51.23,-78.93 51.25,-78.92 > 51.26,-78.91 51.28,-78.91 51.3,-78.91 51.31,-78.9 51.31,-78.89 51.32,-78.88 > 51.32,-78.88 51.33,-78.87 51.34,-78.87 51.35,-78.86 51.35,-78.85 51.35,- > 78.84 51.35,-78.84 51.36,-78.85 51.36,-78.86 51.36,-78.87 51.36,-78.89 > 51.37,-78.9 51.37,-78.9 51.38,-78.9 51.39,-78.9 51.4,-78.89 51.4,-78.87 > 51.41,-78.86 51.42,-78.84 51.43,-78.83 51.44,-78.82 51.45,-78.81 51.45,- > 78.81 51.46,-78.8 51.46,-78.79 51.48,-78.77 51.49,-78.76 51.49,-78.73 > 51.49,-78.72 51.49,-78.71 51.49,-78.7 51.49,-78.69 51.48,-78.68 51.49,-78.69 > 51.5,-78.7 51.5,-78.71 51.51,-78.72 51.51,-78.73 51.5,-78.75 51.5,-78.76 > 51.5,-78.77 51.5,-78.78 51.5,-78.79 51.5,-78.81 51.5,-78.83 51.51,-78.83 > 51.52,-78.82 51.53,-78.82 51.54,-78.82 51.55,-78.83 51.57,-78.83 51.58,- > 78.82 51.59,-78.82 51.6,-78.82 51.61,-78.83 51.61,-78.84 51.61,-78.85 > 51.61,-78.85 51.62,-78.86 51.63,-78.87 51.64,-78.89 51.64,-78.9 51.65,-78.91 > 51.65,-78.93 51.65,-78.94 51.66,-77.83 52.8,-71.37 55.8))'))) order by 1; > st_distance > ----------------- > 92373.36598847 > 126170.82808402 > 364673.04933925 > 407862.08030654 > 529769.79532574 > > > On Tuesday, 9 September 2025 at 18:05, Regina Obe <l...@pcorp.us> wrote: > > > Your smaller polygon is not within the larger one. If it was then this > > would return true > > > > > SELECT ST_Intersects(ST_GeomFromText('POLYGON((-77.933927810506 > > 48.584377886263184,-77.74471476703403 > > 48.69534746901027,-77.69284285304693 > > 48.71525447165988,-77.99039086603969 > > 48.53485501013662,-77.933927810506 48.584377886263184))', > > 4326)::geography, ST_GeomFromText('POLYGON((-81.1755551313531 > > 46.64525933571578,-81.50518756520556 > > 47.21802525065504,-81.6706742747649 > > 47.82323694299835,-81.6602226284047 > > 48.438637358868554,-81.46784223247613 > > 49.04081439308046,-81.09452004412593 > > 49.60598066574395,-80.54928811996326 > > 50.110912115887594,-79.84994871352842 > > 50.53403375725974,-79.02318022750863 > > 50.85659915226637,-78.10377843647761 > > 51.06386274945413,-77.13291403765557 > > 51.14610652092342,-76.15549659681517 > > 51.09937193283258,-75.21696581007667 > > 50.925776760111056,-74.42324583717638 > > 50.655006343959805,-81.12758032107652 > > 46.593233077736336,-81.1755551313531 > > 46.64525933571578))',4326)::geography); > > > > > And it returns false. Your ST_Within compare in planar space is invalid as > > it's > treating the surfaces as planar. > > > > > If you segmentize your geographies you will see the smaller is not within > > the > larger. > > > > > Here is what you get segmentizing in spheroidal space > > > > > SELECT ST_Segmentize(ST_GeomFromText('POLYGON((-77.933927810506 > > 48.584377886263184,-77.74471476703403 > > 48.69534746901027,-77.69284285304693 > > 48.71525447165988,-77.99039086603969 > > 48.53485501013662,-77.933927810506 48.584377886263184))', > > 4326)::geography, 1000) > > > > > UNION ALL > > > > > SELECT ST_Segmentize(ST_GeomFromText('POLYGON((-81.1755551313531 > > 46.64525933571578,-81.50518756520556 > > 47.21802525065504,-81.6706742747649 > > 47.82323694299835,-81.6602226284047 > > 48.438637358868554,-81.46784223247613 > > 49.04081439308046,-81.09452004412593 > > 49.60598066574395,-80.54928811996326 > > 50.110912115887594,-79.84994871352842 > > 50.53403375725974,-79.02318022750863 > > 50.85659915226637,-78.10377843647761 > > 51.06386274945413,-77.13291403765557 > > 51.14610652092342,-76.15549659681517 > > 51.09937193283258,-75.21696581007667 > > 50.925776760111056,-74.42324583717638 > > 50.655006343959805,-81.12758032107652 > > 46.593233077736336,-81.1755551313531 > > 46.64525933571578))',4326)::geography, 1000); > > > > > Here is your planar: > > > > > SELECT ST_GeomFromText('POLYGON((-77.933927810506 > > 48.584377886263184,-77.74471476703403 > > 48.69534746901027,-77.69284285304693 > > 48.71525447165988,-77.99039086603969 > > 48.53485501013662,-77.933927810506 48.584377886263184))', > > 4326)::geography > > > > > UNION ALL > > > > > SELECT ST_GeomFromText('POLYGON((-81.1755551313531 > > 46.64525933571578,-81.50518756520556 > > 47.21802525065504,-81.6706742747649 > > 47.82323694299835,-81.6602226284047 > > 48.438637358868554,-81.46784223247613 > > 49.04081439308046,-81.09452004412593 > > 49.60598066574395,-80.54928811996326 > > 50.110912115887594,-79.84994871352842 > > 50.53403375725974,-79.02318022750863 > > 50.85659915226637,-78.10377843647761 > > 51.06386274945413,-77.13291403765557 > > 51.14610652092342,-76.15549659681517 > > 51.09937193283258,-75.21696581007667 > > 50.925776760111056,-74.42324583717638 > > 50.655006343959805,-81.12758032107652 > > 46.593233077736336,-81.1755551313531 > > 46.64525933571578))',4326)::geography; > > > > > > -----Original Message----- > > > > > > From: nikolai.berkoff via postgis-users > > > <postgis-users@lists.osgeo.org> > > > > > > Sent: Tuesday, September 9, 2025 7:36 AM > > > > > > To: postgis-users@lists.osgeo.org > > > > > > Subject: Geometry / geography woes > > > > > > > > > > > > Hi, > > > > > > > > > > > > I work for a conservation NGO and we deal with lots of species ranges. > > > > > > > > > > > > We have a complicated full geometry (SRID 4326) > > > > > > https://www.iucnredlist.org/species/22733619/139214707 that we have > > > > > > ST_Subdivided. We notice a large difference between the geographical > > > > > > distance from a point (-77 48) to the full geometry and to nearest > > > subdiveded > > > > > > geometry. > > > > > > > > > > > > Here is a toy example with two polygon geometries, one large and the > > > other > > > > > > small and a point https://wktmap.com/?771d2fc9 . First the > > > geographical > > > > > > distance from the point to the large geometry: > > > > > > > > > > > > SELECT ST_Distance(ST_SetSRID(ST_MakePoint(-77, > > > 48),4326)::geography, > > > > > > ST_GeomFromText('POLYGON((-81.1755551313531 46.64525933571578,- > > > > > > 81.50518756520556 47.21802525065504,-81.6706742747649 > > > > > > 47.82323694299835,-81.6602226284047 48.438637358868554,- > > > > > > 81.46784223247613 49.04081439308046,-81.09452004412593 > > > > > > 49.60598066574395,-80.54928811996326 50.110912115887594,- > > > > > > 79.84994871352842 50.53403375725974,-79.02318022750863 > > > > > > 50.85659915226637,-78.10377843647761 51.06386274945413,- > > > > > > 77.13291403765557 51.14610652092342,-76.15549659681517 > > > > > > 51.09937193283258,-75.21696581007667 50.925776760111056,- > > > > > > 74.42324583717638 50.655006343959805,-81.12758032107652 > > > > > > 46.593233077736336,-81.1755551313531 > > > > > > 46.64525933571578))',4326)::geography); > > > > > > > > > > > > st_distance > > > > > > > > > > > > ----------------- > > > > > > 100757.06933552 > > > > > > > > > > > > > > > > > > next the geographical distance from the point to the small geometry > > > > > > > > > > > > SELECT ST_Distance(ST_SetSRID(ST_MakePoint(-77, > > > 48),4326)::geography, > > > > > > ST_GeomFromText('POLYGON((-77.933927810506 48.584377886263184,- > > > > > > 77.74471476703403 48.69534746901027,-77.69284285304693 > > > > > > 48.71525447165988,-77.99039086603969 48.53485501013662,- > > > > > > 77.933927810506 48.584377886263184))', 4326)::geography); > > > > > > st_distance > > > > > > > > > > > > ---------------- > > > > > > 93442.14463111 > > > > > > > > > > > > > > > > > > finally, showing the the small geometry is fully within the large > > > geometry > > > > > > > > > > > > SELECT ST_Within(ST_GeomFromText('POLYGON((-77.933927810506 > > > > > > 48.584377886263184,-77.74471476703403 48.69534746901027,- > > > > > > 77.69284285304693 48.71525447165988,-77.99039086603969 > > > > > > 48.53485501013662,-77.933927810506 48.584377886263184))', 4326), > > > > > > ST_GeomFromText('POLYGON((-81.1755551313531 46.64525933571578,- > > > > > > 81.50518756520556 47.21802525065504,-81.6706742747649 > > > > > > 47.82323694299835,-81.6602226284047 48.438637358868554,- > > > > > > 81.46784223247613 49.04081439308046,-81.09452004412593 > > > > > > 49.60598066574395,-80.54928811996326 50.110912115887594,- > > > > > > 79.84994871352842 50.53403375725974,-79.02318022750863 > > > > > > 50.85659915226637,-78.10377843647761 51.06386274945413,- > > > > > > 77.13291403765557 51.14610652092342,-76.15549659681517 > > > > > > 51.09937193283258,-75.21696581007667 50.925776760111056,- > > > > > > 74.42324583717638 50.655006343959805,-81.12758032107652 > > > > > > 46.593233077736336,-81.1755551313531 46.64525933571578))',4326)); > > > > > > st_within > > > > > > > > > > > > ----------- > > > > > > t > > > > > > > > > > > > > > > > > > Why is the geographical distance to the point and the geometry > > > increasing > > > > > > when we are only making the geometry bigger? Surely as the geometry > > > gets > > > > > > bigger it should get nearer to the point? > > > > > > > > > > > > Thank you very much!
