[postgis-users] Geometries returning zero area with PostGIS 3.1.2.

[Paul Ramsey]

Thanks for the easy-to-use example data.

I strongly doubt (??) this is changed behaviour, because it's behaviour with a reason, and the reasons are this.

First, note the answer I get when I do not add the convex hull function to the query.

  select st_area(ST_Transform(geom, 4326)::geography) from osm;

        st_area       
  --------------------
    4650984117506.695
   33615580446323.664

Correct answers. Also note that, if you look at these shapes, they are essentially just rectangles, but they have a lot of vertices along the edges. In geometry (planar) space, those extra vertices are redundant, they just define the existing straight line between the corners. However in geography space (spherical) those vertices are quite important, they define a path along the parallels which is both (a) defined in edges of < 180 degrees and (b) not the same as the default edge path (a great circle).

So what happens in that convex hull call?

  select st_astext(st_convexhull(st_transform(geom, 4326))) from osm ;

                                  st_astext                                 
  --------------------------------------------------------------------------
   POLYGON((-180 -85.0511287,-180 -60,180 -60,180 -85.05,-180 -85.0511287))
   POLYGON((-180 -85.0511287,-180 -60,180 -60,180 -85.05,-180 -85.0511287))

Urm. The big horizontal "straight line" is reduced to one edge, and it goes "from" (-180 -60) to (180 -60), which is, in spherical space, a zero-distance path. Hence the zero area result.

This is behaviour right at the core of the geography type, so I'm more surprised that you'd *ever* get a difference answer, in any version of PostGIS, running the SQL example you've shown.

P

> On Jul 12, 2021, at 1:03 PM, Marco Boeringa <marco at boeringa.demon.nl> wrote:
> 
> Hi Paul,
> 
> Done:
> 
> https://trac.osgeo.org/postgis/ticket/4951
> 
> Note that I copied the WKT representation straight from DBeaver's display of it in the table view. Seems to work fine though... I tested displaying the files in a browser based WKT renderer, and it appeared OK.
> 
> Marco
> 
> Op 12-7-2021 om 21:46 schreef Paul Ramsey:
>> Marco,
>> Can you open a ticket and attach some WKT to said ticket? That way I can test it without having to learn how to extarct those relations from OSM myself.
>> Thanks!
>> P
>> 
>>> On Jul 11, 2021, at 3:42 AM, Marco Boeringa <marco at boeringa.demon.nl> wrote:
>>> 
>>> Hi all,
>>> 
>>> I am running into an issue with two of OpenStreetMap's polygon geomtries returning a zero area for the following PostGIS SQL clause:
>>> 
>>> ST_Area(ST_ConvexHull(ST_Transform(way,4326))::geography,true)
>>> 
>>> Note that the data was imported using osm2pgsql, with the data stored in WGS1984, not Web Mercator.
>>> 
>>> The two relations that cause the issue are:
>>> 
>>> https://www.openstreetmap.org/relation/2186646 <https://www.openstreetmap.org/relation/2186646>
>>> 
>>> https://www.openstreetmap.org/relation/3394113
>>> 
>>> One represents "Antarctica", the other the "Ross Dependency".
>>> 
>>> Note that both relations appear to be quite old (years), and I am therefor pretty sure I have processed the exact same geometries previously without issues with older versions of PostGIS after planet import. Note also that both geometries test as valid with "ST_Isvalid(way)" after import using osm2pgsql.
>>> 
>>> Has something changed in the latest PostGIS 3.1.2 that might explain this result, e.g. especially related to the "::geography" cast or the "ST_ConvexHull" statement?
>>> 
>>> Of course, in the way these geometries wrap the globe, they may represent a challenge, but as said, I am pretty sure I must have processed the same geometries without issues in previous releases of PostGIS.
>>> 
>>> Marco
>>> 
>>> 
>>> 
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