[postgis-users] Geography/geometry

Birgit Laggner birgit.laggner at thuenen.de
Mon Jan 21 04:17:39 PST 2019

Hi Simon, 

I assume, you are trying to get this all done in one query, right? Did you try to throw ST_MakeValid into the mix? Like: ST_Transform(ST_Union(ST_MakeValid(ST_Transform(a.geog4326::geometry, 3857))), 4326)::geography 


Von: "Simon Greener" <simon at spatialdbadvisor.com> 
An: "PostGIS Users Discussion" <postgis-users at lists.osgeo.org> 
Gesendet: Montag, 21. Januar 2019 06:23:10 
Betreff: [postgis-users] Geography/geometry 


I'm in a situation where I need to ST_Union or ST_Collect some osm_county Polygon (not MultiPolygon) data for Ireland. 

Now, because ST_Union or ST_Collect do not support geography, I cast to geometry before calling. 

select min(a.osm_id) as osm_id, 
count(*) as parts, 
ST_Union(a.geog4326::geometry)::geography as geog4326 -- or ST_Collect 
from data.osm_county as a 
group by a.name; 

Whence I get this: 

ERROR: lwgeom_area_spher(oid) returned area < 0.0 

Investigating I get results like this: 

select distinct st_isvalidreason(a.geog4326::geometry) from data.osm_county as a; 

"Hole lies outside shell[-10.2589459 53.9746452]" 

I guess this is expected because geodetic lines in the source geography are being treated as straight in the cast'd geometry. 

If I use ST_Transform to project a 4326 poly to a 3857 and then call the ST_Union aggregate, or identify a single geography that has the invalidity and execute a self-union, I get the following in both situations. 

ERROR: GEOSUnaryUnion: TopologyException: Input geom 0 is invalid: Hole lies outside shell at or near point -1148162.9982628345 7095296.1166736316 at -1148162.9982628345 7095296.1166736316 

I can't for the life of me work out how to complete the aggregated ST_Union on the 4326 geography data. 

Anyone point out what I am doing wrong or give me a pointer to what I can do to achieve the aggregated union? 
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