[postgis-users] Intersection doesn't work on some data

David Mitchell david.mitchell at telogis.com
Tue Mar 23 21:32:30 PST 2004

I have got two MULTIPOLYGON features, each containing a single POLYGON. 
(attached). When I ask postgis to find the intersection of these, it 
gives the wrong result. It should give:

MULTILINESTRING ((-121.331929 41.18388, -121.34693 41.18388), 
(-121.34693 41.18388, -121.36293 41.18388), (-121.36293 41.18388, 
-121.36544 41.18388), (-121.36544 41.18388, -121.37179 41.18388), 
(-121.37179 41.18388, -121.37608 41.18401), (-121.37608
41.184019, -121.44593 41.18384))

But in fact gives:

MULTILINESTRING((-121.33193 41.18388,-121.34693 41.18388),(-121.34693 
41.18388,-121.36293 41.18388),(-121.36293 41.18388,-121.36545 
41.18388),(-121.36545 41.18388,-121.37179 41.18388),(-121.37179 
41.18388,-121.37608 41.18402),(-121.37608 41.18402,1.7e-308 

Note the final line string has very tiny numbers for its second 
coordinate. I have written a program to run these geometries directly 
through geos and the output is correct, so it must be a postgis problem.

Any help would be appreciated.

David Mitchell
Software Engineer

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