[postgis-users] Strange intersection behavior

[Obe, Regina]

Andreas,

I think you have unearthed one of the cruel lessons of floating point math.

Best described in Isaac Kunen's blog.

http://blogs.msdn.com/isaac/archive/2008/08/07/the-imprecise-nature-of-geometry.aspx

So we see it is not unique to PostGIS.

Hope that helps,
Regina



-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net on behalf of Andreas Albarello
Sent: Mon 12/29/2008 1:06 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Strange intersection behavior
 
Hi list,

I'm at a loss as to what is happening here. I'm using PostgreSQL 8.3.5 
along with PostGIS 1.3.5 on Windows.

So this query works fine:

select astext(intersection(geomfromtext('LINESTRING(8.30234187721807 
46.6802395904018,6.3382169942078 46.3031276124166)'), 
geomfromtext('LINESTRING(7.32028160673789 
46.4916949088309,7.32028323240689 46.491581924505)')));

Now I'd expect this query to work too:

select 
astext(intersection(intersection(geomfromtext('LINESTRING(8.30234187721807 
46.6802395904018,6.3382169942078 46.3031276124166)'), 
geomfromtext('LINESTRING(7.32028160673789 
46.4916949088309,7.32028323240689 46.491581924505)')), 
geomfromtext('LINESTRING(7.32028160673789 
46.4916949088309,7.32028323240689 46.491581924505)')));

i.e., intersecting the intersection with the original line. Weirdly 
enough, I'm getting GEOMETRYCOLLECTION EMPTY. This has nothing to do 
with using geomfromtext, tried the same thing inside plpgsql and got the 
same result. Besides, geomfromtext ought to be deterministic anyways.

Same thing goes for this:

select 
intersects(line_interpolate_point(geomfromtext('LINESTRING(7.32028160673789 
46.4916949088309,7.32028323240689 46.491581924505)'), 0.1), 
geomfromtext('LINESTRING(7.32028160673789 
46.4916949088309,7.32028323240689 46.491581924505)'));

returns false.

Can anyone shed some light on this?

TIA

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Andreas Albarello
Analysis & SW Development

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