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: [email protected] on behalf of Andreas
Albarello
Sent: Mon 12/29/2008 1:06 PM
To: [email protected]
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
Territorium Online srl/GmbH
Via Buozzi Str. 12
I 39100 Bolzano/Bozen
Phone: +39 0471 068611
Fax: +39 0471 068619
email: [email protected]
web: http://www.territoriumonline.com
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