[postgis-users] Strange intersection behavior

[Andreas Albarello]

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: andreas.albarello at territoriumonline.com
web:   http://www.territoriumonline.com
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