[postgis-devel] Two geometries intersects but intersection is empty

[Pierre Racine]

Here is the case:

SELECT st_intersects(geomA, geomB), astext(st_intersection(geomA, geomB))
FROM (SELECT
      st_geomfromtext('POINT(-75.5516150170498 49.2824791927083)',4326) AS geomA,
      geomfromewkb(E'\\001\\003\\000\\000 \\346\\020\\000\\000\\001\\000\\000\\000\\005\\000\\000\\000\\034\\2373\\316i\\343R\\300q\\217\\016\\232\'\\244H@\\347uFeO\\343R\\300g\\322\\255{5\\244H at l\\324vtH\\343R\\300\\374\\177\\323\\251\\000\\244H@\\241\\375c\\335b\\343R\\300\\007=4\\310\\362\\243H@\\034\\2373\\316i\\343R\\300q\\217\\016\\232\'\\244H@') AS geomB
) foo

Sorry I can't provide it as wkt as the wkt version return the expected result (and the wkb version does not (before I read Paul's last FAQ)).

Pierre

>-----Original Message-----
>From: postgis-devel-bounces at postgis.refractions.net [mailto:postgis-devel-
>bounces at postgis.refractions.net] On Behalf Of Martin Davis
>Sent: 3 juin 2010 12:15
>To: PostGIS Development Discussion
>Subject: Re: [postgis-devel] Two geometries intersects but intersection is empty
>
>Can you post the case?
>
>This can happen because the intersects predicate test is more accurate
>than the intersection computation.  (Technically speaking, it's possible
>to detect that two segments intersect, but be unable to compute an
>accurate point that represents the intersection).
>
>Pierre Racine wrote:
>> Hi,
>>
>> Is it possible that st_intersects returns true on two geometries but their st_intersection returns
>"GEOMETRYCOLLECTION EMPTY"?
>>
>> I have a nice case here...
>>
>> Pierre
>>
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>>
>>
>
>--
>Martin Davis
>Senior Technical Architect
>Refractions Research, Inc.
>(250) 383-3022
>
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