[postgis-users] ST_Insersection problem

[Andrea Peri]

>Hello Group,
>We are using "POSTGIS="1.5.1" GEOS="3.2.2-CAPI-1.6.2" PROJ="Rel. 4.7.1, 23 September 2009" LIBXML="2.7.6" USE_STATS" and having a problem with ST_Intersection().
>Given the following query
>select count(st_intersection(tbl_a.the_geom,tbl_b.the_geom)) from tbl_a, tbl_b;
>will result in this error message
>NOTICE:  TopologyException: found non-noded intersection between LINESTRING (62723.7 426635, 62722.5 426634) and LINESTRING (62723.7 426635, 62726.2 426632) at 62723.7 426635
>ERROR: GEOS Intersection() threw an error!
>SQL status:XX000
>
>After the NOTICE message we expect the query to continue but it doesn't.  Both tables have all records st_isvalid='t'.
>
>Does anyone know a solution or workaround for this problem?
>
>Thanks,

Hi,
I give my 2ct. :)
The problemyou report is not really a problem of postgis, but instead
is a problem of the finite arithmetic use by the pcs and of the
methematical algorithm used .

I have every time the error you report.
Please notice I run about 10-20 million records of geometry and some
geometry has also 1 million vertex :)

Is a nightmare.
But this is the beautiful and the hell of the arithmetic finite .

To resolve this you must detect at every step what happened and filter
they using specific "where" clause or CASE operators.

Is pretty easy,
you will see often it return a Collection, and you get only the lines
or the polys from that, again
you can filter out the empty geometry.
After this long path you will have a good procedure to clean all the
problem of a real intersection on a arithmetic finite machine.

Regards,
Andrea Peri.



-- 
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Andrea Peri
. . . . . . . . .
qwerty àèìòù
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