[postgis-users] Snapped polygon

[nguyen liem]

Hi Fabio,

I had a small test with your data. I split your data to 2 tables: blocks(big
polygon) and building(small polygon) table.
Please see the attached file for more details.

The environment is:
"PostgreSQL 8.3.5 on x86_64-unknown-linux-gnu, compiled by GCC gcc (GCC)
4.1.2 20070115 (prerelease) (SUSE Linux)"

I have some green comments bellow:

Cheers.

On Tue, Jul 20, 2010 at 9:59 PM, Fabio Renzo Panettieri <
fpanettieri at xoomcode.com> wrote:

> Hi.
> I'm, having a problem with some geometries.
> I have buildings and blocks represented by polygons, and I have to check
> if buildings are contained in blocks.
>
> The problem is that buildings that are snapped to blocks sides, aren't
> considered inside the block.
>
> These are the validations that I've used so far.
>
> ST_Covers(block, building) => false

    ST_Covers(geometry geomA, geometry geomB); Returns 1 (TRUE) if no point
in Geometry B is outside Geometry A


> ST_Contains(block, building) => false
>
    ST_Contains(geometry geomA, geometry geomB); Returns true if and only if
no points of B lie in the exterior of A, and at least one point of the
interior of B lies in the interior of A.


> ST_Coveredby(building, block) => false
>
    ST_CoveredBy(geometry geomA, geometry geomB); Returns 1 (TRUE) if no
point in Geometry/Geography A is outside Geometry/Geography B


> ST_Within(building, block) => false
>
    ST_Within(geometry A, geometry B); Returns true if the geometry A is
completely inside geometry B

>
>
> The following validations are the ones that confuses me.
>
> ST_Covers(ST_intersection(block, building), building) => true
>
    => false - plz see the explanation in the attached file

> ST_Intersection(block, building) = building => true
>
    =>  true: because "=" operator is a little more naive, it only tests
whether the bounding boxes of two geometries are the same

> ST_isEmpty(ST_Difference(building, block)) => true
> => false - plz see the explanation in the attached file



> For what I've read, here
>
> http://lin-ear-th-inking.blogspot.com/2007/06/subtleties-of-ogc-covers-spatial.html
> "Geometry A covers Geometry B if no points of B lie in the exterior of
> A"
> Covers should return true.
>
> The environment is:
> SELECT version(); => "PostgreSQL 8.4.4 on x86_64-pc-linux-gnu, compiled
> by GCC gcc-4.3.real (Debian 4.3.2-1.1) 4.3.2, 64-bit"
>
> SELECT postgis_version(); => "1.5 USE_GEOS=1 USE_PROJ=1 USE_STATS=1"
>
> I've attached two geometries that you can use to check if I'm doing
> something wrong.
>
> Thanks for your time.
> --
> Fabio R. Panettieri
> Software Architect
> http://www.xoomcode.com
>
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>
>


-- 
*Liem Nguyen (DBA)

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