[postgis-users] bug in st_difference() ?

[Birgit Laggner]

  Hi Ralf,

sorry, I don't understand. I don't want these two points in the result 
polygon of the difference, because they are located in a region that 
should be erased by the difference. I don't need them to become more 
precise. Or am I misunderstanding what you are saying?

Regards,

Birgit.

On 09.03.2011 16:04, Ralf Suhr wrote:
> Hi Birgit,
>
> this is only a topoligie Problem. Your secound polygon is computed outside
> PostGIS and numeric precision are lost by import. In this case ST_Difference()
> and other PostGIS function can have results with very small errors.
>
> You can clean the second polygon by move all points to the borders from first.
> 3597691.18360053 5323329.04452537 becomes
> 3597691.183600531425327 5323329.044525371864438 and so on.
>
> Gr
> Ralf
>
> Am Mittwoch 09 März 2011, 15:05:43 schrieb Birgit Laggner:
>>    Dear list,
>>
>> when I am using ST_Difference(), I often have the problem, that the
>> resulting geometries contain vertexes lying outside of the supposed
>> geometry extent. My impression is that the function tries to preserve
>> vertexes of the input geometries and does not check properly, whether
>> the point lies inside the extent of the new geometry or not.
>>
>> Here an example:
>>
>> select st_astext(st_difference(a_geom, b_geom)) from
>> st_geomfromtext('POLYGON((3580359 5328360,3588529 5330935,3597943
>> 5323120,3606024 5322587,3612951 5315217,3608155 5306691,3603804
>> 5305093,3596166 5306159,3591105 5310244,3584799 5310688,3576540
>> 5317437,3580359 5328360))',31467) as a_geom,
>> st_geomfromtext('POLYGON((3597691.18360053 5323329.04452537,3597943
>> 5323120,3606024 5322587,3611561.81746298 5316695.02444028,3611480
>> 5316582,3605931 5322425,3597573 5323069,3597691.18360053
>> 5323329.04452537))',31467) as b_geom;
>>
>> results in:
>> "POLYGON((3580359 5328360,3588529 5330935,3597943
>> 5323120,3597691.18360053 5323329.04452537,3597573 5323069,3605931
>> 5322425,3611480 5316582,3611561.81746298 5316695.02444028,3606024
>> 5322587,3612951 5315217,3608155 5306691,3603804 5305093,3596166
>> 5306159,3591105 5310244,3584799 5310688,3576540 5317437,3580359 5328360))"
>>
>> Vertexes 4 (3597691.18360053 5323329.04452537) and 8 (3611561.81746298
>> 5316695.02444028) are out of the supposed geometry extent.
>>
>> I am using PostGIS 1.5.2 with GEOS 3.2.2 on a 8.4.5 PostgreSQL-DB on
>> 64-bit SUSE Linux.
>>
>> Maybe, there is a simple explanation, why this behavior might be wanted
>> - if not, I would like to complain this as a bug (or at least suggest an
>> improvement of the function in future versions)...
>>
>> Regards,
>>
>> Birgit.
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>