[postgis-users] Precision problem

[Paragon Corporation]

Maxine,

No this is not really a precision problem.  The ST_AsText finds it within
because yes it truncates thus making the point full within the polygon.

The fact that ST_Dwithin when given 0 returns true and ST_Within returns
false, is because you misunderstand the subtleties of ST_Within.  Most
people do so don't feel bad.

For a geometry to be considered ST_Within, the geometry can not sit wholly
on the boundary of the other.  Its an often misunderstood thing.  You really
want to use ST_Covers or ST_CoveredBy.

http://postgis.refractions.net/documentation/manual-1.4.0SVN/ST_CoveredBy.ht
ml

Leo

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Maxime
van Noppen
Sent: Saturday, July 18, 2009 12:50 PM
To: PostGIS Users Discussion
Subject: [postgis-users] Precision problem

Hello,

I face what I think is a precision problem. I have a point and a polygon and
I test whether the point is within the polygon or not. When both geometries
are in WKB, ST_Within returns false whereas when they are in WKT it returns
true.

WKB :
--------------------------------------------------------------------------
# SELECT ST_Within('010100000013898E0BA7E833C1FF843EA52C212C41', '
0103000000010000000400000000000000A09134C10000000088212A4100000000A0E832C100
00000000812A41000000005E0634C1CDCCCCCC79512C4100000000A09134C10000000088212A
41');
 st_within
-----------
 f
(1 row)
--------------------------------------------------------------------------


WKT :
--------------------------------------------------------------------------
# SELECT ST_Within('POINT(-1304743.04514367 921750.322742611)',
'POLYGON((-1348000 856260,-1239200 868480,-1312350 927932.9,-1348000
856260))');  st_within
-----------
 t
(1 row)
--------------------------------------------------------------------------


The point is on the border of the polygon and I found that using DWithin
with 0 as tolerance makes both test work :

WKB :
--------------------------------------------------------------------------
# SELECT ST_DWithin('010100000013898E0BA7E833C1FF843EA52C212C41',
'0103000000010000000400000000000000A09134C10000000088212A4100000000A0E832C10
000000000812A41000000005E0634C1CDCCCCCC79512C4100000000A09134C10000000088212
A41',
0);
 st_dwithin
------------
 t
(1 row)
--------------------------------------------------------------------------

For now I'll use this solution but I find it quite hacky. Is this a normal
precision-related behaviour ?


Thanks,

--
yabo
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