[postgis-users] (no subject)

[Martin Davis]

Good point, Dave - that would produce the same symptoms.

Hmmm - this sort of thing sounds like it could be a real problem, if
there's extra junk precision which isn't visible textually, but causes
strange results in spatial operations.  Maybe one answer to this is to
allow specifying the desired precision when storing and operating on
PostGIS data?   This would have the added benefit of encouraging people
to store their data with no more than the actual precision contained in
the original observations.

SDE has this capability by default, since it forces you to define a
mapping to its internal integer storage.  Oracle sort of has this by
virtue of its tolerance (although AFAIK there's no way of enforcing
actual storage to a given precision).

Martin Davis, Senior Technical Architect
Vivid Solutions Inc.      www.vividsolutions.com
Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046


> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] On 
> Behalf Of dblasby at openplans.org
> Sent: July 18, 2005 10:36 AM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] (no subject)
> 
> 
> >This sounds like floating point comparisons failing because
> >> of numerical
> >> rounding issues.
> >
> >Highly unlikely - JTS/GEOS is very robust wrt predicates.
> 
> I think he means the 2 geometries arent actually identical - 
> the points maybe "10-7 off".  These differences would be 
> hidden by converting to WKT (since it doesnt show all 64bits 
> of precision).
> 
> Try something like this:
> 
> SELECT encode( asBinary(  geometry1 ) , 'hex'),
>        encode( asBinary(  geometry2 ) , 'hex')
> FROM ... ;
> 
> And see if the two are actually equal.
> 
> dave
> 
> 
> 
> 
> 
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