[postgis-users] (no subject)

[Mark Fenbers]

I had a similar problem when the values were different at the 13th 
decimal place.  This was way too much precision for me, so I was able to 
get around the problem by using the SnapToGrid(0.0000001) function on my 
data before using the Difference() function, etc.
Mark

Martin Davis wrote:

>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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>>    
>>
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>
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