[postgis-users] Polygons: the unstable foundation ofspatialmodeling

Bart van den Eijnden bartvde at xs4all.nl
Fri Mar 26 11:22:16 PST 2004

Hi Chris,

I will try to get in contact with the main author to deliver him the 
comments expressed on this list.

Best regards,

On Fri, 26 Mar 2004 12:04:29 -0000, Chris Faulkner <chrisf at oramap.com> 

> Has anyone been in touch with the authors to ask them to review the
> document ? What happened to peer review in academia ?
> Chris
>> -----Original Message-----
>> From: postgis-users-bounces at postgis.refractions.net
>> [mailto:postgis-users-bounces at postgis.refractions.net] On
>> Behalf Of Martin Davis
>> Sent: 25 March 2004 18:33
>> To: PostGIS Users Discussion
>> Subject: RE: [postgis-users] Polygons: the unstable
>> foundation ofspatialmodeling
>> As Dave Blasby pointed out, PostGIS now uses the GEOS engine
>> (which is a port of the JTS Topology Suite).  JTS/GEOS
>> provides a complete, rigorous implementation of the OGC SFS
>> specification, including of course polygons.  It provides a
>> full validation function for polygons which can not only
>> determine topological validity but returns information on the
>> nature and location of validation failures.  The appeal of
>> the OGC SFS polygon definition is that it is quite well
>> specified and is general enough to be useful  for spatial modelling.
>> As for the paper, it has a good point about there being
>> rather too many different definitions of polygons in the
>> world of GIS.  It would certainly be nice to standardize the
>> definition of polygons across more systems.  It also would be
>> nice if more systems provided rigorous definitions of the
>> polygon topology they support, and equally importantly the
>> precise semantics of the operations they implement.
>> The call for supporting tolerance values is a bit more
>> contentious, in my view.  Allowing tolerance values
>> complicates the semantics and implementation quite a bit.
>> Moreover, I have yet to see a specification of the semantics
>> of tolerance values (for instance, how do they affect the
>> results of spatial overlay operations?).  I suspect that
>> adding such a degree of looseness into a spatial system will
>> result in worse data quality, not better.  The world is
>> pretty used to dealing with fully noded, precise data, so I'm
>> not convinced there is a huge benefit to relaxing the specifications.
>> Martin Davis, Senior Technical Architect
>> Vivid Solutions Inc.
>> Suite #1A-2328 Government Street Victoria, B.C. V8T 5G5
>> Phone: (250) 385 6040 - Local 308 Fax: (250) 385 6046
>> > -----Original Message-----
>> > From: bartvde at xs4all.nl [mailto:bartvde at xs4all.nl]
>> > Sent: March 25, 2004 2:30 AM
>> > To: PostGis Users Mailinglist
>> > Subject: [postgis-users] Polygons: the unstable foundation of
>> > spatialmodeling
>> >
>> >
>> > Hi list,
>> >
>> > the following article dating from October 2003 deals with a
>> > comparison of polygon definitions (and especially the
>> > boundary between valid and
>> > invalid) in several Spatial DBMS's, and also PostGIS 0.6.2.
>> >
>> http://www.vz.geodan.nl/users/bart/17-VanOosterom_Polygons.pdf
>> Any comments on this? I can imagine a lot has changed since
>> version 0.6.2 regarding this subject?
>> Best regards,
>> Bart
>> Summary:
>> --------
>> Spatial models are often based on polygons both in 2D and 3D.
>> Many Geo-ICT products support spatial data types, such as the
>> polygon, based on the OpenGIS 'Simple Features
>> Specification'. OpenGIS and ISO have an agreement to
>> harmonize their specifications and standards. In this paper
>> we discuss the relevant aspects related to polygons in these
>> standards and compare several implementations. A quite
>> exhaustive set of test polygons (with
>> holes) has been developed. The test results reveal
>> significant differences in the implementations, which causes
>> interoperability problems. Part of these differences can be
>> explained by different interpretations
>> (definitions) of the OpenGIS and ISO standards (do not have
>> an equal polygon definition). Another part of these
>> differences is due to typical implementation issues, such as
>> alternative methods for handling tolerances. Based on these
>> experiences we propose an unambiguous definition for
>> polygons, which makes polygons again the stable foundation it
>> is supposed to be in spatial modelling and analysis.
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