[postgis-users] Polygons: the unstable foundation ofspatialmodeling

[Chris Faulkner]

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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