[postgis-users] Polygons: the unstable foundation of spatialmodeling

[Martin Davis]

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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