[Gvsig_english] Modelling 1:n and m:n relationships in GIS

[Squidjag]

Hi Ben,

I teach geospatial database design at what I think you guys call a
polytechnic and I also teach post grad uni students. I'm not a database
design expert but have had a reasonable amount of experience in the 'real
world' designing textual and geospatial databases.

I don't want to appear to be lecturing you and wanted to respond to your
question because I am passionate about the way geospatial professsionals are
taught database design. Your questions are typical, in my experience. My
personal view is that you (and many others) are confusing two concepts. One
is textual database design - e.g. cardinality between relations (1:m n:m
etc) - and the other is spatial analysis - topology.

Relational database design is driven by a desire to maintain data integrity.
Relational actual comes from relational algebra and not relationships as
taught in many MS Access courses! Referential integrity aims to reduce data
anomalies so that (heuristically speaking) database designs are in at least
3rd normal form (a balance between data quality and performance). Because
geospatial systems use RDBMSs these concepts are relevant to the textual
aspects of any geospatial system however, spatial concepts are not exposed
to the same problems because spatial entities are discrete. By this I mean
that if a spatial entity is stored in its own table then it is fully
normalised.

This is conceptually complex but I'm happy to work through this with you. If
it is more appropriate to take this off line I'm happy to do so however, the
best way to do discuss this is by way of an example. 'Address' is one good
example but if you are grappling with the design of something then I'm happy
to discuss this too.

Cheers

Colin


Benjamin Ducke wrote:
> 
> Dear all,
> 
> I keep getting into situations where mapping 1:n and m:n relationships
> in relational DBMS to GIS vector models becomes a problem.
> The toughest restrictions of course are the 1:1 relation between map
> features and attribute table records and the fact that GIS relates
> data by spatial overlay, not foreign fields. I realize that some GIS
> (like GRASS) are somewhat more flexible in that they can attach more
> than one attribute table to a layer, but I am really looking for more
> portable ways to deal with this.
> 
> - How do you deal with 1:n and m:n relations in GIS?
> 
> - What do you do if there are no spatial representations for the
> records on the "n" side? Can that be handled at all?
> 
> Apparently, it would be possible to model some of this using overlapping
> features in Shapefiles, but that would violate topology rules.
> 
> Any ideas are very welcome, indeed!
> 
> Thanks, everyone, 
> 
> Ben
> 
> -- 
> Benjamin Ducke
> Senior Applications Support and Development Officer
> 
> Oxford Archaeology
> Janus House
> Osney Mead
> OX2 0ES
> Oxford, U.K.
> 
> Tel.: ++44 (0)1865 263 800
> benjamin.ducke at oxfordarch.co.uk
> 
> 
> 
> 
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