[OSM-dev] Relations, landuse, and gml2osm mess

Stefan Keller sfkeller at gmail.com
Sun Mar 30 17:04:04 BST 2008


Ivan,

Frederic asked:
>> Out of interest about the whole GML stuff - if you apply the XSLT from
>> here [...] to your OSM file, does the result even remotely look like the
GML
>> that you originally put in?

And you answered:
> I won't even bother running it - it is missing things like linearRings,
> polygons, multiPoligons on the GML side, and relations on the OSM side. It

This XSLT is simply a result of a one-shot-need and never claimed to be a
lossless conversion.

>> I have heard people say that GML supports topology,
> Ha!

I think your attitude to put OSM on one side and GIS in the other won't help
to put OSM any further in the long termn.

If you make judgements over a technology which is about 20 years older than
OSM without even trying to find out it's working you will never find out how
to make it better.

GML uses XML and you can use XLink to encode relationships - even between
layers. And "layers" can be put in one file. GML purposely encodes polygons
as separate object to support object oriented handling. This does not
exclude sharing attributes if you add this as an additional constraint.

Handling of shared topology always was a matter of controversy. The only two
big (!) GIS systems which tried it that way I know of disapeared from the
market. That does'nt mean it's completely wrong what you're trying to do
with relations. Still I think your approach - with node-ways, "predefined"
keys and now relations - will become more and more complicated the more
computational geometry artefacts you try to cope with.

-- Stefan

 (BTW: its funny for me to defend GML as we actually invented a similar -
but of course little bit more clever and still easier - encoding in
Switzerland with Interlis 2 :->).


2008/3/30, Iván Sánchez Ortega <ivansanchez at escomposlinux.org>:
>
> El Domingo, 30 de Marzo de 2008, Frederik Ramm escribió:
> > > If I use a relation of ways (making up a ring) as the outer (or inner)
> > > hull of a multipolygon relation[2], will it work?
> >
> > Don't think it would currently, but it sure could be arranged. Some
> > editors (*cough*) still have problems with nested relations though.[*]
>
> My question was more related to the main renderers (mapnik and
> osmarender).
>
> > While it is true that this would introduce a kind of asymmetry, I
> > think it would be rather pragmatic as you'd seldom need multiple ways
> > on the "inner" ring.
>
> "Seldom" != "never", so it may be needed. Given tie, I'd show you some
> cases
> where it will be needed.
>
> > About the GML:
> >
> > Out of interest about the whole GML stuff - if you apply the XSLT from
> > here [...] to your OSM file, does the result even remotely look like the
> GML
> > that you originally put in?
>
> I won't even bother running it - it is missing things like linearRings,
> polygons, multiPoligons on the GML side, and relations on the OSM side. It
> won't look anything like it, provided that I'm tagging the whole relation
> and
> not the outer rings.
>
> > I have heard people say that GML supports topology,
>
> Ha!
>
> Given my experiences with GML, it supports topology in some alternate
> universe
> only.
>
> Even the implementation of the WFS service for the road network was borked
> - I
> was missing endpoints of lineStrings and had to hack their coordinates by
> the
> bounding box, not to mention how many
>
> > move the forest border, and the road moves as well? Until now I thought
> that
> > any OSM to GML conversion would make two distinct objects, one forest,
> one
> > road, and they'd lose their connection.
>
> They are in two separate layers, which are stored in two separate GML
> files
> (or WFS-T live connections). I don't think most GIS software would allow
> sharing topology between layers.
>
> ( See? That's one thing I really like about OSM: everything is in the same
> layer, everything is topologically connected )
>
>
> --
> ----------------------------------
> Iván Sánchez Ortega <ivansanchez at escomposlinux.org>
>
> Palabras Textuales #307: "Este monitor no me deja imprimir!"
> (Agencia, Ejecutiva de Cuentas, acerca de los mensajes de error de la
> impresora que aparecí­an en la pantalla del monitor)
>
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>
>
>
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