[Tagging] tagging the multipolygon model (was landuse and military)

sly (sylvain letuffe) liste at letuffe.org
Wed Oct 14 21:03:25 BST 2009

(Sorry to break the thread, I'm just subscribed and wanted to give a thought 
on the "not overlapping landuse, and multipolygon model advantages and 
drawbacks" )

Emilie said :
> Landuse should be exclusive by definition. As
> someone pointed out before in a separate message, this is trying to be a
> work around the fact that to some extent landuse is broken in some cases.
> We would like to avoid having two super imposed landuse as much as possible.

That is to say : using more multipolygon relation to model "holes" or, in this 
case, landuse included in landuse.

I've been a great fan for a few time of those (advanced) multipolygon for 
solving the hole case. (Hole case which was, remember, the first solved 
problem by multipolygon relation)

At that time (I think ?) it was created because holes where nothing, but as 
more and more landuse/natural/... tags are created, we could imagine an osm 
database where not even a single area is "nothing".

In the holes continuity, it as been proposed that an area representing 
something inside another area would still be part of a multipolygon relation 
but with it's own tags.

this sounds great, requesting the surface of the big area is strait forward, 
rendering become easy (no "which one is over which one"), such a puzzle makes 
it easy to find problems, etc.

But, this becomes harder and harder for the mapper. A big forest containing 
thousands lakes ? a landuse=residential containing park, cimetary, etc. ?

I fear not every one is gone a make the effort.

Emilie :
>Anyway some of the comments you are making are making sense but it is just
>relying on the renderer to get it right.
+ somewhere else on talk or dev or I don't remember :
"we don't want to create priority hierarchies among landuse/natural"

And after all, is it at all needed ?

In the "area inside area case" (not the partially overlapping areas case)
We can resonably imagine that if a mapper has added such an area inside 
another, then either :
- they can be both (a military area and a forest)
- they can't be both (a lake and a forest)

Maybe if we just define/explain/(do our best not to create same key 
incompatibility, juste like this boundary=military propose to replace the 
ambiguous landuse=military for some cases) 
Same for natural, then what we've left ?

A lake inside a forest, is not a forest
A cimetary inside a residential is not a residential

Let's put the burden on computer programs and free mappers :
why not, instead of the layer=* "tag for the rendering" patch, consider that 
any area inside another should be automatically not considered as part of the 
overlapping area (thus automatically constructing "holes") ?

Sylvain Letuffe liste at letuffe.org
qui suis-je : http://slyserv.dyndns.org

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