[OSM-talk] boundaries vs. is_in vs. relations

[Lester Caine]

Robin Paulson wrote:
> one of the tag proposals i've been working on recently has got me
> thinking about tagging composite items, and the best way to do it.
> 
> in the case of a large site, say a hospital, university or coalmine,
> is there any consensus on how to collectively tag all the buildings,
> sports fields, car parks, etc. so that they are marked individually as
> whatever they are, but also identified as being part of a greater
> whole?
> 
> for instance, say i ask for all the car parks near me, it would be
> good for my routing software to tell me that there's one nearby, and
> for it to implicitly know it's within the boundary of the hospital.
> 
> is this best done with relations, or is that over-complicating things.
> can any part of osm work out all the areas a given point lies within?
> this is subtly different to the problem of administrative boundaries
> we were talking about recently as we generally can get good
> information on the boundaries of entities like hospitals, etc.

is_in being populated and stored is always going to be quicker to search than 
trying to process a multitude of complex boundaries to find out if a node is 
inside or outside the enclosed area? More so on larger boundaries, but non the 
less a university campus or hospital with multiple buildings, car parks and 
other points of interest will return a simple list via correct is_in 
relations, while searching for nodes within boundaries my not actually give 
the right results were nodes are within the boundary area, but not actually 
defined as part of the whole.

That is not to say that a set of is_in relations can't be generated from the 
graphics. Just that there may need to be a means of editing the results. Which 
is why I'm convinced that correctly managed is_in hierarchy is essential long 
term.

-- 
Lester Caine - G8HFL
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