[OSM-talk] Suggestion: fallback tag

[Steve Bennett]

On Fri, Dec 18, 2009 at 12:03 AM, Frederik Ramm <frederik at remote.org> wrote:

> Hi,
>
>
> Steve Bennett wrote:
>
>> Solution: tag it like this:
>> landuse=reserve
>> fallback:leisure=park
>>
>
> This makes only sense if there are certain landuse=reserve areas that you
> want to fall back to leisure=park and other landuse=reserve areas that are
> more like a natural=grass. And this would then mean that landuse=reserve is
> somehow underspecified.
>
> If you have a certain fallback hierarchy that says "dear renderers of the
> world, if you encounter something tagged landuse=reserve and you don't know
> what to do, then treat it as leisure=park", then it makes more sense to
> create this hierarchy externally and feed it to the renderers, instead of
> putting bits and pieces of it all over the database!
>

Yes...but I think it's a fair statement that centralisation of tag semantics
is not working very well, and many people bypass the process altogether.

So, yes, in a perfect world, we would simply define these fallbacks
centrally. But in the OSM world, it would be useful to do them case by case.
One benefit is no one needs to argue over them. You want to tag your
fallback as landuse=nature_reserve? Go ahead.

There are some technical problems, too. Mapnik, for example, renders by
> going through the rules one by one, fetching the matching objects, and
> rendering them.
>

Interesting. Probably solvable.

One way that comes to mind is to generate/store a list of supported tags,
and make "fetching the matching objects" include fetching objects that have
a fallback tag corresponding to the current rule, but for which the main tag
is not supported. You know...there are ways. :)

Steve
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/talk/attachments/20091218/aaa2b04a/attachment.html>