[OSM-talk] Suggestion: fallback tag

Frederik Ramm frederik at remote.org
Thu Dec 17 13:03:38 GMT 2009


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!

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.

Your thinking is obviously the other way round: Take each object in turn 
and decide how to render it.

With your approach it is quite easy to have an "else" case that renders 
things differently if none of the other rules apply. With Mapnik that is 
not so easy.

Bye
Frederik




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