[OSM-dev] Coastlines and water - contact and technical

Ben Supnik bsupnik at xsquawkbox.net
Sun Jul 19 21:04:09 BST 2009


Hi Jon,

I started to summarize this on the wiki and realized something...

> documented. The algorithm osm2pgsql uses is to gather together all the
> tags which are related to the polygon feature (e.g. landuse=, natural=
> etc) and then compare those against the outer ways. If the way and
> polygon match then it will exclude the way. If they do not match then
> the way will be included.

What is the precise definition of "match"?

> As above, the way needs to be exluded if it is untagged or is the same
> as the polygon. If it is different then it does need to be rendered.

The implication of this is: a way might be "its own area" with regarding 
to one set of tags and "part of a multipolygon" with another.

If I tag an outer boundary as "park" and it forms the outer boundary of 
a forest multipolygon that has holes, the park covers the area of the 
forest, plus the hole.

I think I need to understand the tag-matching rules to figure out the 
right thing to do.

cheers
Ben

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