[mkgmap-dev] Commit: r1555: Ignore multipolygons with fewer than 2 resolved elements.

Fri Feb 5 15:41:19 GMT 2010

>
>     The definition of the multipolygon says that the tags from the
>     multipolygon are used for the outer polygons ("Tags describing the
>     multipolygon (e.g. landuse=forest) should go on the relation. The outer
>     way(s) should be left untagged, unless they describe something in their
>     own right."). If these multipolygons are dropped the tag information of
>     the multipolygon is lost.
>
>
> there are 2 common forms of tags on myltipolygon. both are supported in
> Mapnik and Osmarender.
> Ideally mkgmap should work the same way
>
> 1) tag is on the relation and will overwrite any tag with same key on
> the outer way. A different tag can still be applied to the ways.
> 2) tag is on the outer way, again additional tags can be set
>
> in both cases I think the way must be duplicated

All ways are duplicated in mkgmap.
When single ways are joined to polygons the new polygons are tagged with 
a tag merge of the single ways. The merge uses the "first way wins" 
algorithm (which produces a bit random results, but I don't know a 
better algorithm).

In the end there is a decision which tags are removed:
- If the original OSM way is a closed polygon then all tags are removed 
from the original OSM way.
- Otherwise only tags for polygons are removed (at the moment these are 
"boundary", "natural", "landuse", "land_area", "building", "waterway"). 
This keeps all line tags (like highway) so the original still make use 
of their line-tagging.

If the multipolygon contains any of the polygon tags, then all tags of 
the multipolygon are copied to the resulting polygons (additionally to 
the merge of tags)

WanMil