[mkgmap-dev] Multipolygones and tags in outer line

Tue Feb 22 18:38:44 GMT 2011

>
> Hi,
>
> I found now several buildings/areas, which where constructed
> with multipolygons, but where only the inner polygons where
> ignored for rendering.
>
> As an example see at:
> http://www.openstreetmap.org/browse/relation/56430
>
> This is a building with tree inner polygons. As long as
> the outer-polygone
> http://www.openstreetmap.org/browse/way/10700238
> has tags, the whole building is rendered as one whole area.
> If I move the tags from the outer-polygone to the multipolygone
> (for example like done with
> http://www.openstreetmap.org/browse/relation/6187)
> it's rendered correct.
>
> Is this a mkgmap bug? Or is it possible to fix this with the
> style file?
>
>   Thanks,
>     Thorsten

Hi to all,

mkgmap tag handling of multipolygons is a compromise between the strict 
rule to tag the mp-relation only and the real OSM world.

Assume the following scenario:
relation: type=multipolygon
way 1: role=outer
way 2: role=outer
...
way 7: role=inner
way 8: role=inner
...

1st case: the mp contains more than the type=multipolygon tag.
The multipolygon (space between outer and inner ways) is tagged with the 
mp tags. The mp tags are removed from the original outer ways.

2nd case: the mp contains only type=multipolygon
The multiypolygon is tagged with the intersection of tags of the outer 
ways. These are removed from the original outer ways.

What does that mean for your mp 
http://www.openstreetmap.org/browse/relation/56430? In the original 
version the mp was tagged with
name=Germanisches Nationalmuseum, so the multipolygon contained the name 
tag only. The name tag was removed in the outer way.
So in the end the outer way was rendered without holes and without the 
name. The mp (with holes) was not rendered because there is no rule for 
a polygon with a name tag only.

In the current version 3 the mp (outer way with holes) is tagged with
building=yes
name=Germanisches Nationalmuseum
tourism=museum
wikipedia:de=Germanisches_Nationalmuseum.

These tags are removed from the outer way 10700238 so it is left with
source=yahoo
Probably there is no rule for the source tag so the "holeless" outer way 
is not rendered.

The tags of the inner ways are not changed.

WanMil