[mkgmap-dev] Proposal: Multipolygon handling and line based multipolygons

Tue Sep 14 21:41:12 BST 2010

>
> That sounds like a good proposal. Another, possibly
> conceptually-related problem with the mp code is as follows:
> Consider a multipolygon with one outer way having the tags
> barrier=fence
> landuse=farm
> and one inner way having the tag
> natural=water
>
> The mp splits this into two outer polygons with landuse=farm and
> barrier=fence and one inner polygon with natural=water.  The problem
> is that now the fence no longer follows the (outer) ways it was
> originally designed to follow, but also follows the synthetic
> orthogonal ways created by the mp code.  If the style rules process
> both landuse=farm (in the polygon ruleset) and barrier=fence (in the
> lines ruleset) then you get an unwanted, synthetic fence being created
> where none really exists.

Yes, that's a nice side effect of the patch I posted.
Although strictly speaking the tagging of your example is wrong. If a 
member way should have non polygon related tags the multipolygon must be 
tagged instead of the members. So the mp should have landuse=farm and 
the outer way should be tagged with barrier=fence. This already works 
without the patch. But I know that lots of such taggings exist in the 
OSM data.

>
> Having the mp code add a new tag to synthetic ways created as part of
> multipolygon splitting would be one way of tackling this problem (by
> subsequent re-writing of the style rules).  The other would be to
> avoid adding barrier=fence (or other such linear tags) to synthetic
> ways created as part of polygon splitting.

Good news: you don't need to rewrite your style rules. This is done 
automatically by the patch.

Could you please test the patch and give some feedback?

Thanks!
WanMil