[OSM-talk] area_with_holes as alternative to multipolygon relation

[Robert Vollmert]

On Mar 3, 2008, at 22:45, Sven Grüner wrote:
> Robert Vollmert schrieb:
>> I think not. I'm pretty sure for osmarender, but may not have had
>> enough patience to test out the various combinations with mapnik.
>

I was wrong about osmarender: What I remembered is actually code from  
mapnik (or rather, osm2pgsql):

static int pgsql_out_way
[...]
     const char *multipolygon = getItem(tags, "multipolygon");

     // If the way has been part of a multipolygon then skip
     if (multipolygon)
         return 0;

The "multipolygon" tag is set in pgsql_out_relation for ways with  
role=inner. Furthermore, pgsql_out_relation appears to aggregate tags  
of all ways involved, so you should end up with  
landuse=forest,natural=water on the multipolygon. I wonder what  
happens when both have differing natural= tags?

Or am I reading this all wrong? I haven't really been able to test  
because so far, my mapnik install doesn't appear to render any areas...

> Nah, that island is just natural=water and role=inner in a  
> multipolygon.
> The green comes from the park encasing the lake. But it only works  
> when
> drawn counter-clockwise.
>
> http://www.informationfreeway.org/?lat=52.249&lon=10.52656&zoom=17

And it also works in mapnik! Maybe you're just lucky with the drawing  
order? :)

I'll keep experimenting.

Cheers
Robert