[OSM-talk] Mapnik now not supporting layer=1 on golfcourses

[Jon Burgess]

On Mon, 2007-05-14 at 19:26 +0100, Jon Burgess wrote:
> On Mon, 2007-05-14 at 17:24 +0100, Andy Robinson wrote:
> > We appear to have lost some golf course support in Mapnik in the most
> > recent tiles rendered. Previously (as per osmarender still) golf
> > course areas within parkland where the course has a layer=1 set were
> > rendered one on top of the other for all to see, the current Mapnik
> > tiles are no longer showing these golf courses. Example being:
> > 
> >  
> > 
> > http://www.openstreetmap.org/index.html?lat=52.5740&lon=-1.8576&zoom=13
> > 
> 
> It seems that the current code only uses the layer= property to
> determine the drawing order (z_order) on lines and not on polygon areas
> like the golf course & park.
> 
> This is the case both for the old setup_z_order.sql script and the
> current osm2pgsql. If it rendered OK before in Mapnik then it was
> probably by luck.
> 
> Adding this feature needs changes to both define a z_order for polygons
> and also to use this to sort the data in the osm.xml style.
> 
> 	Jon
> 

I have just implemented the necessary changes to osm2pgsql and osm.xml
to add a z_order for polygons. I have updated the render database to set
the z_order values on the polygon table and use this new osm.xml file.

The 4 tiles which make up the park at level 13 have been re-rendered and
the golf course now shows up on the link above.

I there there may be an outstanding issue with the layer= support
however. I think the mapnik code rendering process renders everything in
the order: { polygon, line, point } regardless of layer=. This means it
is possible to place one area over another area, but is it not possible
to place an area over the top of a road. I think I've seen people do
this for tunnels and I don't think this will work with mapnik.

	Jon