[OSM-talk] Nested areas

Jon Burgess jburgess777 at googlemail.com
Thu Jun 5 20:14:02 BST 2008


On Thu, 2008-06-05 at 19:49 +0200, spaetz wrote:
> > > osmarender rules pay attention to the layer tag even when dealing with
> > > areas. In this case the river is on layer=-1, and the industrial area
> > > has no layer tag (so defaults to 0). osmarender is rendering all -1
> > > objects first, then moves on to the layer 0 objects.
> > 
> > This seems wrong to me.  An easy fix would be to subtract a number (e.g. 
> > 10) from the layer value of areas so they always get rendered under 
> > non-area objects.  Maybe I'll look at doing this when I don't have a 
> > hundred and one other things to do. :)  I suspect there's no easy way of 
> > doing the surface-area calculation to keep small areas on top though.
> 
> Why should it work differently? If I want a tunnel under a forest, a
> layer=-1 *should* draw the tunnel under the forest. Why do you think
> it's doing something wrongly? 

If the tunnel becomes invisible because the forest is drawn on top then
that does not look good. If however the tunnel was drawn over the top
with an appropriate style then that may be more useful. Mapnik would
render the tunnel on top of the area but using a dashed style.


> tagging a river with layer=-1 seems wrong to me on the other hand.

Buildings are often constructed over the top of rivers. Whether the
building is +1 or the river is -1 surely just depends on where you take
your ground reference. If the river really flows through the building
then I guess the layer tag is not the right answer.

A similar issue must occur frequently with railway lines through train
stations.

	Jon






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