[OSM-dev] Topologicial correctness in OSM

80n 80n80n at gmail.com
Wed Jan 17 22:25:36 GMT 2007

On 1/17/07, Nicola Ranaldo <ranaldo at unina.it> wrote:
> On Wednesday 17 January 2007 20:33, Nick Black wrote:
> > I know that shapefiles store topology, but I dont know much about the
> > details of it.
> >
> > http://www.esri.com/news/arcuser/0401/topo.html
> Pleae check
> http://en.wikipedia.org/wiki/Shapefile#Topology_and_shapefiles,
> it seems some formats does not support topology.
> [...]
> > I'm loathed to start another format war, but OSM's model has a lot
> > going for it - and has considerably more documentation than Mapinfo's
> > format!
> You are right, howewer we lack:
> 1) hierarchycal ways
> 2) advanced support for areas
> As the first may be a long term goal, the second is necessary to elevate
> osm
> to a full featured model. Actually we cannot define areas with inner holes
> while the other formats do.

I think maybe you can.  Because ways can be non-contiguous, you can create
an area that comprises an outer circle and an inner circle.  If the outer
circle is clockwise and then inner anti-clockwise then using something like
the SVG non-zero rule, you get a definition of areas with holes.

If you use the SVG even-odd rule, which is the other way of doing it, then
you don't even need to ensure that the inner circle is anti-clockwise.

The osmarender rule files currently implement the non-zero rule.  See:
http://dev.openstreetmap.org/~ojw/Tiles/tile.php/16/32674/21824.png for an
example of some lakes containing islands.


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