[OSM-dev] dropping segments

Nigel Magnay nigel.magnay at gmail.com
Sat Jul 28 18:13:20 BST 2007


>
>
> We have the concept of "layers" already, for denoting something built at
> another height level (i.e. a bridge over a stream would be layer=1, the
> stream, by default, layer=0).


Yes, I'm thinking of layers like photoshop, rather than layers as a proxy
for height (or render ordering) - but it seems to me that both are valuable.


> Using this for areas with holes would be am ugly hack, because the
> clearing in the woods is not on another height level as the woods. If
> you have a donut-shaped park and a building in the middle, and you
> simply make the park a circular area and count on the building being
> "painted" above, you're creating a distorted model of reality. For
> example:
>
> "Paint a map without buildings" - would show a circular park and not a
> donut-shaped park.
>
> "Calculate the total parkland area" - would include the area of the
> donut hole.
>
> "Tell me which objects are at lat=..., lon=..." - would return building
> AND park for a point inside the building.
>
> As for flooding information and so on, you're talking about logical
> layers here not the existing ones. (But logical layers, too, are
> unsuitable for solving the donut-shaped-park problem.)
>

The more I think of it, the more I wonder if it's really neccessary to have
areas with holes rather than just 1 area made up of 2 child areas, each of
which would be the 2 halves of the donut. A renderer could understand that
common edges could be removed relatively easily. The only other alternaltive
that I can think of would be to have parent areas that contain logical
operations (e.g. add, subtract, union) - this sounds much more complex for a
renderer to cope with and makes the calculations that you describe above
much more complex as well...
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openstreetmap.org/pipermail/dev/attachments/20070728/f64c3741/attachment.html>


More information about the dev mailing list