[OSM-talk] Valid geometry for closed ways

[Frederik Ramm]

Hi,

Iván Sánchez Ortega wrote:
> If you want polygons to behave as in "paleo" GIS, you should refer to the 
> industry standars. Specifically, to 
> http://www.opengeospatial.org/standards/sfa , version 1.2.1, page 26:

Agree with all, except that, for simplicity, we tend to make an 
exception in

> c) No two Rings in the boundary cross and the Rings in the boundary of a 
> Polygon may intersect at a Point but only as a tangent

in that we allow touching inner rings. The multipolygon

+------------+
|  +--+--+   |
|  |  |  |   |
|  +--+--+   |
+------------+

with two touching inner rings is often expressed in OSM as three closed 
ways, one for the outer ring and two for the inners, whereas in true 
paleo notation you would have to have four closed ways - one for the 
exterior and one for the combined hole, making up the "inner" of the 
multipolygon, and then two independent ways for each of the inner areas.

Of course this only happens if you actually have two different inner 
areas, e.g. outer=forest, inner1=meadow, inner2=water.

Bye
Frederik