[OSM-talk] area topology
Shaun McDonald
shaun at shaunmcdonald.me.uk
Tue May 13 14:30:49 BST 2008
On 13 May 2008, at 14:06, Ulf Mehlig wrote:
> Please excuse me if this is a FAQ, I just didn't find an answer in the
> wiki (possibly because it is obvious) ...
>
> In topological GISses like grass, borders are shared between adjacent
> (vector) areas. However, I wonder how adjacent areas should be
> digitized
> in OSM (let's say, forests and farmland, or riverbanks and the
> associated wetlands). Am I right in assuming that OSM ways always
> belong
> to one single area? If this is so, do I have to duplicate ways along
> the
> common border? JOSM informs about double ways when it validates data
> so
> I had the slight impression that they are not really wanted.
>
> I see a similar problem with line data (e.g. roads, streams) which may
> happen to be an area border. From reading
>
> http://wiki.openstreetmap.org/index.php/Editing_Standards_and_Conventions#Tagging_Areas
>
> I think that using a street as a border of a plaza (or a block of
> buildings) is not wanted. Does one digitize double ways, leading along
> the same nodes, or does one make a separate area in a small distance
> to
> the existing line (street/stream), which might be topologically
> incorrect and is more difficult to maintain?
>
If you have a road and stream running parallel they would be entered
as 2 ways that are parallel. The same happens for the carriageways of
a motorway that are separated by a barrier.
The way that I look at it, is if there is a barrier, such as a fence
of wall, then the node in the way should not be shared. Otherwise the
sharing of nodes isn't a problem, and can produce better results,
especially for routing.
Shaun
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