[OSM-talk] Data primitives (was: The segments vs ways vs superways question again...)

[guy at graviles-reynolds.org]

Quoting Jochen Topf <jochen at remote.org>:
> 
> You thought this was complicated? It gets worse...
> 
> Lets consider a primary road that goes through a larger town, you'll
> probably
> have sections with different speed limits and other characteristics. Some
> parts
> may be single-lane, some multi-lane, some divided, some not. But it is
> still
> only one road with one name. Or even worse: A motorway, crossing a whole
> country. Currently we draw both parts of a devided motorway as different
> paths,
> but thats not right. You'll never be able to render motorways properly on
> all
> scales doing that. They follow, after all, the same polyline. Sure one part
> is
> a few meters to the left and one a few meters to the right, but thats the
> same
> for any track (or a road with cycle lane). If you look on any map, you'll
> see
> that they draw a motorway or trunk road as one polyline, typically rendered
> as
> a colored line with a black line on each side and a black line in the
> middle.
> And if you are not confused yet, think about how to model exits and
> motorway
> links in this case.

Not strictly true, I know of several sections on dual carriageways (trunk and 
primary not just motorways) where the carriageways are divergent by several 
hundred meters, well beyond the margin of error for GPS. 

Also with dual carriageways if entered as a single polyline how do you 
differentiate between a side road that has access to both carriageways or a 
side road that has access to just one of the carriageways or for that matter 
a road which runs East/West forms a junction with a dualed road which runs 
North/South: 2 polylines meeting at a node, but there is no crossover i.e. 
someone coming from the West travelling East has to first travel North to the 
next junction, then back south before rejoining the East/West road and 
continuing eastward.  

Guy 'Batchoy' Reynolds