On 2015-11-24 13:20, Martin Koppenhoefer wrote:
> 2015-11-24 12:43 GMT+01:00 Colin Smale <colin.sm...@xs4all.nl>: > >> One issue with dual carriageways (and now I think about it, also railway >> lines) is about generalisations at certain zoom levels. If you zoom out >> beyond a certain level, both halves of a DC (or the individual tracks of a >> railway) would be better modelled as a single line. The renderer/consumer >> needs an algorithm, and/or hints from the tagging, to know what belongs to >> what. > > yes, generalization would be nice sometimes for zoomed out levels, but a > relation would not help much. You still have to judge which part can be > omitted and which not (or how to combine the two (or more) into one), at > least for situations where both parts are not strictly parallel. > > See here how it works not too bad like it is done now: > http://www.openstreetmap.org/#map=16/42.1322/13.1322 > http://www.openstreetmap.org/#map=15/42.1314/13.1323 I see how the casing on the lines gets suppressed where they overlap, but you can see the name being rendered individually on the two halves. And I suspect the shields with the road numbers are more numerous than they need to be. That casing-suppression looks a bit wierd sometimes where two roads are geometrically adjacent WITHOUT being logically related, for example here (the north-south road right in the middle of the map, where it is labelled "Canterbury Way"). Two parallel but unrelated roads, of different classes, are being blended into one: http://www.openstreetmap.org/#map=15/51.4347/0.2426 >> They can provide certainty where otherwise complex heuristics would be >> needed which may or may not work in all cases. Two streets with the same >> name isn't good enough on its own IMHO. The two halves of a dual carriageway >> may have different names and still be the same road. > > Given that there is no such unambiguous thing in the real world like "the > same road", you'd always have to judge based on a common definition. There > will likely be arguments for both ways of looking at it: same road or > different roads. A relation could make it clear how the mapper saw it, but > I'd already question the concept on a more general level: what is "the same > road"? The definition of "the same road" will probably depend on the specific use case. The use case may be rendering, to produce a nice-looking map on the screen or on paper, or it MIGHT be navigation-related. My idea is to remove the constituents of the relation and replace them with a simplified version, which looks/behaves more suitably for the given zoom level. Keeping all the roads properly connected will be a helluva job, so it might not be viable as a simplification of a routing network, but giving the resulting turn directions based on the simplified model may have some benefits; not sure about this though. //colin
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