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 or you'd want to see actually 2 separate ways still in z14: http://www.openstreetmap.org/#map=14/42.4821/13.6116 or even in zoom 10: http://www.openstreetmap.org/?mlat=49.6009&mlon=11.3557#map=10/49.6014/11.3551 > On the subject of man_made=bridge, that relies on geometry to associate > the parts. That loose association only works because bridges are > straight(ish). > yes, it would eventually create problems in case e.g. a bridge spirals around itself (guess improbable case) similar to spiral stairs (maybe these wouldn't be called bridges but ramps). It worked for all bridges I have mapped so far. It is a very intuitive and easy approach, leads to very simple mapping results (typically just a rectangle of the bridge outline), i.e. it is easily adoptable and maintainable, is supported by even the more simple editing programs. You can call that a loose association because it "only" relies on spatial properties, but I bet it is more sustainable and reliable than any solution that requires mappers to explicitly search for a relation and add parts to it when they modify some road. And it stores the outline of the bridge you'd need anyway if you wanted to add some significant detail. More complex cases can still be modelled with a bridge relation anyway, but these are very rare compared to the rest of bridges. > No need for relations where there is no need for them? Sounds a bit like a > circular definition. If a relation is the right way to model reality, we > should also not run away from them. > yes, of course. Routes, multipolygons, turn restrictions, boundaries, etc. There are lots of good reasons for relations in our current data model. > 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"? Cheers, Martin
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