On Thu, Oct 1, 2009 at 3:25 AM, SteveC <[email protected]> wrote: > I want to revive a very old idea - tag equivalences. It might be > solving a problem that doesn't exist or someone might have done it and > I've missed it. >
Closest I got was the osmosis tagtransform plugin [1]. I made that because people keep insisting on tagging things like highway=path;bicycle=yes|designated, which is essentially like a highway=cycleway and we want those rendered on the cycle map. Now for various reasons, mostly due to laziness, version mismatches, and not being entirely sure about what the different path combinations mean, this has never been deployed on the cycle map for real, but anyway. What I was originally thinking was that what would be nice (and therefore almost certain to be never developed) would be a proper tagging app on the OSM website which let people describe their tagging schemes. Part of that would then be how to get from one scheme to another. In thinking how you might do that some of the weird and wonderful ways that schemes differ come to mind and tagtransform was born. Back to the likenesses, saying an autobahn is like a motorway makes sense, is a good start, but very limited. One of the first problems you hit is what "like" actually means. In the UK a lot of footpaths look a lot like a lot of bridleways. So therefore highway=footway is like highway=bridleway, one allows horses but the other doesn't otherwise they're mostly the same. But a lot of foot paths are like cycle paths, which are a lot like tracks which are a lot like service roads which are a lot like unclassified roads. Anyway it turns out that just about all roads and paths are transitively like each other which is a little confusing and not a very useful result. With the likenesses XML you could sort this out with overlapping likeness groups but it quickly becomes a problem for renderers if they're trying to find things which are like a footpath, and things which are like a cycle path, because lots of things are like both and you don't know why. So the way in which something is alike is actually very important. What may be a better idea is object hierarchies -- ie: is-a links or mediawiki-like categories. This way you end up doing things the other way round... <feature> <tag k="highway" v="motorway"/> <is-a>road</is-a> <is-a>major-road</is-a> <is-a>biggest-roads-we've-got</is-a> <is-a>road-with-speedlimit</is-a> <is-a>road-which-meets-uk-reg-blah15</is-a> <is-a>road-which-meets-eu-reg-blah01</is-a> </feature> <feature> <tag k="<insert german for road or whatever>" v="autobahn"/> <is-a>road</is-a> <is-a>major-road</is-a> <is-a>biggest-roads-we've-got</is-a> <is-a>road-which-meets-de-reg-blah97</is-a> <is-a>road-which-meets-eu-reg-blah01</is-a> </feature> (that's very flat, there's no reason you couldn't actually structure it a bit by introducing the categories as objects with categories, and those is-a parts could easily be OSM tags specifying actual speedlimits etc with the expectation these represent the defaults) So now the renderer can just go for the "biggest-roads-we've-got" category, and render anything that matches. And surprise surprise it ends up looking a heck of a lot like an editor presets description file [2]. And that fact is interesting because of the question: why didn't we just tag all the motorways like that in the first place? Screw highway=motorway, just add road=yes;major_road=yes;biggest_road_we_have=yes... I can think of a couple of reasons. - we're all lazy and don't want to type it -- editor presets can negate that one - the german government decides to impose a speedlimit so we immediately get to change the autobahn definition rather than retag all the objects using an evil bot One of the big questions is whether all of this would make life easier or harder for the renderers -- it would definitely raise the tech start bar. Dave [1] http://wiki.openstreetmap.org/wiki/Osmosis/TagTransform [2] See the potlatch 2 map-features.xml http://trac.openstreetmap.org/browser/applications/editors/potlatch2/resources/map_features.xml -- but there you're limited to one category because it's more for menu structure than proper categorisation. _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
