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Bridge vs Tunnel?

 17 4 So, how do you decide if a way is a bridge over another way, or if it has a tunnel going under it? Here are some examples, all involving a railway track that's a cutting, with roads going over it at various points: Would this be a bridge? It's only the width of the road (which in my mind would be too short for a tunnel), but the road doesn't raise up to go over the railway, as the track is at a lower level.. How about this? I thought this made sense being marked as a tunnel, as it's clearly going under various roads, and it's a relatively long section. This seems borderline again.. bridge or tunnel? I suppose layers come into this as well don't they? I assume the track should be marked as `cutting=yes` and `layer=-1`. The bridge could be explicitly marked as `layer=0` to signify that there is no change in level as it crosses the other way. Or would that not make sense to do? Should the track just be marked as having a section of tunnel? asked 21 Jun '11, 21:28 Sekai 291●5●5●8 accept rate: 0% dieterdreist 3.7k●11●35●67 1 I always do it on method of construction; therefore, the above three would likely all be bridges. Definitely, 1 and 3 are.... 2 might be considered a cut-and-cover tunnel.... survey might be needed (o; (22 Jun '11, 01:17) c2r 1 I changed level to layer in your post, because the commonly used tag is layer. actually layer=0 is default and can be avoided (you might still want to use it to emphasize that you didn't simply forget to put it). Layers in OSM don't actually say if something is below or above ground, they only come to play in relation to other objects crossing them: the higher the layer is the higher is the feature in the order of overlapping features. (22 Jun '11, 13:04) dieterdreist Using bride or tunnel, you can completely get rid of the layers=* They really only play a role if some features cross WITHOUT a bridge or tunnel or if there is more than one bridge/tunnel crossing at the same point. They are only a hint to the renderer what to draw on top and carry no other meaning in terms of height. But for bridges/tunnels, the renderer already knows what to do. (19 Aug '13, 16:18) Chaos99 1 youll get a complaint adding a bridge over a way, without a tag layer 1 or -1. (19 Aug '13, 21:56) Hendrikklaas

 9 That's always going to be a little subjective. To me the presence/absence of a slope on the top/bottom highway doesn't make a difference. The construction type is tempting, but I can see many cases were it will not yield a definite answer. I'd say if the bottom highway crosses a single feature (however broad the feature may be) it's a bridge, otherwise it's a tunnel. So I'd rate your 1&3 as bridges, 2 as tunnel. YMMV. In the end it's not a big issue, the main thing is to have no connectiing nodes and to have the layers right. answered 22 Jun '11, 10:55 Vincent de P... ♦ 17.3k●18●152●249 accept rate: 19% 4 +1 The difference between tunnel and bridge is really somewhat arbitrary, and not really important for most applications. The important thing (especially for routing) is to use `layer`s (because that indicates that it's not a level crossing). If it's ambiguous, just choose one; the rule of thumb given looks fine. (22 Jun '11, 11:29) sleske Sorry, in my comment above "levels" should have been "layers", as that is the right tag to use. (29 Apr '15, 12:21) sleske @sleske: I've edited it – better now? :-) Then please delete your and my (this) comment. (29 Apr '15, 12:29) aseerel4c26 ♦
 5 Technically these structures are all bridges (because not dug in the massive earth but constructed openly and then later covered, and not longer then 80 / 160 metres). I would still tag 1 and 3 as bridge and 2 might be better represented as tunnel in osm (because otherwise you would miss the start and end of the covered part of the way, and you might end up with lots of parallel bridges what I would consider a worse representation compared to a simple tunnel for the way below). answered 22 Jun '11, 12:58 dieterdreist 3.7k●11●35●67 accept rate: 3% 3 +1 good point about the "multiple parallel bridges". That's something that often bothers me with OSM renderings. (06 Jul '11, 02:10) sleske I'd agree they are all bridges. Surely a tunnel implies tunneling through some sort of natural feature? (29 Jul '11, 00:04) EdLoach ♦ I solved the paralel bridges with highway areas & railway areas. Then went on to map road widths with areas too. The routeing lines stay to as they are linked and tagged like in comercial products to aid simplifed routing algorithiums. (01 May '14, 21:50) Govanus
 1 Alas even knowing the structure may still be ambiguous, a cut and cover tunnel is in essence a very wide bridge. Adding to the abiguity are some larger culverts which typically I'd consider a tunnel, but some may consider a bridge answered 21 Jul '11, 21:43 Sundance 467●5●11 accept rate: 3% 3 So you're saying most of the NYC subway isn't really underground, the street above is just a bridge? (29 Jul '11, 07:18) Baloo Uriza 1 cut and cover constructions are technically considered tunnels if they are very long (e.g. at least 80m in Germany, 160m in the USA). I'd consider the box culvert in your first image a bridge. (29 Jul '11, 08:44) dieterdreist 1 No I'm saying the construction techniques for a some cut and cover tunnel can be identical to a bridge. A good example of this would be the Seattle freeway lids on Mercer Island and soon to be on State Route 520. The longer curved one, okay tunnel, the shorter ones, bridge or tunnel is more ambiguous http://www.openstreetmap.org/?lat=47.5881&lon=-122.23165&zoom=15&layers=M And yes I would map a cut and cover tunnel as a tunnel. There is some vagueness in some wider bridges as in this example, or in some culverts. (04 Aug '11, 17:10) Sundance cut and covers are very common for underpasses and metros alike seem bottom catch all solution. (01 May '14, 21:43) Govanus
 1 Imagery is not the main way data should ideally be collected for OSM, ground surveying is. The best way to get data into OSM is to go to the place and check it out. Take your GPS with you, collect tracks, make observations. If you want to know if it's a bridge or a tunnel, head over there and check it out. If it's not near you, then focus on what is! You should be mapping what's local to you, what you can observe yourself, and use the imagery as a backup. answered 22 Jun '11, 03:29 emacsen 1.2k●16●23 accept rate: 13% 6 While what you say is true, it's offtopic I think. Going on-site probably won't make things clearer in this case. For all we know, the OP might have been on-site and know the area well, but used aerial imagery to illustrate his question. (22 Jun '11, 10:58) Vincent de P... ♦ 1 You hit the nail on the head Vincent. Thanks for the advice though emacsen! (22 Jun '11, 12:03) Sekai I try to to do this mostly seeing the site and possibly taking down notes before using the corrected offset satelight/arial pics to line things up at a drawing out stage. Sometimes when I try using JOSM with no internet and no imagry my attempts are delibratly errored to make it more obvious that the fetures like building outlines still need correction. Still I'd still be pondering after having stared at a long overbridge about this issue. (01 May '14, 21:20) Govanus
 10 Down the pub one time we suggested the simple rule: "If the road passing under is longer (underneath) than the gap is wide, then it should be a tunnel on the under way, rather than a bridge on the over way." ....But we were all completely drunk at the time :-) By that rule I'd say your second and third images are both tunnels (and that is how I would map them), but your first one is 50/50. Could be a bridge. See comments on my diary for some further discussion. I've also just added a specific section to the wiki where we should one day answer this question. [update] : The wiki page was modified to carry some more specific advice, which is good I guess. See the discussion here But on the whole I agree with Vincent de Phily. If you have a case where you can't decide between bridge or tunnel... well then it probably doesn't matter too much. Getting levels right matters more (mainly to help the renderers). Avoiding connections where there are none is important (To avoid routing problems) answered 21 Jul '11, 18:58 Harry Wood 9.5k●25●88●128 accept rate: 14%
 0 These should all be tagged as bridges, because they are bridges. Don't tag something as a tunnel just because it looks nicer in mapnik. answered 21 Jul '11, 15:27 Janjko 323●3●9 accept rate: 0%

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question asked: 21 Jun '11, 21:28

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last updated: 29 Apr '15, 12:29

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