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Can anyone advise me how to map the bridge where the M27 motorway crosses the Wallington River ( A single bridge carries the motorway across both the river and a footpath immediately west of the river. The map shows the two main carriageways of the motorway, and the start/end of two slip roads, as four separate ways at that point. Mapping four separate bridges would be seriously misleading, not least because it is possible to swap between the main carriageways and the slip roads at that point. The usual solution would be to show the river running in a tunnel (culvert), but then how could the map show that the footpath runs through the same tunnel? Any help would be appreciated.

asked 17 Sep '12, 22:08

Madryn's gravatar image

accept rate: 13%

At the moment there is only little support in tagging multiple ways over a single bridge. You can create a bridge relation (the proposal is more detailed) and group all ways belonging to the same bridge with a relation but I don't know of any renderer or other tool supporting this so far.

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answered 18 Sep '12, 07:27

scai's gravatar image

scai ♦
accept rate: 23%


I can tell you that no tool supports this relation although the proposal exists since a while (the poor wiki content you forward as a link being a good indicator).

(18 Sep '12, 09:54) Pieren

At the moment these situations are often mapped with multiple bridges for every road, see for example here:

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answered 18 Sep '12, 20:46

mmehl's gravatar image

accept rate: 15%

If it is possible to swap between the slip roads and the main road over the brigde, please branch off the slip ways later.

In general, a way with multiple lanes is logically a single way as alon as you can (legally) change between the different lanes, even if they are exit lanes, to allow precise routing. This is independent of the bridge problem.

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answered 18 Sep '12, 08:12

Roland%20Olbricht's gravatar image

Roland Olbricht
accept rate: 36%

When I started to contribute in OSM, one person said that when a bridge was wider than longer, he tagged the underways as "tunnels". I'm still following this hint altough a civil engineer will tell you here that a bridge has to be mapped as a bridge, always. (Un)fortunatelly, we are not all civil engineers and you are still able to decide yourself the level of your contributions accuracy.
In your case, I would also tag both the river and the footpath as tunnels separately (tunnel=culvert and tunnel=yes). I don't think that modelizing a group of tunnels in a relation is very popular today (probably not, like for bridges). The wiki about culvert suggests that you could simply add a "foot=yes" on the waterway culvert which is maybe an alternative solution.

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answered 18 Sep '12, 09:46

Pieren's gravatar image

accept rate: 15%

edited 18 Sep '12, 09:51

Hi, my reaction / opinion on the subject of bridges was a wrong one according to the Wiki, where a viaduct is a long bridge. Every now and then its feels like Im walking in a mist of rules. I need a Wiki to find my way around. I would like to work on the spot but I wont do and just watch what Madryn makes of it. Pieren you send me if Im not mistaken of in the wrong way, according to the Wiki, a way over a way gets a tag layer = +1, but he stated that the underlying way should get -1 (tunnel) even if its not a tunnel in reality ? Excuse me Im flabbergasted.

Greetz Hendrik

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answered 18 Sep '12, 11:59

Hendrikklaas's gravatar image

accept rate: 5%


You seem to mix two things. layer=-1 is not automatically a tunnel and layer=1 is not automatically a bridge. This tag just specifies the relative position to other crossing ways, nothing more. That's why we have separate bridge and tunnel tags (which are usually combined with a layer tag).

(18 Sep '12, 12:40) scai ♦

Hi Madryn, Were talking bridges or viaducts. A road over a road is a viaduct not a bridge. A road over a river is a bridge, but since theres a way next to it its a viaduct again. Both crossings look like a tunnel but are viaducts. The 4 highway tracks are getting viaduct +1 and the earth body next to it gets the tag embankment, because its leveled higher over the countryside. Greetz Ps look at this situation node 179043561 Netherlands (IJsselstein N 210) Hoge Biezendijk. And look at Google maps vieuw at the situation.

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answered 18 Sep '12, 00:21

Hendrikklaas's gravatar image

accept rate: 5%


Please be more clear with your explanations. Noone can know what "getting viaduct +1" means, but you probably meant bridge=viaduct and layer=1. Also try to provide wiki links to the tags you are talking about and provide links for your way IDs.

(18 Sep '12, 06:42) scai ♦

I've never heard before that a road over a raod is a "viaduct" not a bridge. The freedictionnary says about bridge:"A structure spanning and providing passage over a gap or barrier, such as a river or roadway.". Viaduct : "A series of spans or arches used to carry a road or railroad over a wide valley or over other roads or railroads."

(18 Sep '12, 09:36) Pieren

In English English, a road over a road is still a bridge.

(18 Sep '12, 10:48) SomeoneElse ♦

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question asked: 17 Sep '12, 22:08

question was seen: 5,005 times

last updated: 18 Sep '12, 20:46

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum