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Is anyone able to explain the use of layers, further then bridge 1, 2, 3 and tunnel -1, -2 ? The rest of the situation is 0, but key nor value anywhere. A bridge over water gets 1, the surrounding area no value but the water lower then the surroundings none either. Whats the logic in this ? And how should a culvert beiing tagged, key layer _ value -1 ?


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asked 11 Aug '12, 11:42

Hendrikklaas's gravatar image

accept rate: 5%

layer defines the relative vertical position of elements that intersect or overlap. In other words: Of two intersecting elements, the one with the larger layer value is above the other one.

This is the only purpose of layer. In particular, it does not define an absolute elevation. An element with layer=1 might very well higher than another element somewhere else that has layer=2, as long as they don't intersect.

So, with your example, it is perfectly ok that water lower than the surroundings is on the same layer as elements around it, as long as it is not directly below those elements - i.e. there is no overlap/intersection. A culvert, on the other hand, tends to run below something, e.g. a road, and will therefore usually need a layer=-1 tag to place it below that road (in addition to tunnel=culvert).

By convention, positive layer values are used for features above the ground, and negative layer values below the ground. It is also good practice to use the smallest (by absolute value) layer that describes the current situation - so even though tagging a "normal" bridge as layer=5 would still correctly place it above the ground-level road it crosses, this should be avoided. As a result, layers larger than 1 and lower than -1 are quite rare.

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answered 11 Aug '12, 13:09

Tordanik's gravatar image

accept rate: 35%

Imagine 2 roads A and B that cross each other over a river and a third road C that is a tunnel under the river. The river would be at level/layer 0. The 2 aerial roads would have layers 1 for A and 2 (for the upper road B) and the tunnel C would have -1. The rendering would show that road B crosses over road A, over the river. And tunnel C would be rendered with dashed lines.

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answered 11 Aug '12, 13:07

gerdami's gravatar image

accept rate: 15%


Thanks Guys, Ill have some work to do. I didnt tag all my bridges and culverts correctly. But intersections and subway systems could go deeper or higher then + or - 3.

(11 Aug '12, 22:01) Hendrikklaas

Potlatch2 has a layer ruler that goes from - 5 to + 5

(12 Aug '12, 09:30) gerdami

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question asked: 11 Aug '12, 11:42

question was seen: 5,377 times

last updated: 12 Aug '12, 09:30

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