[Talk-GB] Sorting out layering in East Anglia, Essex, London and Kent

Peter Miller peter.miller at itoworld.com
Tue Apr 19 15:26:35 BST 2011

On 19 April 2011 15:20, Andy Allan <gravitystorm at gmail.com> wrote:

> On Tue, Apr 19, 2011 at 3:05 PM, Matt Williams <lists at milliams.com> wrote:
> > On 19 April 2011 15:50, Andy Allan <gravitystorm at gmail.com> wrote:
> >> I'm assuming his map layers view has some logic that layers tags only
> >> apply to ways that cross but I don't believe that to be true.
> >
> > Actually, that's exactly how I understood the layer tag to be used. It
> > is simply there to disambiguate cases where there would otherwise be
> > "z-fighting".
> But my point is that ways don't need to actually cross, they can just
> be pretty close together (e.g. parallel), for the layer tags to be
> useful and required.

That is not what the wiki says (and said before my edits). Before my edits
it said:

"The layer Key can be used to mark if a way/node/area is above or under
another one.

"This tag should only be used for height differences that are real, like
bridges over a street or tunnels under another object

"When tagging things, try to avoid the layer tag most of the time.
Especially do not use it in these circumstances:
* Do not tag areas like landuse, natural etc. with a layer
* Do not tag waterways like rivers, streams etc. with a layer just because
you have a bridge running above them and do not want the bridge to be

"Remember: The layer tag has no meaning for absolute heights. The bridge
within a perfectly flat street should be layer=1 even if the stream is as
far below it as the Grand Canyon. The track on top of Mount Everest would be
layer=0 even though it is 8848 meters above sealevel. In other words, the
ground level, as would be shown on a topographic map, is always layer=0.

"if two roads intersect in mid-air, they must both have the same layer to
display properly. This means that it may be necessary to break one of them
at a nearby point.

In general I have be interpreting those rules across the area I mentioned.



> Cheers,
> Andy
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