[OSM-talk] Indiscrimate layering

Elena of Valhalla elena.valhalla at gmail.com
Sun Dec 21 09:36:21 GMT 2008


On Sat, Dec 20, 2008 at 5:05 PM, Sven Rautenberg <sven at rtbg.de> wrote:
> Elena of Valhalla schrieb:
>> not so true everywhere: in mountain areas rivers are usually at the
>> bottom of valleys,
> The bottom of a valley is layer=0.

if it is a wide valley with other features of course yes; if it is a
narrow strip as wide as the river and any other feature (e.g. houses)
is on level with the road, above the river, to me the basic ground
level is the road one, even if the place where the river is was cut in
the ground by water and not by men.

>> and I've seen many small rivers covered and
>> practically brought underground in urban areas.
> These streams should be tagged tunnel=yes, layer=-1.

exactly, and that's another river that is not on layer=0

> If you come across an existing stream which has no layer tag, do you add
> "layer=-1" to the whole stream and then check the whole stream if your
> change may conflict e.g. with a tunnel which is already at layer=-1? I
> don't think so.

no, I usually make layer changes local, because that way they tend to
reflect reality better

but if I now that a river lies below ground for a long part, and I'm
checking the map of an area I have just surveyed, I may move it to
layer=-1 and check for conflicts, if that describes reality better
than the alternative. Definitely not something I'd do in a quick
editing session to add a couple features, however.

> Tagging a part of a way with a non-default layer does not mean anything
> about it's elevation. It is all relative, but it is a good idea to
> assume "natural surface level" to be layer=0 in most cases.

Of course the important part is the relative value, and I'd sort of
agree with the layer=0 guideline to be followed whenever possible, but
I would use if for "basic" or "mean surface level", instead of
natural; i.e. I would put the distintion on what can be seen on the
ground and not on the origins of the feature.


> The stream therefore is the natural surface wherever its water flows.
> The bridge is above the water, so it is layer=1. And it is less
> dangerous when just tagging the bridge compared to tagging the whole stream.

It would just be as safe to split the stream and tag that part as
layer=-1, when that describes reality better

of course there are lots of rivers where reality is better described
by a layer=0; my examples were just to show that there can't be an
hard rule on the matter.

-- 
Elena of Valhalla

homepage: http://www.trueelena.org
email: elena.valhalla at gmail.com




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