[OSM-talk] Underground / hovering buildings

[M∡rtin Koppenhoefer]

2011/2/15 Nathan Edgars II <neroute2 at gmail.com>:
> Jacek Konieczny wrote:
>> layer=-1 tells only that the thing is under layer=0 and over layer=-2,
>> nothing in relation to 'ground level' (some rivers or roads may have
>> layer=-1 or layer=1 on most of its length).
>>
> No, ground level is layer 0. A nonzero layer on a ground-level feature is an
> error.


-1,
http://wiki.openstreetmap.org/wiki/Layer


> In addition, it's not always clear what ground level is in dense urban
> environments.


+1, but as there is not tag for "ground-level" that's not a big problem for us.


> In part of downtown Chicago, the pre-civilization ground level
> is now under two levels of elevated streets. But there are buildings that
> fill the formerly-open spaces, so in some sense ground level has moved up
> two layers.


yes, that's a frequent phenomenon, in other parts of the world with an
older history you can find stuff as deep as 50 or maybe even 100
metres (e.g. in China), another famous example is
http://en.wikipedia.org/wiki/%C3%87atalh%C3%B6y%C3%BCk , where 14
levels in this former settlement were identified (hence it's
impossible to bring them all in OSM ;-) ).


> Here it's probably best to explicitly label layer=0 on
> whatever's chosen as the current ground level.


no, layers don't express something like ground-level. You can tag
everything with layer=0 but it is pointless, as that's one of the very
few defaults in OSM.

cheers,
Martin