[Tagging] The actual use of the level tag

Tobias Knerr osm at tobias-knerr.de
Mon Jan 21 22:19:46 UTC 2019

On 21.01.19 22:38, Roland Olbricht wrote:
> I do consider both to be SIT compliant.

I'm not sure if it's clear from the written text of SIT, but neither
fractional levels nor indoor features outside of a building outline were
part of SIT's design. (And yes, these are obvious omissions that will
need work.)

> In particular, there is no object that could or should carry tags
> applying to all involved objects.

Levels only have meaning within a single structure, though. Level "2" in
the neighbouring building (or even building part) might represent an
entirely different absolute elevation.

An important assumption (and limitation, because this isn't always true
in reality) of SIT is, therefore, that there is some kind of outline
within which a level number refers to a single, fixed elevation.

I didn't really raise that point so far because I don't have a better
solution to offer yet, but your station mapping does contradict the
assumptions we had when writing SIT, and I feel we should collaborate to
define the semantics of such a situation.

> But I do not see yet why there should be a mapping of levels to integers
> at all.

One reason that's of particular interest to me is that SIT is intended
to be compatible with 3D rendering, allowing for the creation of 3D
models that represent both the inside and outside of buildings at the
same time.

At the moment, Simple 3D Buildings has no support for "half" levels, so
if we want to preserve that feature of SIT, we would need to update both
tagging standards at the same time.

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