[Tagging] The actual use of the level tag

[Tobias Zwick]

>> - also the building with "ground_level=G" to define which level is
>>   the ground level. If ground_level is missing, 0 is assumed.
>>
> Do we really need a ground level? I think not. We need connections to
> outside ways and entrances.

Well, all of which I mentioned is optional. But I can come up with two
use cases for wanting to know which level is the ground level:

1. Localization

In an application, it is much nicer to be able to write
"ground floor" (en-GB), "first floor" or "basement floor 2"
than
"level 0", "level M" or, worst, "level -1"

To refer to a "level" may be even less understandable when trying to
translate that into other languages.

But for this to work, it is necessary for the application to know which
level is the ground floor.

2. 3D indoor levels

When displaying the floors of a multi-storey building in 3D on top of
each other, you need to know where on the Z-Axis you have to place it.

Tobias