[Tagging] The actual use of the level tag

Tobias Zwick osm at westnordost.de
Mon Jan 21 19:16:15 UTC 2019

On 20/01/2019 23:39, Tobias Knerr wrote:
> The main challenge I see with your proposal, though, is that the
> levels=* tag on the building would be utterly required to make any sense
> of the order of floors. Without it, applications would have no idea
> whether "M" is above or below "P2", for example. So at the very least,
> adding levels=* should explicitly be documented as mandatory when using
> non-standard levels.

Yes, the levels=* would be mandatory for proper ordering in those cases
where the floor levels are not numerical. For places where it is
numerical, nothing changes. It'd be an extension after all.

Consider: For places where they are not numerical, only
levels="LG,UG,M,1,2" on the building (part) outline is required to allow
tagging shops with a non-numerical level, nothing more.

In the current SIT scheme*, in this case, it is required to add 5
additional outlines, one for each floor (indoor=level), and tag each
with a level-index (level) and floor denomination (level:ref) in order
to create the relation between the level-indices as tagged on each
individual shop and the actual floor denomination.
In other words, to map a mall with non-numerical floor numbers already
requires to use the stuff described under "advanced modelling", and it
is more than one tag, it is 5 times "level" and 5 times "level:ref".

Additionally, one has to tag each shop not with the actual floor
denomination, but with the said level-index referenced in each floor
In the described case of non-numerical floors, the level-index becomes a
purely virtual ("programmer's") value which only becomes meaningful when
all the data is put together. And apart from making the machine
understand the relation between index and ref, the mapper himself needs
to make the conversion (in his head - .oO("UG is 0", "M is 1", "1 is
2",...)) for every shop he maps as well, which is prone to error,
especially with sporadic contributors.
In a nutshell, mappers and software are expected to know and use the
(advanced) SIT scheme if they "just" want to map on which floor a shop
is located.

I do not want to sound so combative or negative here - to reason for why
a new tag may be useful, I just need to describe the problem. I want us
all to be on the same page.

* as far as I understand it, correct me if I am wrong

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