[Tagging] The actual use of the level tag

Tobias Zwick osm at westnordost.de
Sun Jan 20 13:49:16 UTC 2019


Hi there,

In the wiki, the level tag is defined to be a 0-based-index so that
level=0 is the ground floor, i.e. at the street level. In other words, a
two-storey mall with no basement will have shops at level=0 and level=1.

This is intuitive for (at least) Europeans, people from Commonwealth
countries and parts of Asia as this coincides with common language.
However, in at least the USA, Canada, Japan, Korea, China and most
probably more regions, the floor at street level is denominated as the
"1st floor" in common language. In other words, it is a 1-based-index.

To my knowledge, no editor (neither iD nor JOSM) and no user-end
application takes this into account and localizes the level tag to
common language (properly). Instead, it is assumed that everybody knows
about this and adheres to this scheme.

This is why I suspect that despite it being mentioned several times in
the wiki, the level tag is not used the way it was defined in those
regions where the definition of the level tag to be 0-index-based does
not coincide with common language. Deliberately, or unwittingly.

So, I did a little research.

Via overpass, I searched for multi-storey malls in some metropolitan
regions where the first floor is at street level and looked at with
which levels the shops inside were tagged.

Results
=======

Region                        | likely zero-based | likely one-based
------------------------------|-------------------|-----------------
Washington, Philadelphia, NY  |         3         |        2
Silicon valley, Los Angeles   |         4         |        4
Quebec, Montreal, Ottawa      |         6         |        0
Moscow                        |        23         |       51
Tokyo                         |        14         |       16
Seoul                         |         0         |        2
Bangkok                       |         8         |        5
------------------------------|-------------------|-----------------

For comparison:
whole Phillipines             |        23         |        4
whole Netherlands             |        11         |        2
Berlin                        |        19         |        2

*likely zero-based: there was at least one shop tagged with level=0
*likely one-based: there was no shop tagged with level=0 but shops at at
                   least two different other levels

Note that cases where mappers did not add level=0 to shops at street
level because they thought level=0 is the default and should not need to
be specified would be counted as "likely one-based" in above table.

Conclusions
===========
Two immediate conclustions can be made:

1. user-end software cannot reliably assume that level=0 is the ground
level in certain regions because the data is not mapped consistently
according to the wiki definition

2. generally, tagging definitions that are not intuitive to use (in a
region) will not be used consistently (in that region), leading to
ambiguous data.

Regards
Tobias



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