[Tagging] The actual use of the level tag

Tobias Zwick osm at westnordost.de
Wed Jan 30 13:44:50 UTC 2019

I stumbled upon a real-world example yesterday that may make the attempt
to have the level-tag describe a "global" order (as used in OpenLevelUp,
JOSM etc.) somewhat impractical -  with that level-selector UI element:

So, Hamburg is a really flat city. And even still, the mall "Europa
Passage" ...
... does have two "ground floors". If you enter through the northern
entrance, you are in the lower ground floor (level=0), if you enter
through the southern entrance, you are in the upper ground floor
(level=1). If you walk alongside the mall though (Bergstra├če), there is
no (apparent) elevation.

So, imagine there are more indoor-mapped places both around the northern
and the southern end of that mall (there is, partly, but I am sparing
you the details), and as a matter of fact, sprinkled throughout the
entire city centre.
Then, to have a global order of things, the ground floor of all the
buildings South of that mall would need to be tagged with level=1 and
the ground floor of all the buildings North of this mall would need to
be tagged with level=0. In other words, must be relative to the level
order used in this building.

I see two problems with this:

1. Where to stop? How global is this global order? Going further South,
at what point does the level-value for the ground floor revert back to
0? What then if two such places collide? (A mall and a multi-level train
station were mapped separately and they built a tunnel to connect the
two, but they connect on different level=X - or not even a tunnel, let's
say they are just next to each other)

2. It is somewhat confusing for the user (and the mapper) because if you
look at such a map, at for example level=0, you would see an unexpected
split through the city centre where seemingly no building is
indoor-mapped South of that mall ... oh wait, it's all on level=1+.  ???

I, too, hope that the example was understandable.


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