Hi.
As Initiator and author of the proposal you refer to, let me explain, WHY:
Am 18.02.2011 11:16, schrieb M∡rtin Koppenhoefer:
2011/2/18 David Murn<da...@incanberra.com.au>:
Because the use of (min_)levels,height is in use by 3D renderers and
IMHO this min_level-part of the advanced building proposal is not
working (is using wrong semantics), at least for the illustration you
can find in the wiki.
Is not working and "is using wrong semantics" is not the same.
building_levels should be the amount of building
levels. If a building forms a "bridge" like in the illustration, where
adjacent buildings have 7 levels, the "bridge" has only 2 levels and
the 5 levels below are void, the proposal states you should still
apply building_levels=7 and count the voids as levels.
You are an architect and from that perspective you are completely right.
My purpose with this design of the tagging scheme was something often
applied in OSM: backwards compatibility.
Most people tagging level counts of buildings I think would not think as
you describe for "bridges".
As bridges do not appear alone and instead are always part of a building
including the sides of the bridge, the building as a whole would have
been tagged with building_levels=7.
So far this obviously don't describe the bridge of course.
Now let's think about this bridge as a whole through the building block
of 7 levels height.
And that's what the min_level does: it raises the bottom about the
min_level count - in this example, it raises the bottom about 5 levels.
I played with the alternative more common for you, but what would that mean?
Only being able to interpret building_levels and not supporting
min_levels would lead to false assumptions about the height of the
building. A building with a roof above 7 floors would be interpreted as
a building with height of 2 floors.
Let's stay at 3D-Rendering as the example application, you would have a
U-shaped instead of a A-shaped building.
This is against any common practise and definition in architecture,
yes, but not against interception of most people: Even a building with a
tunnel at the basement level with 6 levels above is seen as a
7-level-building.
And if you want to have the number of levels inside the building you can
simply calculate it by building_levels - min_level.
building law and the definition of building_levels in the wiki.
The definition of building_levels in the wiki
(http://wiki.openstreetmap.org/wiki/Proposed_features/Building_attributes)
states "Number of stories, including the ground floor".
My interpretation is: There is no explicit definition of the handling of
"missing levels" like "tunnels".
But it's clearly stated "including ground floor". So I would say: it
does not depend on the existance of a raised first floor.
Even for you as an architect I don't think that you define the first of
the 2 raised floors of the example bridge is a "ground floor".
Please tell me, if I'm wrong.
regards
Peter
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