[Tagging] RFC - Feature Proposal - area of steps for pedestrians.

Christoph Hormann osm at imagico.de
Fri Mar 29 11:41:31 UTC 2019

On Friday 29 March 2019, Warin wrote:
> Hi,
> This one has been sitting for a long while! Still not certain about
> some aspects of it.
> See what you make of it.
> https://wiki.openstreetmap.org/wiki/Proposed_features/Area-steps

First agreement with Martin that you need to decide if you want to make 
a proposal for defining some polygon-like 3d surfaces in general of 
some sort or specifically to deal with steps only.  In the latter case 
this should be clear from the tagging, i.e. something like 
type=steps_area and nothing generic.

Two more general notes on the more fundamental underlying problem:  

* you should be aware that you can't uniquely define the shape of a two 
dimensional surface in three dimensions exclusively through the shape 
of its outline.  You can do that in 2d (provided what you have has a 
defined outline) but not in 3d.  That is simple mathematics.  So you'd 
have to document what assumptions you make regarding the shape of the 
surface, otherwise the meaning of your proposal would be ill-defined.

* tying yourself to the idea of polygons being the ideal way to map 
anything in OSM could prevent you from finding a both mapping and data 
use efficient way to document things.  You should keep in mind that >90 
percent of stairs that exist are simple constant width + strait step 
stairs that can perfectly accurately be mapped with a linear 
highway=steps and a width tag.  If you approach the problem with how to 
extend this method in ways that are (a) optional and backwards 
compatible, (b) make use of the information already present in the 
linear way mapping and (c) that cover most of the other <10 percent of 
the cases without the idea that the solution *has to be in the form of 
a polygon variant* that would more likely yield an efficient solution.

Christoph Hormann

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