[talk-ch] Sidewalk position precision in absence of dedicated osm-ways (Re: Pls, don't delete sidewalks, visual impaired people might use them)

Raphael Das Gupta (das-g) lists.openstreetmap.ch at raphael.dasgupta.ch
Fri Aug 25 09:49:12 CEST 2017

On 25.08.2017 08:13, Marc Mongenet wrote:
> I don't understand the sidewalk accuracy argument.
> With street width, the sidewalk location can accurately computed, no?

In theory, yes. In theoretical practice though, this would require that
the street width would be tagged accurate at every running meter of the
street and errors in that width would add up with errors in the street's
position, so drawing the sidewalk separately from, aerial imagery might
be more precise. In actual practice, the offsets in aerial imagery and
noise/error in GPS positions tend to be much larger than this and the
navigation way's topology tend to be much more important than the
precise location of the ways.

So yes, if you're worried about sidewalk location, and the sidewalk
isn't physically (by space or barrier) separated from the street, do tag
street width or at least the number of lanes and make sure the street's
way has a reasonable location. Only where the sidewalk is physically
separated draw them as separate ways as precise as feasible and make
sure you connect them properly to the streets where the separation ends
or is broken (gates in barriers, road intersections, pedestrian
crosswalks, asphalted gaps in the space between road and sidewalk, ...)
so that navigation can work properly.

Unfortunately, the "Sidewalks" article on the wiki
<https://wiki.openstreetmap.org/wiki/Sidewalks> doesn't currently
mandate that convention, merely hints at it, and I'm not sure whether
there's strongly enough consensus to change that document accordingly.
(See also this old (2012) discussion
on that article's discussion page.)


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