[Tagging] sidewalks and tagging for the renderer

Tobias Knerr osm at tobias-knerr.de
Wed Apr 11 16:21:59 BST 2012


On 11.04.2012 02:04, Martijn van Exel wrote:
> On 4/10/2012 4:38 PM, Tobias Knerr wrote:
>> A sidewalk=left/right/both fails when you want to define the relative
>> ordering, and separate footway=cycleway fail in practice because no
>> renderer is actually able to puzzle the highway back together from
>> unconnected parallel ways.
> 
> What is the use case for being able to do that? What can you do that you
> can't with a separate geometry for a sidewalk that may be as much as 6
> feet from the main roadway?

For one, you can render them without overlaps and gaps between the
sidewalk and roadway.

Around here, sidewalks are usually just the width of a kerb (~ 15 cm)
away from the main roadway. That's not wider than a white line on the
road and isn't much of a physical separation (which contributes to my
reluctance to treat them as separate ways).

This also means that, with separate ways for the sidewalk, the mapper
would have to draw with unlikely precision to avoid graphical glitches -
a few pixels too far from the road, and there's a very noticeable gap
between road and sidewalk that does not exist in reality. A few pixels
less, and the sidewalk disappears below the road - or the other way round.

And of course: As soon as you don't render road and/or sidewalk not to
scale, rendering breaks down even with centimetre-precise mapping.
With tags on a highway, on the other hand, the sidewalk is part of the
render style of the highway and is always placed perfectly.

Besides rendering, other reasons why a program would want to associate
the sidewalk with the highway include: routing instructions; adding the
possibility to cross the road anywhere to the routing graph where this
is allowed; and accessing the name or other attributes of the highway
(unless all relevant tags are copied to the sidewalk).

Tobias



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