[talk-ch] Semantics through abstraction (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 00:08:27 CEST 2017


marc bring up good and valid points.

On 24.08.2017 23:12, marc marc wrote:
> 3) It is only afterwards that one can draw the sidewalks separately 
> without sending the people into a dangerous situation.

In what I assumed up today to be the consensus, there are two cases when
sidewalks should already now be drawn separately:

 1. (as already mentioned) when separated from the street by space
    (i.e., not actually laterally connected to the street)
 2. when separated from the street by a barrier (e.g. as seen here
    <https://www.openstreetcam.org/details/8505/3981> on the right-hand
    side)

In all other cases, for the time being, the presence (or absence) of
sidewalks should indeed be represented by additional tags on the
street's way rather than dedicated ways for the sidewalks, and I'm not
even sure whether we should pursue

> 1) have a tag or relation with a meaning "This sidewalk is separated for 
>   geographical precision, but for routing, it is permanently connected 
> to the road".
> 2) Have at least one routing algorithm that can use it.
as that would complicate things further for both mappers and data users.
(I do agree though, that if we /were/ to map all sidewalks separately,
whether separated or not, these steps would have to be taken first.)

Maps (and map data) must abstract (and therefore, to some degree,
simplify, aggregate and interpret) reality. It must do so, not (or at
least not primarily) as a means to save data size and mapping effort,
but to be adequately useful at all. Taking it to the extreme, to make
that point obvious: The information "there is, here on this street, a
rectangular yellow paint application on the ground, and next to it
another one and another one and several more"* is much less useful than
the information "here on this street, there's a marked pedestrian crossing".

For sidewalks this might be less obvious, but there's an important
semantic difference between "there are a street and a sidewalk next to
each other, with unknown lateral relation to each other" and "there's a
street with a sidewalk at the right-hand side", because in the latter
case, you can (even though you might not be allowed to) walk from the
sidewalk onto the street or (even though you might not be allowed to)
park a car on the sideway or let someone unboard the car right onto the
sidewalk or let them board the car right from the sidewalk. Or you might
inadvertently trip and step onto the street. When they are physically
separated by space or a barrier, this isn't usually possible and thus
this warrants mapping the lateral adjacent and the physically separated
case so differently.

Given tools and maps that know about and use this convention to
interpret the data (and given data that actually sticks to the
convention) this will help blind or otherwise visually impaired people
just as much (or even more) than fully seeing-capable ones.

Regards,
Raphael

*remember the cantonal GIS that had street ground markings mapped with
the actual outlines of the colored area (i.e., dashed lines as series of
rectangles, arrows as 9-or-more-vertice polygons, "STOP" markings as
vectorization of the actual letters "S", "T", "O" and "P", etc.) instead
of their semantics in traffic? While very accurate, that probably isn't
useful, unless the road traffic department had planned to renew those
markings with a street-wide inkjet printer vehicle or something.

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