[OSM-talk] Area-type objects and ways along its boundaries

[Ed Avis]

Nathan Edgars II <neroute2 <at> gmail.com> writes:

>>I am confused, what is the preferred approach to map boundaries of areas,
>>that in reality are bounded by a street or another way. Whether to draw the
>>boundary over this way (possibly using the same nodes), or to draw them in
>>parallel but close to this way?
> 
>Depends what the way is. If it's a street, the area most likely stops
>at the right-of-way line, and does not extend to the middle of the
>street, so it would be incorrect to extend the area into the street.
>On the other hand, if it's something like a fence or wall that forms
>the de facto edge of the area, it probably should be connected (if not
>using the same way with a relation for the area).

I agree with this, and would add that for 'pedestrian areas' such as squares
or plazas, which are often mapped as an area tagged with highway=pedestrian, it
makes sense for the area to extend up to the surrounding road, since you can walk
all the way to the middle of the road or even cross it.

>If the area is an administrative boundary (or a useless
>census-designated place), it shouldn't be joined to the way, even if
>it's drawn along it. Boundaries (at least in the US) are usually
>defined such that neither minor nor major realignments of highways
>will affect them.

That may be so, but if the boundary is defined to be in the same place as the
current line of the road, and the road is slightly inaccurate in OSM, then you
would expect that adjusting the road position by a few metres to reflect more
accurate survey data would also adjust the boundary.  If there were roadworks
and the road were changed in the real world, then of course it would need to be
moved separately from the administrative boundary.  But that is a fairly rare
event, whereas minor tweaks to object positions in OSM are quite common.

Similarly, if a boundary is defined to follow the course of a river, I would
expect it to share the same nodes as the river's centre line, so that tweaks
to the river's surveyed position would be reflected in the boundary.  Otherwise
you end up with a river and a boundary line that criss-crosses it, which can
be confusing.

-- 
Ed Avis <eda at waniasset.com>