[OSM-talk] is_in

[Abigail Brady]

On 5/19/07, David Earl <david at frankieandshadow.com> wrote:
> Ideally we'd just have
>   Fulbourn is_in=South Cambridgeshire
>   South Cambridgeshire is_in=Cambridgeshire
> etc and let the system work it out (I could certainly do so in the name
> finder if it was like this).

I would note that presently we have no good way of representing South
Cambridgeshire...

> But ideally the information shouldn't be needed at all, if we had
> administrative boundaries represented. Point in polygon tests would then
> enable this. But getting administrative boundaries in is hard (we can't
> trace them from modern maps). Actually the other way round might be
> interesting - estimate the bounaries by examining is_in, which wouldn't
> establish the boundary, but at low zooms could produce a useful map.

This could use the same mechanism that the existing postcode map
generator is using.  Additionally, it could use boundaries in OSM
where available - maybe show known boundaries as a solid line and
interpolated boundaries as a dashed line.  There are quite a lot of
areas where it's possible to come up with administrative (or indeed
postcode) boundaries just by surveying on the ground - especially when
the boundary runs along a road or other identifiable linear feature.
I can imagine an algorithm picking up the known good boundaries, the
is_in tags, and interpolating boundaries between these - I guess it'd
want to not just draw straight lines but would avoid crossing roads as
far as possible - so if you tag all the city/county borders it'd know
that estates extruding from the city should be politically within it.
Such a beast would be very complex though.

> Is is_in actually hierarchical anyway? Are district level authority
> boundaries always enclosed within regional ones (District, County in the UK
> and whatever equivalents there are elsewhere). For example, currently the
> South Cambridgeshire parliamentary constituency overlaps both Cambridge and
> South Cambridgeshire districts but encloses neither completely. (Not that
> I'm suggesting is_in should record parliamentary consituencies).

In England, districts don't cross shire county boundaries, and
districts and shire counties don't cross region boundaries.  There's a
couple of weird cases where this doesn't hold true with ceremonial
counties - in paticular where Stockton (unitary) is split between
Durham (ceremonial) and North Yorkshire (ceremonial).

-- 
Abi