[OSM-talk] administrative boundaries and is_in

[Robin Paulson]

can someone explain a few things about the way boundaries work, and
their relation to the is_in key?

as far as i can tell, when a location (say the suburb of balham, in
london) is added to the map, the is_in tag needs to be set, multiple
times. in this case, it would be set as follows:

is_in:Westminster (...i think)
is_in:greater london
is_in:england
is_in:united_kingdom
is_in:British_Isles
is_in:Great_Britain
is_in:Europe
...etc.

which seems counter-intuitive, not to mention requiring huge amounts
of work. do we set this for every item - roads, churches,
supermarkets,....thousands of other items?
is there anything underway to enable OSM to calculate where an object
is, based upon knowledge of administrative boundaries - after all,
they are only a polygon-shaped bounding box?

if i set is_in of balham to london, and the is_in of london to
england, does osm know that balham is therefore in england, by
cascading the is_in values? and so on, for as many levels as we
define?

my second, related, point concerns boundaries that coincide with
coastlines: do we need to trace over the coastline of a
country/city/suburb to define an unbroken loop for each administrative
areas, or can OSM work out for itself that the coastline forms the
rest of the boundary? what about if the entire boundary is defined by
coastline? are these questions only relevant if and when items are
automagically aware of their location?

thanks for any help