[Talk-us] Differences with USA admin_level tagging

Frederik Ramm frederik at remote.org
Tue Jul 11 19:03:56 UTC 2017


Hi,

On 07/11/2017 08:18 PM, OSM Volunteer stevea wrote:
> I'm glad Adam brings up the topic of Gores, as I'm also unclear on how such "holes" get "punched into" larger (multi)polygons via tagging.  For example, I am "sort-of-sure" (but not positive) that in Vermont, a "gore" (or grant, location, purchase, surplus, strip...usually the result of "leftovers" from survey errors) get a tag of admin_level=4 to accurately reflect that the governmental administration happens via state-level bureaucracy.

I think there might be a misunderstanding here and I would like to chip
in before this gets out of hand, even if I don't have any specialist USA
knowledge:

If you have an admin_level 4 entity - like a state - then the boundaries
with admin_level 4 are the outer demarcation of that, i.e. they separate
the area where the state is responsible from the area where the state is
not responsible.

The only reason to have an admin level 4 boundary inside a state, would
be if there was somehow a piece of *federal* territory inside the state.
Only then would the state have a "hole" in it that would be tagged with
admin level 4! An area inside the state that is state-governed because
of a lack of admin_level 5+ entity does not need its own boundary. It is
defined by the boundaries of the admin_level 5+ entities that surround it.

> without using a multipolygon relation, 

You will be using boundary relations which are practically identical to
multipolygon relations. Any attempt to create a "lower 48 states"
polygon without relations would hit the 2000 node limit.

> is it correct within OSM to tag, say a very large "lower 48 states" polygon with admin_level=2 AND ALSO tag admin_level=2 on, say, a national_park inside of it

That would only be correct if the national park was *not* part of the
lower 48 states but somehow part of another nation.

I'm not 100% sure what you want to achieve but think of it like coloured
polygons. If you have an admin_level 2 area for the USA, think of that
as one colour, and then you have a lot of states, each with a different
colour. In those areas where the "USA colour" shines through, because
they're not covered by any state, that's automatically federal territory
and you do *not* want an admin_level 2 boundary surrounding that
(because then not even the "USA colour" would shine through, there would
be nothing there).

> Guidance by knowledgable people with real answers might guide us on a number of these situations, not just "Gores" (et al) but other kinds of "hole" tagging without multipolygons.  

If you mean not only "without multipolygons" but "without boundary
relations" too then I think you should stop right here and leave it to
people who can work with relations.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"



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