[Tagging] Boundary Relations. What's a subarea used for?

André Pirard A.Pirard.Papou at gmail.com
Mon Jan 12 00:22:54 UTC 2015


Hi Fred,

I have time for one topic tonight.

On 2015-01-11 22:04, Frederik Ramm wrote :
>> Moreover, it is straightforward for a consumer program like OSM.org
>> to use the subareas to draw the outline of the regions, provinces,
>> etc. inside the country map or to do other things like measuring
>> borderlines. 
> This can be done without a subarea hierarchy; a plain admin_level
> tagging is fully sufficient for that.
I didn't want to speak of admin_level, but if you do...

First, let us note that the question is "What's a subarea used for?" and
not "can it be done otherwise?".
I do not reject other (each one's own) solutions but I claimed that
subareas are clearly easier.
Note that the subarea (more easily) identifies the underlying relations
clearly by number while the admin_level method sends the programs
fumbling into the map in search for levels to be determined by
comparison (I'm skipping details(2)).

But secondly, what I wanted to say, the admin_level is an unnecessary
concept (1).
Using a number is a burden: how do we insert a level between 6 and 7,
and the number of discussions about level numbers is a proof of that.  I
think that the only use of a number is for the borderline width on the
map and I'm surprised that no one yet said that it's tagging for the
renderer (kidding;-) ). So, that's one thing to keep.
But, as I said, otherwise (almost) unnecessary when subareas are used,
because the subareas themselves define the nesting.
If a boundary relation is a subarea of another, it has the next higher
"former-level" to put it in a figurative way.
Now, why "almost"?
Because a level slot might be empty somewhere and there must be one way
to spot the gap.
Here we can  kill two birds with one stone with what I'll put in the
self-explanatory namespace language:
administrative:type:name=province  (not a proposition)
- all the relations named "province" etc. must be at the same level, so
we can detect gaps in between two levels.
- and 'ray we finally can write  "province Liège  arrondissement Liège 
commune Esneux" beside a borderline without any confusion about what the
names are.

This was a part of the answer to the question "What can a subarea be
used for".

André.


(1) BTW, it sounds like it applies only to boundary
<https://wiki.openstreetmap.org/wiki/Key:boundary>=administrative
<https://wiki.openstreetmap.org/wiki/Tag:boundary%3Dadministrative> so,
how do we nest other types of boundaries?
(2) BTW2, a _level=N+n boundary must be topologically inside a _level=N
boundary.  A subarea boundary needs not.


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