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

[Frederik Ramm]

Hi,

On 01/11/2015 09:42 PM, André Pirard wrote:
> Look at the Belgium relation
> <http://www.openstreetmap.org/relation/52411> and, while hiding the
> subareas (in left pane), try to figure with that map the *administrative
> tree* (regions, provinces) using the borderlines.  You won't.

www.openstreetmap.org is not a browser for administrative hierarchies,
so it doesn't surprise that browsing administrative hierarchies is not
easy on www.openstreetmap.org.

There are services optimised for browsing adminstrative hierarchies and
which do the required preprocessing, totally independent of a "subarea"
relation:

http://forum.openstreetmap.org/viewtopic.php?id=24132

(live site down for maintenance as I write this, but:
https://osm.wno-edv-service.de/boundaries)

> 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.

> So, it looks like masochism somehow to tag borderlines for anything else
> than the lowest level. From it, one may have that program compute the
> borderlines of every relations upwards.

This is very likely true only for a few countries. Many countries have,
for example, special administrative regions around their capital cities
which are not part of any sub-national admin tree. Another thing is sea
boundaries; the territorial waters of a country are sometimes only part
of the country itself, sometimes also part of lower-level administrative
units, but this stops at some point - a country is, therefore, usually
more than the "sum of its parts".

> I won't write too much in one article but I'll add this.  What's that
> aversion against redundancy?
> Redundancy used as crosscheck is used in many place. For example, TCP,
> which is the transport protocol of the Internet, uses redundancy to make
> sure that these words I wrote came to you intact.  Same on a disk drive
> surface to make sure the recording is correct.  Etc.

True, but this also means that the person doing the mapping has more
work to do.

Something that I have often discussed with people but that has never
really materialised is: I would like to have an admin tree *outside* of
OSM. A structure that tells me what you're writing above - that there's
a country named Belgium, that it consists of two regions named Flanders
and Wallonia, that these in turn consist of ... and so on. Perhaps one
day Wikidata can give me such a list.

Then I could run a program that tries to match the geographic OSM data
set which tells me similar things but through geography instead of
explicit assignment, to the hierarchical and non-geographic dataset I
have from the other source, and anything that doesn't match is probably
a bug ("oops, there should be N cities in Wallonia but I have only found
M, and some unclaimed area...").

In terms of redundancy, that would be much better IMHO, we'd have an
external source to cross-reference, and someone who only knows that A is
a city in B but not where its boundary lies could nonetheless add this
information (to Wikidata or whatever).

Bye
Frederik

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