Boundary relations [1] can have members that are the boundary itself,
thus the geometrical part of this boundary, as well as further details,
in particular an admin_centre and a label, which are both extremely well
accepted, and (disputed) a subarea.
The valid geometrical members are 'outer' (5M) and 'inner' (45T), and
the deprecated <empty> (62T), enclave (0) and exclave (0) [2]. enclave
and exclave are extinct since 2013, according to wiki discussion.
admin_centre is used on nodes (184T), ways (142) and relations (16).
It is defined in the wiki so far as a node for the "capital, county seat
etc., usually a town, city or village (depending of the boundary level,
see place=*)"
Now, for admin levels relating to counties and towns, the seat of the
admin_centre is typically the town hall, which is often mapped as a
building outline, thus a way. It carries further useful tags such as the
name of the particular administration and its address.
There is no logical reason why this townhall should not be added to the
boundary relation as a way. That was proposed on the wiki discussion
page a few months ago.
However, mapping it this way triggers a problem in carto [3], which
relates upstream to an issue in osm2pgsql [4]. Apparently any way member
in a boundary relation is treated as part of the geometry (catch-all),
instead of processing only those that are defined as such
(white-listing). Catch-all rules are frowned upon in different
situations as they are prone to errors.
Tu summarize, I propose to allow admin_centre as ways in boundary
relations, and fix the catch-all software issue.
[1] https://wiki.openstreetmap.org/wiki/Relation:boundary
[2] http://taginfo.openstreetmap.org/relations/boundary#roles
[3] https://github.com/gravitystorm/openstreetmap-carto/issues/2559
[4] https://github.com/openstreetmap/osm2pgsql/issues/678
--
tom
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