[Tagging] Mapping time zones as geometries (relations)

Frederik Ramm frederik at remote.org
Tue Mar 7 06:43:20 UTC 2017


On 03/07/2017 02:06 AM, Kevin Kenny wrote:
> There are reasonable use cases for wanting to take a timezone name and
> get back a multipolygon

Question is, does (the core database of) OSM have to fulfil these use cases!

> or to get a list of name and multipolygon for
> all the timezones (or all the timezones on a given map). That's hard to
> do unless there are relations for the timezones. Not impossible, we
> could query administrative boundaries for the tag, but that's likely to
> be SLOW since the tag will at best be in hstore and not indexed.
> Grouping disjoint items, such as "all the administrative regions in a
> time zone" seems tailor-made for relations.

In my opinion, no. We've been there - people (ab)using relations like
some kind of bookmark, to make data retrieval easier (e.g. a relation
"all cycleways in XY city" just because it was slow/difficult to
retrieve them otherwise).

You would typically use a relation to group things when they can be in
more group than one, and therefore tagging the membership on the
individual object becomes cumbersome - cycle, bus, or hiking routes are
a prime example, since the same street/way can be part of several of them.

Not so with time zones, I should hope; a region can only be associated
with exactly one (not 0, not more than one) time zone. Hence adding a
time zone tag to the object is the easiest way to maintain the data and
to ensure that you don't accidentally have two!

I agree it might make things a little more difficult for the consumer
but an overpass query can quickly find every boundary relation tagged
with a certain time zone, and I'm sure sooner or later specialist sites
will take the data and prepare daily updated json files or whatever that
one can overlay on a map.


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

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