[Tagging] Mapping time zones as geometries (relations)

Frederik Ramm frederik at remote.org
Mon Mar 6 00:30:33 UTC 2017


   I would like to start a discussion about the mapping of time zones.

There are currently 80 relations in OSM tagged as boundary=timezone
using between them over 10,000 ways.

I think that mapping these time zones as geometries has several issues:

1. It introduces yet another un-observable object type into OSM. When I
asked about the source of the data, one person quipped "well you can
observe it on a local clock", but the people adding time zone boundaries
don't seem to actually do that - they copy the information from somewhere.

2. It is for the most part unnecessary noise; almost all time zone
boundaries run along administrative boundaries, so you could just add a
timezone tag to the administrative entity instead of mapping a whole
separate geometry.

3. It complicates editing; anyone touching one of the aforementioned
10,000 ways (which are for the most part also used by admin boundaries)
will be confronted with the time zone relation and will have to decide
what to do with it.

4. It creates very large polygons that may be difficult to edit.

Here's a list of time zones currently mapped in OSM (with relation ID)

3563652 Московское время
3563678 Екатеринбургское время
3563726 Красноярское время
3563727 Омское время
3563885 Иркутское время
3563886 Якутское время
3563937 Владивостокское время
4297465 Самарское время
6426325 Cherry County - Chicago TZ part
6426796 Idaho Denver TZ part
6426797 Idaho Los Angeles TZ part
6428675 Alaska - Timezone America/Adak
6428690 Alaska - Timezone America/Anchorage
6428730 Alaska - Timezone America/Yakutat
6428802 Alaska - Timezone America/Juneau
6428803 Alaska - Timezone America/Sitka
6428944 Canada - Timezone America/Pangnirtung
6430296 Malaysia - Kuala Lumpur Timezone
6430370 Kiribati - Kiritimati timezone
6430371 Kiribati - Tarava timezone
6430484 Greenland - timezone America/Danmarkshavn area
6430503 Greenland - timezone America/Thule area
6430514 Greenland - timezone America/GodThab area
6430515 Greenland - timezone America/Scoresbysund area
6430658 Хандыгское время
6440860 America/Tijuana Timezone
6440942 America/Merida Timezone
6440955 America/Cancun Timezone
6440975 America/Bahia_Banderas Timezone
6441066 America/Mazatlan Timezone
6442501 America/Mexico_City Timezone
6442514 America/Santa_Isabel Timezone
6442531 America/Hermosillo Timezone
6442686 America/Matamoros Timezone
6442687 America/Ojinaga Timezone
6442693 America/Chihuahua Timezone
6442726 America/Monterrey Timezone
6442820 America/Edmonton Timezone part
6442821 America/Regina Timezone
6442822 America/Winnipeg Timezone part
6445625 America/Eirunepe Timezone
6445626 America/Manaus Timezone
6445682 America/Belem Timezone
6445683 America/Santarem Timezone
6445860 Asia/Dushanbe Timezone part
6446092 Arizona - America/Phoenix Timezone
6446108 America/Atikokan Timezone
6446109 America/Winnipeg Timezone
6446170 America/Cambridge_Bay Timezone
6446189 America/Rankin_Inlet Timezone
6446190 America/Resolute Timezone
6451096 Atlantic/Azores Timezone
6451097 Atlantic/Madeira Timezone
6451098 Europe/Lisbon Timezone
6451458 America/Coral_Harbour Timezone
6451459 America/Iqaluit Timezone
6483867 Ontario - America/Toronto Timezone
6483978 America/Creston Timezone
6483979 America/Dawson_Creek Timezone
6483980 America/Edmonton Timezone
6483981 America/Vancouver Timezone
6485969 Asia/Sakhalin Timezone
6491030 America/Argentina/Buenos_Aires Timezone
6491031 America/Argentina/Catamarca Timezone
6491032 America/Argentina/Rio_Gallegos Timezone
6491033 America/Argentina/Salta Timezone
6491034 America/Argentina/Ushuaia Timezone
6491133 Atlantic/Canary Timezone
6491134 Europe/Madrid Timezone
6491494 America/Chicago Timezone
6496417 America/New_York Timezone
6496587 America/Indiana/Indianapolis Timezone
6496665 America/Boise Timezone
6496694 America/Los_Angeles Timezone
6498476 America/Menominee Timezone
6498563 America/Detroit Timezone
6498592 America/Nipigon Timezone
6498593 America/Thunder_Bay Timezone
6594626 Магаданское время
6594635 Камчатское время

And here's them in a map:


Now when you overlay the administrative boundaries on levels 2 and 4,
only very few lines remain:


And when you go up to admin level 6:


Assuming that these boundaries are actually correct, the only places in
which time zone boundaries don't follow admin boundaries are:

* In South America, the Western boundary of the Manaus timezone and the
Eastern boundary of the Santarem timezone;

* In North America, the line between the Hermosillo and Santa Isabel
timezones, the Phoenix timezone, a small part of the boundary between
the Los Angeles and Boise time zones in Malheur county, the Chicago
Timezone in Cherry County, a piece of the Los Angeles timezone in Idaho,
the line between Vancouver and Dawson Creek respectively Creston
timezones, two incursions into the Regina timezone for Edmonton and
Winnipeg, two micro timezones called Nipigon and Thunder Bay, as well as
the Iqaluit and Rankin Inlet timezones far North. (Strangely, around
Alaske the timezone boundary seems to include a few miles of water that
are not included in the admin boundary - a quirk in mapping or a quirk
in time zone definitions?)

As you can see in the images, these lines make up just a fraction of
time zone boundaries (and it is quite possible that some of these just
shine up because the matching admin6 boundaries haven't yet been mapped).

I fear that the presence of *some* time zone relations will entice
people to map even more of them, unnecessarily introducing extra
geometries into our database when you could easily build time zone
geometries by either (a) keeping an external database that says "Time
Zone X consists of the following administrative units" and then
collecting the units from OSM, or (b) adding a time zone tag to
adminsitrative entities in OSM - namely, the highest entitiy that still
has the same time zone which will often be a whole country, sometimes an
admin-4 entity and very rarely an admin-6 entity.

If there are really some time zone bits and pieces that don't follow
admin boudnaries (maybe they're marked with signs on the ground
"caution, wind back your clock all ye who pass here"?) then maybe those
few bits can be mapped as individual objects... but please, don't
inflate OSM with time zone geometries just because "it's easier to
consume that way" (which is the gist from a discussion I had with a time
zone mapper).

I am worried because with the same reasoning, you could start mapping
things like "states that have signed up to the XYZ treaty", "states in
which XYZ is legal/illegal", or anything else. It's a slippery slope,
and *if* time zones are mapped in OSM at all, I'd rather not have them
mapped as giant geomtries in their own right, but as properties.

I have recently deleted a few time zone relations in the US and,
grudgingly, reinstated them because I had to accept that a prior
discussion was called for. (Changeset discussion here
https://www.openstreetmap.org/changeset/45644359), and I also requested
the deletion of
http://wiki.openstreetmap.org/wiki/Tag:boundary%3Dtimezone which
prompted verdy_p to write a small novel on the talk page there.

I find the subject of time zones hugely interesting and seeing the
quirks mapped above has already sent me on a journey reading up on the
details, but I still think that the mapping of time zones as geometries
should be discouraged in OSM.

What do you think?


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

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