Tag:boundary=timezone

A time zone boundary represents the extent of a region that observes a uniform standard time.

Rationale

Time zone geometries are useful in a number of applications that handle both location and time. For example:

• Calendaring software needs to correctly offset the time of an event or appointment based on the time zone at the location where it will take place, especially when a user in one time zone sends an invitation to a user in a different time zone.
• A search engine or router may need to convert an opening_hours=* value from local times to the user's time zone.
• A router needs to factor in time zone changes when calculating the estimated time of arrival, especially when the route uses a public transit service that runs on a published schedule.
• A mobile or embedded operating system needs to automatically adjust the clock when traveling across a time zone boundary.

The inclusion of time zone boundaries in OpenStreetMap has been controversial because some see them as unverifiable constructs or redundant to other data already mapped in OpenStreetMap. [1][2] The most well-known time zone boundaries follow national or regional administrative boundaries (boundary=administrative), and some time zone boundaries are exactly coincident with a single administrative boundary. In principle, this data is adequately captured by the timezone=* tag on the administrative boundary.

However, some time zone boundaries follow very local administrative boundaries. For example, Elko County, Nevada, generally observes Pacific Time, but the border city of West Wendover observes Mountain Time as an exception. A standard time zone boundary sign is posted at the city limits. The area outside West Wendover does not lie inside any city-level boundary that could be tagged with a different timezone=*. Therefore, in the absence of time zone boundaries, a data consumer would have to inspect every administrative boundary for potential exceptions.

Moreover, some time zone boundaries do not follow administrative boundaries at all but instead follow on-the-ground features that are mapped in OpenStreetMap. For example, in the Standing Rock Indian Reservation, the centerline of North Dakota Highway 31 divides the Mountain and Central time zones. In Florida, the main channel of the Gulf Intracoastal Waterway divides the Central and Eastern time zones. Time zones affected by these idiosyncrasies are more likely to be mapped as boundary relations than those that strictly follow administrative boundaries.

How to map

Create a relation and add the type=boundary boundary=timezone tags to it. See Relation:boundary for more information on how to map a boundary.

Tags to use in combination

• Name of the time zone could be specified by name=* (Central European Time, Moscow Time, Pacific Time Zone...)
• For abbreviation code for time zone use ref=*, e.g. ref=AST for Atlantic Standard Time Zone, ref=CET for Central European Time etc.
• Use utc=* to tag Coordinated Universal Time offset of the time zone in ±hh:mm format, e.g. utc=+01:00 for Central European Time, utc=-07:00 for Mountain Time Zone etc.
• Use timezone=* to specify which IANA time zone database time zone applies.

Using the data

Evan Siroky's Timezone Boundary Builder project derives geometries from OpenStreetMap's boundary=timezone relations and timezone=* tags on boundary=administrative relations. It is the successor to Eric Muller's tz shapefile, which was discontinued in 2016. Timezone Boundary Builder is the principal open data source used with the tz database, a public domain, collaborative database of time zone metadata sponsored by ICANN. The tz database is an industry standard used by most modern operating systems, standard libraries of programming languages, database servers, and firmware.

OpenHistoricalMap

OpenHistoricalMap is also adding historical time zone boundaries, beginning with comprehensive coverage of the United States since 1919. Unlike in OSM, these time zone boundaries correspond one-for-one with legally defined boundaries, independently of daylight saving time or other deviations from standard time. Boundary adjustments are modeled as overlapping boundary relations corresponding to the resulting time zone, rather than the tz database convention of smaller boundaries corresponding to the adjusted region.

External links

• List of time zones by country