Advancing our ability to tell time

Contributed by: 
Mark Hedley and Chris Little

Last summer, members of the OGC Technical Committee’s Temporal Domain Working Group (Temporal DWG) had a long and interesting dialog about representation of coordinate reference systems (CRSs), the need for aligned spatial and temporal CRSs, and the eventual need for dynamic datums[1]. As a result, the Temporal DWG recommended the formation of a new Standards Working Group (SWG) to address such issues. The Temporal Well Known Text (WKT) for Calendars  Standards Working Group (“Temporal WKT SWG”) was officially chartered in December 2015 and now welcomes membership and encourages public participation. Those who are interested can subscribe to the Temporal WKT SWG mailing list. Those who are interested can also subscribe to the Temporal DWG mailing list.

 

As revealed in that long discussion and as explained on the Temporal DWG wiki page, there is a need in the climate domain and other domains for data structures for modeling and encoding time. Such data structures need to be expressive enough for:

  • Coordinate systems and datums/epochs (years/days since 0 BCE, milliseconds since 1/1/1970, days since last Sunday, ...);

  • Calendars (Proleptic-Gregorian, Julian, Mayan, 360 day year, Hijri Calendar, etc);

  • Notation (ISO 8601 or restrictive profile, etc); and

  • Proxies for time (number of tree rings, mm of ice or sediment core, etc).

 

Cutting across these are also the ideas of:

  • Multiple time dimensions (e.g. ‘real’ time vs forecast time);

  • Relative and absolute time; and

  • Points, periods and durations.

 

The purpose of the new Temporal WKT SWG is to adapt a set of existing standards to enable the capability to represent and encode temporal metadata within data sets and protocols that use a customised calendar. The Temporal WKT SWG will deliver:

  • An OGC standard defining string representations of temporal entities, valid for various calendars, and

  • An OGC extension to ISO 19162: Well Known Text for Coordinate Reference Systems (CRSs), to enable the establishment of Temporal CRSs for various calendars.

 

The purpose of this SWG is to adapt a set of existing standards to provide the capability to represent and encode temporal metadata within data sets and protocols that use customised or non-Gregorian calendars of various kinds. This work will address a range of communities who use time representations based on calendars which are not the Gregorian calendar as recognised by the International Standards Organisation (in ISO 8601).

 

In order to provide interoperable representations of this temporal metadata, extensions to the current metadata standardisation provision in ISO 8601 are required. This has particular business value for the Climate Science community, who make extensive use of such temporal representations, and for the wide range of OGC communities who are looking to make use of climate data to interact with their own data sets to facilitate further applications.

 

It is expected that this work will also involve proposing adaptations and extensions to other existing OGC standards and ISO standards besides ISO 8601. The ISO standards that influence this domain and may require updating include:

  • ISO 8601 - Data elements and interchange formats – Information interchange – representation of dates and times;

  • ISO 19108 - Geographic information – Temporal schema;

  • ISO 19111 - Geographic information – Spatial referencing by coordinates;

  • ISO 19162 - Well Known Text for Coordinate Reference systems; and

  • OGC 13-102r2 Name type specification – Time and index coordinate reference system definitions.

 

The new SWG work programme will deliver these Key Capabilities.

  • Formal definitions of calendars and temporal coordinate systems.

  • Encoding of calendar based temporal coordinate reference system instances in ISO 19162 Well Known Text.

  • Formal definitions of temporal entities (instant, period) with respect to calendars and temporal coordinate systems. Implementations of the standard will be able to formally define: calendars, an instant in time, a customised temporal coordinate reference system, a customised temporal CRS in Well Known Text, and a time period.

 

Defining a “no leap second” Gregorian calendar will be required, as many software implementations handle standard time this way. Conversion between date-times in different calendars is out of scope.

 

The SWG’s work will be complete when use cases defined in the SWG Charter are able to be realized by applications that implement the candidate standards and extended standards.


 

Guest Blog post by Mark Hedley and Chris Little of the UK Met Office

[1]Geodetic datums define the size and shape of the earth and the origin and orientation of the coordinate systems used to map the Earth. “Dynamic datums” assume changes in the size and/or shape of the Earth. The Earth can be described as an “oblate ellipsoid”, but it is neither a perfect nor a stable oblate ellipsoid.