temporalReferenceSystems.xsd See ISO/DIS 19136 15.5. A value in the time domain is measured relative to a temporal reference system. Common types of reference systems include calendars, ordinal temporal reference systems, and temporal coordinate systems (time elapsed since some epoch). The primary temporal reference system for use with geographic information is the Gregorian Calendar and 24 hour local or Coordinated Universal Time (UTC), but special applications may entail the use of alternative reference systems. The Julian day numbering system is a temporal coordinate system that has an origin earlier than any known calendar, at noon on 1 January 4713 BC in the Julian proleptic calendar, and is useful in transformations between dates in different calendars. In GML seven concrete elements are used to describe temporal reference systems: gml:TimeReferenceSystem, gml:TimeCoordinateSystem, gml:TimeCalendar, gml:TimeCalendarEra, gml:TimeClock, gml:TimeOrdinalReferenceSystem, and gml:TimeOrdinalEra. A reference system is characterized in terms of its domain of validity: the spatial and temporal extent over which it is applicable. The basic GML element for temporal reference systems is gml:TimeReferenceSystem. Its content model extends gml:DefinitionType with one additional property, gml:domainOfValidity. A temporal coordinate system shall be based on a continuous interval scale defined in terms of a single time interval. The differences to ISO 19108 TM_CoordinateSystem are: - the origin is specified either using the property gml:originPosition whose value is a direct time position, or using the property gml:origin whose model is gml:TimeInstantPropertyType; this permits more flexibility in representation and also supports referring to a value fixed elsewhere; - the interval uses gml:TimeIntervalLengthType. A calendar is a discrete temporal reference system that provides a basis for defining temporal position to a resolution of one day. gml:TimeCalendar adds one property to those inherited from gml:TimeReferenceSystem. A gml:referenceFrame provides a link to a gml:TimeCalendarEra that it uses. A gml:TimeCalendar may reference more than one calendar era. The referenceFrame element follows the standard GML property model, allowing the association to be instantiated either using an inline description using the gml:TimeCalendarEra element, or a link to a gml:TimeCalendarEra which is explicit elsewhere. gml:TimeCalendarEra inherits basic properties from gml:DefinitionType and has the following additional properties: - gml:referenceEvent is the name or description of a mythical or historic event which fixes the position of the base scale of the calendar era. This is given as text or using a link to description held elsewhere. - gml:referenceDate specifies the date of the referenceEvent expressed as a date in the given calendar. In most calendars, this date is the origin (i.e., the first day) of the scale, but this is not always true. - gml:julianReference specifies the Julian date that corresponds to the reference date. The Julian day number is an integer value; the Julian date is a decimal value that allows greater resolution. Transforming calendar dates to and from Julian dates provides a relatively simple basis for transforming dates from one calendar to another. - gml:epochOfUse is the period for which the calendar era was used as a basis for dating. gml:TimeCalendarPropertyType provides for associating a gml:TimeCalendar with an object. gml:TimeCalendarEraPropertyType provides for associating a gml:TimeCalendarEra with an object. A clock provides a basis for defining temporal position within a day. A clock shall be used with a calendar in order to provide a complete description of a temporal position within a specific day. gml:TimeClock adds the following properties to those inherited from gml:TimeReferenceSystemType: - gml:referenceEvent is the name or description of an event, such as solar noon or sunrise, which fixes the position of the base scale of the clock. - gml:referenceTime specifies the time of day associated with the reference event expressed as a time of day in the given clock. The reference time is usually the origin of the clock scale. - gml:utcReference specifies the 24 hour local or UTC time that corresponds to the reference time. - gml:dateBasis contains or references the calendars that use this clock. gml:TimeClockPropertyType provides for associating a gml:TimeClock with an object. In some applications of geographic information — such as geology and archaeology — relative position in time is known more precisely than absolute time or duration. The order of events in time can be well established, but the magnitude of the intervals between them cannot be accurately determined; in such cases, the use of an ordinal temporal reference system is appropriate. An ordinal temporal reference system is composed of a sequence of named coterminous eras, which may in turn be composed of sequences of member eras at a finer scale, giving the whole a hierarchical structure of eras of verying resolution. An ordinal temporal reference system whose component eras are not further subdivided is effectively a temporal topological complex constrained to be a linear graph. An ordinal temporal reference system some or all of whose component eras are subdivided is effectively a temporal topological complex with the constraint that parallel branches may only be constructed in pairs where one is a single temporal ordinal era and the other is a sequence of temporal ordinal eras that are called "members" of the "group". This constraint means that within a single temporal ordinal reference system, the relative position of all temporal ordinal eras is unambiguous. The positions of the beginning and end of a given era may calibrate the relative time scale. gml:TimeOrdinalReferenceSystem adds one or more gml:component properties to the generic temporal reference system model. Its content model follows the pattern of gml:TimeEdge, inheriting standard properties from gml:DefinitionType, and adding gml:start, gml:end and gml:extent properties, a set of gml:member properties which indicate ordered gml:TimeOrdinalEra elements, and a gml:group property which points to the parent era. The recursive inclusion of gml:TimeOrdinalEra elements allow the construction of an arbitrary depth hierarchical ordinal reference schema, such that an ordinal era at a given level of the hierarchy includes a sequence of shorter, coterminous ordinal eras. gml:TimeOrdinalEraPropertyType provides for associating a gml:TimeOrdinalEra with an object.