TimeCalendar, TimeCalendarEra Calendars and clocks .1 Overview

164 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. gml:TimeCalendar may be used in any position that a gml:TimeReferenceSystem is valid. Its content model is defined as follows: complexType name=TimeCalendarType complexContent extension base=gml:TimeReferenceSystemType sequence element name=referenceFrame type=gml:TimeCalendarEraPropertyType maxOccurs=unbounded sequence extension complexContent complexType 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 implements ISO 19108 TM_CalenderEra see D.2.5.8 and ISO 19108:2002, 5.3.2.1 and is declared as follows: element name=TimeCalendarEra type=gml:TimeCalendarEraType Its content model is defined as follows: complexType name=TimeCalendarEraType complexContent extension base=gml:DefinitionType sequence element name=referenceEvent type=gml:StringOrRefType element name=referenceDate type=gml:CalDate element name=julianReference type=decimal element name=epochOfUse type=gml:TimePeriodPropertyType sequence extension complexContent complexType gml:TimeCalendarEra inherits basic properties from gml:DefinitionType see 15.2.1 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. Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 165

14.4.4.3 TimeCalendarPropertyType, TimeCalendarEraPropertyType

gml:TimeCalendarPropertyType provides for associating a gml:TimeCalendar with an object: complexType name=TimeCalendarPropertyType sequence minOccurs=0 element ref=gml:TimeCalendar sequence attributeGroup ref=gml:OwnershipAttributeGroup attributeGroup ref=gml:AssociationAttributeGroup complexType gml:TimeCalendarEraPropertyType provides for associating a gml:TimeCalendarEra with an object: complexType name=TimeCalendarEraPropertyType sequence minOccurs=0 element ref=gml:TimeCalendarEra sequence attributeGroup ref=gml:OwnershipAttributeGroup attributeGroup ref=gml:AssociationAttributeGroup complexType

14.4.4.4 TimeClock

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 implements ISO 19108 TM_Clock see D.2.5.8 and ISO 19108:2002, 5.3.2.4 and is declared as follows: element name=TimeClock type=gml:TimeClockType substitutionGroup=gml:TimeReferenceSystem gml:TimeClock may be used in any position that a gml:TimeReferenceSystem is valid. Its content model is defined as follows: complexType name=TimeClockType final=all complexContent extension base=gml:TimeReferenceSystemType sequence element name=referenceEvent type=gml:StringOrRefType element name=referenceTime type=time element name=utcReference type=time element name=dateBasis type=gml:TimeCalendarPropertyType minOccurs=0 maxOccurs=unbounded sequence extension complexContent complexType 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. 166 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved.

14.4.4.5 TimeClockPropertyType

gml:TimeClockPropertyType provides for associating a gml:TimeClock with an object: complexType name=TimeClockPropertyType sequence minOccurs=0 element ref=gml:TimeClock sequence attributeGroup ref=gml:OwnershipAttributeGroup attributeGroup ref=gml:AssociationAttributeGroup complexType 14.4.5 Ordinal temporal reference systems 14.4.5.1 Overview 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 varying resolution. An ordinal temporal reference system whose component eras are not further subdivided is effectively a temporal topology 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 topology 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.

14.4.5.2 TimeOrdinalReferenceSystem, TimeOrdinalEra

gml:TimeOrdinalReferenceSystem implements ISO 19108 TM_OrdinalReferenceSystem see D.2.5.10 and ISO 19108:2002, 5.3.4 by adding one or more gml:component properties to the generic temporal reference system model. It is declared as follows: element name=TimeOrdinalReferenceSystem type=gml:TimeOrdinalReferenceSystemType substitutionGroup=gml:TimeReferenceSystem gml:TimeOrdinalReferenceSystem may be used in any position that a gml:TimeReferenceSystem is valid. Its content model is defined as follows: complexType name=TimeOrdinalReferenceSystemType complexContent extension base=gml:TimeReferenceSystemType sequence element name=component type=gml:TimeOrdinalEraPropertyType maxOccurs=unbounded sequence extension complexContent complexType gml:TimeOrdinalEra implements ISO 19108 TM_OrdinalEra see D.2.5.10 and ISO 19108:2002, 5.3.4. Its content model follows the pattern of gml:TimeEdge see 14.3.2.8, inheriting standard properties from gml:DefinitionType see 15.2.1, and adding gml:start, gml:end and gml:extent properties, a set of