Overview Temporal reference systems

162 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved.

14.4.2 Basic temporal reference system, TimeReferenceSystem

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 see 15.2.1 with one additional property, gml:domainOfValidity. It is implemented as follows: element name=TimeReferenceSystem type=gml:TimeReferenceSystemType substitutionGroup=gml:Definition gml:TimeReferenceSystem may be used in any position that a gml:Definition is valid. Its content model is defined as follows: complexType name=TimeReferenceSystemType complexContent extension base=gml:DefinitionType sequence element name=domainOfValidity type=string sequence extension complexContent complexType EXAMPLE This element might appear in an instance document as follows: gml:TimeReferenceSystem gml:id=JulianCalendar gml:description xlink:href=http:aa.usno.navy.mildatadocsJulianDate.html gml:nameJulian Calendargml:name gml:domainOfValidityWestern Europegml:domainOfValidity gml:TimeReferenceSystem

14.4.3 TimeCoordinateSystem

A temporal coordinate system shall be based on a continuous interval scale defined in terms of a single time interval. gml:TimeCoordinateSystem implements ISO 19108 TM_CoordinateSystem see D.2.5.9 and ISO 19108:2002, 5.3.3 with the exceptions specified below and is declared as follows: element name=TimeCoordinateSystem type=gml:TimeCoordinateSystemType substitutionGroup=gml:TimeReferenceSystem gml:TimeCoordinateSystem may be used in any position that a gml:TimeReferenceSystem is valid. Its content model is defined as follows: complexType name=TimeCoordinateSystemType complexContent extension base=gml:TimeReferenceSystemType sequence choice element name=originPosition type=gml:TimePositionType element name=origin type=gml:TimeInstantPropertyType choice element name=interval type=gml:TimeIntervalLengthType sequence extension complexContent complexType Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 163 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 see 14.2.2.7, or using the property gml:origin whose model is gml:TimeInstantPropertyType see 14.2.2.4; this permits more flexibility in representation and also supports referring to a value fixed elsewhere;  the interval uses gml:TimeIntervalLengthType, defined in 14.2.2.8. EXAMPLE Coordinate systems might be described in data instances as follows: gml:TimeCoordinateSystem gml:id=Laser36 gml:descriptionTime scale used during a laser experimentgml:description gml:nameLaser timescale 36gml:name gml:domainOfValidityLaser laboratorygml:domainOfValidity gml:origin gml:TimeInstant gml:timePosition2002-11-28T12:50:00+08:00gml:timePosition gml:TimeInstant gml:origin gml:interval unit=―second‖ radix=―10‖ factor=―12‖1.0gml:interval gml:TimeCoordinateSystem gml:TimeCoordinateSystem gml:id=geologyMa gml:nameGeological time systemgml:name gml:domainOfValidityEarthgml:domainOfValidity gml:origin gml:TimeInstant gml:description xlink:href=http:www.c14dating.comagecalc.htmlConventional origin used for carbon dating. Equivalent to present for other radiometric dating techniques which have much lower precision.gml:description gml:timePosition1950gml:timePosition gml:TimeInstant gml:origin gml:interval unit=―year‖ radix=―10‖ factor=―-6‖1.0gml:interval gml:TimeCoordinateSystem 14.4.4 Calendars and clocks 14.4.4.1 Overview Calendars and clocks are both based on interval scales. A calendar is a discrete temporal reference system that provides a basis for defining temporal position to a resolution of one day. 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. Calendars have a variety of complex internal structures. This schema defines a simple external calendar interface. Every calendar provides a set of rules for composing a calendar date from a set of elements such as year, month, and day. In every calendar, years are numbered relative to the date of a reference event that defines a calendar era. A single calendar may reference more than one calendar era.

14.4.4.2 TimeCalendar, TimeCalendarEra

A calendar is a discrete temporal reference system that provides a basis for defining temporal position to a resolution of one day. gml:TimeCalendar implements ISO 19108 TM_Calender see D.2.5.8 and ISO 19108:2002, 5.3.2.3 and is declared as follows: element name=TimeCalendar type=gml:TimeCalendarType substitutionGroup=gml:TimeReferenceSystem