Basic temporal reference system, TimeReferenceSystem TimeCoordinateSystem

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 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.