TimePeriodPropertyType Temporal geometry .1 Introduction

154 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. Values based on calendars and clocks use lexical formats that are based on ISO 8601, as described in XML Schema Part 2:2001. A decimal value may be used with coordinate systems such as GPS time or UNIX time. A URI may be used to provide a reference to some era in an ordinal reference system 3 . In common with many of the components modelled as data types in the ISO 19100 series of International Standards, the corresponding GML component has simple content. However, the content model gml:TimePositionType is defined in several steps the details of the mapping to ISO 19108 TM_Position are described in D.2.5.5: complexType name=TimePositionType final=all simpleContent extension base=gml:TimePositionUnion attribute name=frame type=anyURI default=ISO-8601 attribute name=calendarEraName type=string attribute name=indeterminatePosition type=gml:TimeIndeterminateValueType extension simpleContent complexType Three XML attributes appear on gml:TimePositionType: A time value shall be associated with a temporal reference system through the frame attribute that provides a URI reference that identifies a description of the reference system. Following ISO 19108, the Gregorian calendar with UTC is the default reference system, but others may also be used. Components for describing temporal reference systems are described in 14.4, but it is not required that the reference system be described in this manner, as the reference may refer to any resource that may be identified with a URI. For time values using a calendar containing more than one era, the optional calendarEraName attribute provides the name of the calendar era. Inexact temporal positions may be expressed using the optional indeterminatePosition attribute. This takes a value from an enumeration defined as follows: simpleType name=TimeIndeterminateValueType restriction base=string enumeration value=after enumeration value=before enumeration value=now enumeration value=unknown restriction simpleType These values are interpreted as follows:  ―unknown‖ indicates that no specific value for temporal position is provided;  ―now‖ indicates that the specified value shall be replaced with the current temporal position whenever the value is accessed;  ―before‖ indicates that the actual temporal position is unknown, but it is known to be before the specified value;  ―after‖ indicates that the actual temporal position is unknown, but it is known to be after the specified value. 3 e.g. a geological epoch.