The method for identifying a temporal position is specific to each temporal reference system.  gml:TimePositionType supports the description of temporal position according to the subtypes described in ISO 19108.<br/>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 . <br/>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.<br/>Three XML attributes appear on gml:TimePositionType:<br/>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, as the reference may refer to anything that may be indentified with a URI.  <br/>For time values using a calendar containing more than one era, the (optional) calendarEraName attribute provides the name of the calendar era.  <br/>Inexact temporal positions may be expressed using the optional indeterminatePosition attribute.  This takes a value from an enumeration.<br/>