Quantity Class Requirements Class: Basic Types and Simple Components Packages Requirements Class

SWE Common Data Model OGC 08-094r1

7.2.9 Time Class

The “Time” class is used to specify a component with a date-time representation and whose value is projected along the axis of a temporal reference frame. This class is also necessary to specify that a time value is expressed in a calendar system. This class derives from “AbstractSimpleComponent” and is shown below: AbstractSimpleComponent «Type» Time «property» + referenceTime: DateTime [0..1] + localFrame: TM_TemporalCRS [0..1] + uom: UomTime + constraint: AllowedTimes [0..1] + value: TM_Position [0..1] Figure 7.13 – Time Class Time is treated as a special type of continuous numerical quantity that can be either expressed as a scalar number with a temporal unit or a calendar date with or without a time of day. Consequently, this class has all properties of the “Quantity” class, plus some others that are specific to the treatment of time. As time is always expressed relative to a particular reference frame, the “referenceFrame” attribute inherited from the parent class “AbstractSimpleComponent” shall always be set on instances on this class unless the default ‘UTC’ is meant. Requirement http:www.opengis.netspecSWE2.0requml-simple-componentstime-ref-frame-defined Req 27. The “referenceFrame” attribute inherited from “AbstractSimple Component” shall always be set on instance of the “Time” class unless the UTC temporal reference system is used. Note that specifying the frame of reference is required even when using ISO notation because there can be ambiguities between several universal time references such as UTC, TAI, GPS, UT1, etc… Differences between these different time reference systems are indeed in the order of a few seconds and increasing, that is to say not negligible in various situations. Example J2000 is a well known epoch in astronomy and is equal to: - January 1, 2000, 11:59:27.816 in the TAI time reference system - January 1, 2000, 11:58:55.816 in the UTC time reference system - January 1, 2000, 11:59:08.816 in the GPS time reference system These offsets are not always constant and depend on the irregular insertion of leap seconds in UTC Copyright © 2011 Open Geospatial Consortium 33 OGC 08-094r1 SWE Common Data Model The “axisID” attribute inherited from the parent class does not need to be set since a time reference system always has a single dimension. However it can be set to ‘T’ for consistency with spatial axes. The “referenceTime” attribute is used to specify a different time origin than the one sometimes implied by the “referenceFrame”. This is used to express a time relative to an arbitrary epoch i.e. different from the origin of a well known reference frame. The new time origin specified by “referenceTime” shall be expressed with respect to the reference frame specified and is of type “DateTime”. This forces the definition of this origin as a calendar datetime combination. Requirement http:www.opengis.netspecSWE2.0requml-simple-componentstime-ref-time-valid Req 28. The value of the “referenceTime” attribute shall be expressed with respect to the system of reference indicated by the “referenceFrame” attribute. Example This class can be used to define a value expressed as a UNIX time i.e. number of seconds elapsed since January 1, 1970, 00:00:00 GMT by: - Specifying that the reference frame is the UTC reference system - Setting the reference time to January 1, 1970, 00:00:00 GMT. - Setting the unit of measure to seconds See definitions of some commonly accepted time standards at http:en.wikipedia.orgwikiTime_standard or http:stjarnhimlen.secomptime.html The optional “localFrame” attribute allows for the definition of a local temporal frame of reference through the value of the component i.e. we are specifying a time origin, as opposed to the referenceFrame which specifies that the value of the component is in reference to this frame. Requirement http:www.opengis.netspecSWE2.0requml-simple-componentstime-local-frame-valid Req 29. The “localFrame” attribute of an instance of the “Time” class shall have a different value than the “referenceFrame” attribute. This feature allows chaining several relative time positions. This is similar to what is done with spatial position in a geopositioning algorithm and which is also supported by this standard using the “Vector” class. Example In the case of a whiskbroom scanner instrument, the “sampling time” is often expressed relative to the “scan start time” which is itself given relative to the “mission start time”. It is important to properly identify the chain of time reference systems at play so that the adequate process can compute the absolute time of every measurement made 34 Copyright © 2011 Open Geospatial Consortium