The 'Observable Property' model enables observed physical properties (also termed "Quantities" within the 'International vocabulary of metrology' [http://www.bipm.org/utils/common/documents/jcgm/JCGM_200_2008.pdf]) to be aggregated into composite properties and for any qualification or constraint relating to those observed physical properties to be explicitly described.<br /></p><p><br /></p><p>A data publisher may need to use the Observed Property Model  to explicitly define any qualification or constraint applied to the observed property, or, where the observation results in the evaluation of multiple properties, to explicitly define each of those individual physical properties.<br /></p><p><br /></p><p>Usage:<br /></p><p><br /></p><p>The Observable Property Model may be used to specify constraint or qualification of a physical quantity (or set of physical quantities) within the context of a specific observation or measurement event; e.g. via OM_Observation/parameter Attribute (from ISO 19156, type Class "NamedValue"):<br /></p><p>    - name: http://def.wmo.int/opm/2013/observable-property#property [1]<br /></p><p>    - parameter: instance of Class "AbstractObservableProperty" or concrete subclass thereof [2]<br /></p><p><br /></p><p>Note that Class "OM_Observation" asserts a constraint such that the value of the parameter/NamedValue/name Attribute shall not occur more than once within a given instance of Class OM_Observation. In situations where multiple physical quantities are observed or measured, the value of the associated parameter/NamedValue/value Attribute may be specified as an instance of Class "CompositeObservableProperty" thus allowing a set of observable properties to be specified. <br /></p><p><br /></p><p>Also note that, whilst Class "OM_Observation" provides Attribute "observedProperty" for specifying the (physical) property observed or measured the cardinality of this Attribute is one (e.g. only a single property can be referenced). Furthermore, this Attribute is implemented "by reference" (serialised as gml:ReferenceType) implying that one cannot define an instance of Class "CompositeObservableProperty" in-line within the data. Whilst it is permissible to make a local reference to an instance of Class "CompositeObservableProperty" defined elsewhere within the data, it is preferable to reference an external definition. The reason for this is that the value of Attribute "observedProperty" is used by the OGC Sensor Observation Service as a key on which data is indexed; use of internal references are likely to inhibit discovery and retrieval of data. <br /></p><p><br /></p><p>Alternatively, if the constraint or qualification of the physical quantity (or set of physical quantities) is consistent within a repeatable procedure, it may be defined within the context of that procedure; e.g. via the Process/parameter Attribute (from "«Application Schema» METCE", type Class "NamedValue").<br /></p><p><br /></p><p>Note that in the case of Class "Process", there is no constraint on the number of occurences of a given parameter name. However, recommended practice indicates the use of an instance of Class "CompositeObservableProperty" where a set of observable properties are to be specified.<br /></p><p><br /></p><p>[1] Association Role "property" is specified to have type Class "AbstractObservableProperty"<br /></p><p>[2] Class "AbstractObservableProperty" has concrete sub-classes "ObservableProperty", "QualifiedObservableProperty" and "CompositeObservableProperty".<br /></p>