valueObjects.xsd

See ISO/DIS 19136 17.5.
The elements declared in this Clause build on other GML schema components, in particular gml:AbstractTimeObject, gml:AbstractGeometry, and the following types:  gml:MeasureType, gml:MeasureListType, gml:CodeType, gml:CodeOrNilReasonListType, gml:BooleanOrNilReasonListType, gml:IntegerOrNilReasonList.  
Of particular interest are elements that are the heads of substitution groups, and one named choice group. These are the primary reasons for the value objects schema, since they may act as variables in the definition of content models, such as Observations, when it is desired to permit alternative value types to occur some of which may have complex content such as arrays, geometry and time objects, and where it is useful not to prescribe the actual value type in advance. The members of the groups include quantities, category classifications, boolean, count, temporal and spatial values, and aggregates of these.  
The value objects are defined in a hierarchy. The following relationships are defined:
-	Concrete elements gml:Quantity, gml:Category, gml:Count and gml:Boolean are substitutable for the abstract element gml:AbstractScalarValue.  
-	Concrete elements gml:QuantityList, gml:CategoryList, gml:CountList and gml:BooleanList are substitutable for the abstract element gml:AbstractScalarValueList.  
-	Concrete element gml:ValueArray is substitutable for the concrete element gml:CompositeValue.  
-	Abstract elements gml:AbstractScalarValue and gml:AbstractScalarValueList, and concrete elements gml:CompositeValue, gml:ValueExtent, gml:CategoryExtent, gml:CountExtent and gml:QuantityExtent are substitutable for abstract element gml:AbstractValue.  
-	Abstract elements gml:AbstractValue, gml:AbstractTimeObject and gml:AbstractGeometry are all in a choice group named gml:Value, which is used for compositing in gml:CompositeValue and gml:ValueExtent.  
-	Schemas which need values may use the abstract element gml:AbstractValue in a content model in order to permit any of the gml:AbstractScalarValues, gml:AbstractScalarValueLists, gml:CompositeValue or gml:ValueExtent to occur in an instance, or the named group gml:Value to also permit gml:AbstractTimeObjects, gml:AbstractGeometrys.