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.
Application Data
valueObjects.xsd More information at: urn:ogc:specification:gml:schema-xsd:valueObjects:3.2.0.
gml:AbstractScalarValue is an abstract element which acts as the head of a substitution group which contains gml:Boolean, gml:Category, gml:Count and gml:Quantity, and (transitively) the elements in their substitution groups.
gml:AbstractScalarValueList is an abstract element which acts as the head of a substitution group which contains gml:BooleanList, gml:CategoryList, gml:CountList and gml:QuantityList, and (transitively) the elements in their substitution groups.
gml:AbstractValue is an abstract element which acts as the head of a substitution group which contains gml:AbstractScalarValue, gml:AbstractScalarValueList, gml:CompositeValue and gml:ValueExtent, and (transitively) the elements in their substitution groups.
These elements may be used in an application schema as variables, so that in an XML instance document any member of its substitution group may occur.
gml:CompositeValue is an aggregate value built from other values . It contains zero or an arbitrary number of gml:valueComponent elements, and zero or one gml:valueComponents property elements. It may be used for strongly coupled aggregates (vectors, tensors) or for arbitrary collections of values.
An XML attribute uom (“unit of measure”) is required, whose value is a URI which identifies the definition of a ratio scale or units by which the numeric value shall be multiplied, or an interval or position scale on which the value occurs.
A Value Array is used for homogeneous arrays of primitive and aggregate values.
The member values may be scalars, composites, arrays or lists.
ValueArray has the same content model as CompositeValue, but the member values shall be homogeneous. The element declaration contains a Schematron constraint which expresses this restriction precisely. Since the members are homogeneous, the gml:referenceSystem (uom, codeSpace) may be specified on the gml:ValueArray itself and inherited by all the members if desired.
Application Data
<sch:pattern name="Check either codeSpace or uom not both">
This is a convenience choice group which unifies generic values defined in this Clause with spatial and temporal objects and the measures described above, so that any of these may be used within aggregate values.
The XML Instance Representation table above shows the schema component's content as an XML instance.
The minimum and maximum occurrence of elements and attributes are provided in square brackets, e.g. [0..1].
Model group information are shown in gray, e.g. Start Choice ... End Choice.
For type derivations, the elements and attributes that have been added to or changed from the base type's content are shown in bold.
If an element/attribute has a fixed value, the fixed value is shown in green, e.g. country="Australia".
Otherwise, the type of the element/attribute is displayed.
If the element/attribute's type is in the schema, a link is provided to it.
For local simple type definitions, the constraints are displayed in angle brackets, e.g. <<pattern = [1-9][0-9]{3}>>.
If a local element/attribute has documentation, it will be displayed in a window that pops up when the question mark inside the attribute or next to the element is clicked, e.g. <postcode>.
Abstract(Applies to complex type definitions and element declarations). An abstract element or complex type cannot used to validate an element instance. If there is a reference to an abstract element, only element declarations that can substitute the abstract element can be used to validate the instance. For references to abstract type definitions, only derived types can be used.
Collapse Whitespace PolicyReplace tab, line feed, and carriage return characters with space character (Unicode character 32). Then, collapse contiguous sequences of space characters into single space character, and remove leading and trailing space characters.
Disallowed Substitutions(Applies to element declarations). If substitution is specified, then substitution group members cannot be used in place of the given element declaration to validate element instances. If derivation methods, e.g. extension, restriction, are specified, then the given element declaration will not validate element instances that have types derived from the element declaration's type using the specified derivation methods. Normally, element instances can override their declaration's type by specifying an xsi:type attribute.
Nillable(Applies to element declarations). If an element declaration is nillable, instances can use the xsi:nil attribute. The xsi:nil attribute is the boolean attribute, nil, from the http://www.w3.org/2001/XMLSchema-instance namespace. If an element instance has an xsi:nil attribute set to true, it can be left empty, even though its element declaration may have required content.
Prohibited Derivations(Applies to type definitions). Derivation methods that cannot be used to create sub-types from a given type definition.
Prohibited Substitutions(Applies to complex type definitions). Prevents sub-types that have been derived using the specified derivation methods from validating element instances in place of the given type definition.
Replace Whitespace PolicyReplace tab, line feed, and carriage return characters with space character (Unicode character 32).
Substitution GroupElements that are members of a substitution group can be used wherever the head element of the substitution group is referenced.
Substitution Group Exclusions(Applies to element declarations). Prohibits element declarations from nominating themselves as being able to substitute a given element declaration, if they have types that are derived from the original element's type using the specified derivation methods.
Target NamespaceThe target namespace identifies the namespace that components in this schema belongs to. If no target namespace is provided, then the schema components do not belong to any namespace.