Property metadata in GML
... overloading href, and use of other xlinks
GML Property pattern and xlinks
The implementation of properties
as "child elements" of elements representing features is described in GmlImplementation
The standard pattern for properties with complex values is that the value may appear
- by representation (inline) as a child element of the property element, or
- by reference as the resource denoted by the value of the xlink:href attribute.
The standard rule is that one of these should occur, and neither or both is invalid.
This rule is not enforceable using W3C
XML Schema, but it may be formally expressed and enforced using the Schematron pattern provided in gmlBase.xsd.
However, there is a case for using xlink attributes for the additional purpose of property value metadata
In particular, this leverages the other (rarely used) simple link attributes other than xlink:href to augment a link.
Kinds of metadata
Metadata may apply to geospatial data at various levels.
The ISO 19115 MD_ScopeCode codelist identifies 16 scope levels:
In terms of data instances, we have most experience with dataset
metadata, and related parts of a hierarchy such as series
The possibility of seamless databases with incremental update also implies a requirement for feature
level metadata, which is supported in GML.
However, although attribute
is one of the scope levels, implementations are rare at this scope level.
The well known metadata standards (FGDC, ISO 19115, ANZMETA) are primarily focussed on dataset metadata exchanged using the so-called transfer model
which is primarily centered on the transfer of complete metadata records as artefacts in parallel with the datasets described.
We have recently encountered two use-cases for property level
- the need to link to the observational basis of a property value, often in the form of an Observation
- construction of a denormalized representation of a feature or other record (i.e. everything inline), while still preserving an indication of the fact that a value is normatively available from another source
These use-cases can be supported by use of the xlink attributes together with
A property metadata pattern for GML property elements
We propose to relax the contraint that prohibits the occurrence of both an xlink:href as well as an inline value, so that the xlink:href can link to property-value metadata.
In order to implement this in a GML Application Schema,
- where the property is a "normal" property, using the either inline/or byReference pattern, no change is required
- where the property is tagged inlineOrByReference="inline", or where the property has a "simpleContent" type, then xlinks must be added to the type in the element declaration, for example following this pattern:
<element name="size" minOccurs="0">
This property is stereotyped //estimatedProperty//
so needs to provide a means to link to the Observation
that acts as the propertyValueProvider
- any property element on which a metadata link may appear must not provide context for the Schematron "refAndCOntent co-occurrence prohibited" pattern. An alternative rule applies as follows:
Rule for co-occurence of link and property value: if a property element both contains an inline value and carries an xlink:href attribute, the xlink:arcrole attribute must also be present, with a value selected from the following list:
urn:ogc:def:arcrole:OGC:evidence indicates that the target of the accompanying xlink:href provides evidence for the property value
urn:ogc:def:arcrole:OGC:original indicates that the target of the accompanying xlink:href provides the normative value for the property; i.e. if the local and remote values differ, then the remote value is correct
- any others?
urn:ogc:def:arcrole:OGC:quality indicates the the target of the accompanying xlink:href provides a description of the quality of the property value (e.g. using a gmd:AbstractDQ_Element instance)
The xlink:role attribute may (optionally) be used to indicate the type of the target resource, for example
urn:ogc:def:featureType:OGC:OM:Observation may indicate that the target of a link to evidence is actually an om:Observation instance
urn:ogc:def:identifiableObjectType:ISOTC211:CI_ResponsibleParty may indicate that the target of a link to an original is a CI_ResponsibleParty instance.
- 21 Nov 2006