Database Server Models for WMS and WFS Geographic Web Services - Geographical Information Systems: Trends and Technologies - page 149

Table 1. mdWFS specifi cations.

Extensions Description

Service (parameter) In order to defi ne a protocol, a new parameter in a query is

defi ned: service = mdWFS.

GetCapabilities operation The GetCapabilities operation is extended to obtain a

SchemaList. This list contains all the available schemas in
this service.

DescribeFeatureType
operation

The DescribeFeatureType operation is extended to provide
the XMI format (i.e., metadata exchange format in UML
using XML) to transfer information on models.

DoTransform operation A new operation. Its aim is to transfer the conceptual

schema representation towards mdWFS and to set up the
semantic transformations.

Table 2. Elements of the UMLT language.

Elements Description

SelectionCriteria Allows a data selection upon a logical expression.

VirtualAssociation Allows the management of objects defi ned as being not linked

with an association object. Imported objects may have some
attributes or foreign keys (evaluated in real time) in order to
provide calculated relationships.

TransformationAction Defi ned as a heritage of an Opaque Action UML. It provides an

element of activity that cannot be anymore structured. This is
an elementary action of a transformation.

AssignmentDefi nitions Identifi es expressions and primitive types as specifi cations.

MappingRule Built as a composition of rule defi nitions, it provides

cartographic objects.

AssociationBinding Selecting associated objects, it is possible to defi ne the way

these associations are evaluated.

JoinType An enumerate type to specify the type of join within an

association link.

This new service provides a better semantic interoperability in the
sense that it describes a methodology. The semantic transformations are
formalized at the conceptual level. During a semantic transformation, virtual
associations may be associated with, using a virtual link if necessary. The
virtual association is defi ned to provide an explicit link for the join operation
(in opposition to a conventional derived association). The joinCriteria
property specifi es the join conditions. The new service, mdWFS, allows a
higher semantic interoperability. The relevant level is a transformation at
the semantic level using a conceptual level of abstraction. The results of
the data schema transformation process can be queried using a GetFeature
operation. For the time, this specifi cation is not an OGC normalized service.
Foerster et al. (2010) and Wiemann et al. (2012) present different kinds of
transformation based on the same principles.