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Feature Model and Encoding

Contents

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Overview

The 'Feature' is the fundamental unit of geospatial information, so the Feature Model is the fundamental meta-model used for developing an Application Schema.

Features are typed objects with identity. This is often referred to as "vector" data in traditional GIS.

Feature types are defined by a characteristic set of properties (i.e. their attributes, associations, operations). A feature type is usually specific to an application domain, and will be part of a Feature Type Catalogue (FTC) that describes a key part of the language of a domain. Features often correspond with objects that are recognisable in the real world, such as road, mine, truck, storm. However, spatial properties are not mandatory, so a feature type could be defined for any item of interest within a domain. This potentially allows data access for both spatial and non-spatial information to be unified through a common interface.

The General Feature Model is formally defined in ISO 19101 and ISO 19109. For a more detailed discussion, see FeatureModel.

The OGC Web Feature Service is the primary interface for feature data.

General Feature Model

Preliminaries

In accordance with ISO 19103, UML is used as the Conceptual Schema Language.

Meta-model

The General Feature Model is a "meta-model" for the definition of a feature type (i.e. the schema for feature instances) .

GFM.png
From ISO 19109, annotated

This class diagram says that specific property-types are the "carrier of characteristics" for a particular feature type, and that these property-types may be operations, attributes or associations. Each of those are characterized appropriately in turn.

Formalizing a feature-type

The definition of a feature-type may be formalized in a UML model. A UML profile is provided that constrains the variation that is allowed by generic UML, and adds some customization in the form of stereotypes and tagged values (i.e. the standard UML extensibility points) - see SchemaFormalization#ISO_TC_211_Profile_of_UML.

This model for "Borehole" feature-type shows many of the standard patterns:
GFMBorehole.png

This class diagram says that Borehole is a feature-type that specializes SamplingCurve. Two attributes are inherited: length which is of type Measure (a scaled number), and shape which is of type GM_Curve (a bent line). Two navigable associations are added: indexData holds the BoreholeDetails in a data structure; collarLocation provides a link to another feature-type, BoreholeCollar.

Note the following:
  • properties are shown as class attributes and associations
    • no operations are shown; all properties are realized as static values
  • the set of properties is characteristic of the feature type.
  • in the case of class attributes
    • the attribute name instantiates the memberName attribute of GF_PropertyType
    • the attribute type instantiates the valueType attribute of GF_AttributeType
  • in the case of class associations
    • every navigable association-end carries a rolename
    • the association rolename instantiates the memberName attribute of GF_PropertyType
  • the type of a property may be structured
    • if the value of a property varies in the scope of a feature-instance, then it may be expressed as a function or coverage - see OGCInformationModels#Coverages.

For more detail on the mechanics of modeling feature-types, and a UML template pre-loaded with types from the ISO 19100 specifications, see HollowWorld.

For an example of a large domain-specific feature model, see GeoSciML.

Encoding and transfer

A feature model described using the patterns outlined above may be converted to a GML-conformant XML Schema. This defines a document-model for serializing instances of geospatial data, suitable for transfer. The UML-XML encoding rule results in an encoding within which the model structure is shown explicitly - see GmlImplementation.

The OGC Web Feature Service defines an interface for requesting feature data, in which the query is expressed in terms of the GML representations, and services are required to provide a GML-encoded response, alongside other formats if desired - see OGCServiceInterfaces#Web_Feature_Service_OGC_WFS.

Information Communities and feature catalogs

Communication within a specific information community is characterised by the information model and the catalogue of feature types that is agreed by its members (see ISO 19110, 19126).

Mapping to other modeling formalisms

Features and conventional GIS

The General Feature Model takes an object-oriented view of the world. To describe an object its "type", or the "class" to which it belongs, must be determined. This fixes what properties are associated with it, one or more of which may be geometric or spatial.

This approach contrasts subtly but importantly with the conventional vector-GIS and CAD approach. In GIS and CAD, the entitites of interest are characterized primarily as "points", "lines" or "polygons", with additional attributes attached. Objects are often typed by the name of the layer to which they belong, which may correspond with a single feature type, though detail may also be provided by a "type" attribute per geometry instance. In such conventional systems, identity is associated with geometry, and usually only one geometry (and scale) per feature is available.

Tables, Objects and XML

See SchemaMapping.

-- SimonCox - 30 Oct 2007
Topic attachments
I Attachment Action Size Date Who Comment
GFM.pngpng GFM.png manage 50.2 K 30 Oct 2007 - 13:48 SimonCox Extract from General Feature Model
GFMBorehole.pngpng GFMBorehole.png manage 10.4 K 30 Oct 2007 - 13:56 SimonCox Model for "Borehole" feature-type
Topic revision: r25 - 28 Oct 2013, SimonCox
 

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