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OGC Information (meta) models

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The Open Geospatial Consortium (OGC) provides specifications for components for interoperable geospatial information systems. The main focus is on service interfaces, in support of service-oriented architectures - see OGCServiceInterfaces. However, in order to use these, it is necessary to understand the scope of the model for the information types that are the primary focus of each interface.

This topic provides a brief summary of each of the main information meta-models or viewpoints that underly the data that is transferred on each of the main interfaces. Note that formal definitions of some of these models come from the ISO 19100 series of standards. An analysis of transformations between the standard models is provided at InformationViews.

Also see a GIGAS report to GEOSS AIP-3 on Data Harmonization which discusses the relationship between features, coverages and observations, in the context of environmental monitoring.

Features

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.

Coverages

A coverage describes the value of a property as a function of location. When implemented in digital systems, it is usually represented as either a grid or as a tesselation of polygons in 2-D GIS, which expresses a sampling strategy.

The region of space-time within which a coverage is defined is called its domain, and the set of coverage values is its range. A coverage domain is often a grid of points or pixels (e.g. imagery), but may also be a network of curves (e.g. roads or streams), a tesselation of polygons, or a collection of other geometries. A coverage has values at all positions within the domain, but it is not mandatory for the domain to be continuous within its bounds.

The intention of the coverage viewpoint is to support assessment of the variation of a property within the domain of interest. The variation may be continuous or may contain discontinuities. In digital representations, the value of a coverage is usually sampled at discrete locations, then a rule or function (e.g. nearest-neighbor, bilinear-interpolation, piecewise-constant) is specified to obtain values at intermediate locations.

The General Feature Model (above) specifies that properties occur in the context of features. Hence, a coverage, which describes the variation of a property, occurs as the description of a property that varies within a feature. Examples include the colour (space-varying property) of a scene (feature); temperature (time-varying property) at a weather station (feature); rock-type (position-varying property) within a borehole (feature). In many conventional applications the feature that owns the coverage is implied by the application or context, and might not be encoded in the data.

The model for coverages and grids is formally defined in ISO 19123.

The OGC Web Coverage Service is the primary interface for coverage data.

Maps

A "map" as used in OGC refers to a portrayal of a number of (usually related) features and coverages in a single artefact. Maps are often encoded as images (e.g. GIF, JPEG, PNG etc) though "vector" portrayals are also in the same category (e.g. KML, PDF). Maps convey information to humans using colour and symbolization, and are generally not re-usable for other purposes.

Coverages are distinguished from "maps" in that the values are data in a coverage are on a meaningful scale, while in a map they are colours and shapes.

The OGC Web Map Service_OGC_WMS (also known as ISO 19128) is the primary interface for maps.

Metadata records

Documents that describe datasets and services, but do not contain the actual data, are known as metadata. Such a document typically includes information related to discovery, access, and maintenance. Standards for the content and sometimes also encoding of metadata documents include Dublin Core, FGDC, ANZLIC, ISO 19115 and 19119. These documents are typically provided through an indexed catalogue interface.

The OGC Catalogue Service is the primary interface for metadata records.

There is some consideration of using the ebXML Registry framework for implementing GI catalogues. This is contentious - see MetadataAndEbRIM.

Observations and sensors

An observation is a data-capture event, so the description of an Observation provides property-value metadata (see ObservationsAndSampling). In the OGC context, an Observation may be modeled as a feature-type with some standard properties, related to
  • the procedure used,
  • the time when it was applied,
  • the observed property, and
  • the feature-of-interest.
The result of an observation is an estimate of the observed property on the feature of interest. If the property is not constant, then the result may be a Coverage, including time-series.

A "sensor" is a procedure which is used to make observations. The description of a sensor can usually be decomposed as a processing chain, with each step characterized by inputs, outputs, and a method with any parameters. The Sensor Model Language (SensorML) is a generic framework for the description of observation procedures.

Observations and Measurements is formally defined in a two part OGC Specification: The SensorML model is formally defined in an OGC Specification.

The OGC Sensor Observation Service is the primary interface for observation and sensor data.

Geography Markup Language

Geography Markup Language (GML) (also known as ISO 19136) is a set of encoding patterns and utility components to be used in the definition of an XML document format for feature types, for an application domain. GML is provided as an XML Schema that is intended to be imported into an application schema implementation.

Note: GML cannot be used "out of the box" - it is a framework for the development of an application language.

Note: the purpose of GML-encoding is to transfer data for re-use. Cartographic portrayal requires additional styling.
GML supplies standard components for geometry, coordinate reference systems, time, measures and some other cross-domain elements. The GML specification also includes rules for construction of the application schema by extending certain base classes, in particular gml:AbstractFeatureType. The patterns and rules are discussed here: GmlImplementation.

The OGC Web Feature Service is the primary fine-grained GML data access interface.

A Wikipedia page provides links to a number of GML Application Schemas.

KML

The specification for KML (formerly Keyhole Markup Language) has been introduced into the OGC standardization process, with KML v2.1 available as an OGC Best Practice document.

KML is a presentation language for geospatial data, supported by the Google Earth visualization application. The KML data model is relatively simple - essentially a small set of geometry types, encoded using GML. Additional information is attached, including labels, styling (symbols, color, etc), and annotation (which appears in a pop-up "balloon" in the Google Earth interface) - all related primarily to portrayal for visual use. The "features" may be organized in a set of hierarchical folders (which often provides an implication of semantic type), but support for explicit description of relationships between feature instances is limited.

The annotation is encoded in HTML, and is thus the main extensibility point. Since HTML may carry links, this means that a suitable KML viewed in Google Earth can provide a portal to web services, including geospatial services conforming to OGCServiceInterfaces. In order to use this approach consistently it will be necessary to develop conventions ("standards") for how to embed specific link types in KML.

Some OGC-WMS systems support KML as an output format.

See also

 
Topic revision: r19 - 22 Dec 2010, SimonCox
 

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