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Spatial Data Infrastructures

The term spatial data infrastructures comprises the techologies and policies that facilitate the availabilitiy and acces to spatial data. It provides the basis for publishing, discovering and using spatial data within the public sector, the commercial sector, non profit sector, universities and by the citizens in general in a services (web) architecture.

Specifications and standards

The entity behind the definition of the specifications for building spatial data infrastructures is the Open Geospatial Consortium (OGC). The OGC is a consortium of over 300 companies, public organisations and universities and defines its mission as:

The process to develop publicly available interface specifications to support interoperable solutions that "geo-enable" the Web

The main OGC interface specifications are:

• WMS: Web Map Service
• WFS: Web Feature Service
• WCS: Web Coverage Service
• WMC: Web Map Context
• W3DS: Web 3D Service
• CS-W: Catalogue Service for Web

The OGC has defined GML (Geography Markup Language) as the format for interchanging geographic datasets.

Also ISO standards play a role. Important ISO standards is the geospatial domain are:

• ISO19110: Standard for registration of feature type definitions
• ISO19115: Metadata standard for geographic datasets
• ISO19119: Metadata standard for services van de services
• ISO19139: XML encoding of ISO19115 metadata standard

Publish-find-bind paradigm

In the OGC reference architecture the following roles exist:

• Provider
• Broker
• User

The publish-find-bind paradigm describes the interaction between the different roles. A data provider publishes its geographic dataset as a service (eg. WMS or WFS). The service and the geographic dataset are being described by metadata (using ISO19115 and ISO19119 standards). The metadata is being harvested into a catalogue (broker). A user queries the catalogue for a specific geographic dataset through the catalogue service (CS-W) by specifying keywords and search area. Then the user retrieves the geographic dataset through a service and is able to use the data in its own business process.

Technology

The OGC only defines the specifications but it does not develop the components (eg. WMS, WFS or CS-W) to implement these specifications. For the technical realisation of an SDI either components of a commercial company or components developed by the open source community have to be used. A well-known company that deploys a complete sets of OGC components is ERDAS. Their suite of products ranges from components to deploy geographic datasets as WMS or WFS to a Catalogue service and database for the storing and quering metadata. Well-known open source initiatives are UMN MapServer for WMS, GeoServer for WFS and GeoNetwork for CS-W. Deegree offers a full set of WMS, WCS, WFS and Catalogue service.

Web Map Service (WMS)

The WMS is a service that returns a georeferenced map (layer). In this context a map is being considered as a two-dimensional visualisation (according to a predefined style) of geo-feature instances. The service does not return the actual geo-features.

Default a WMS serves one or more styles per layer. By defining a style on the clientside and sending this style as part of the WMS request to the server, a map with a user-defined style is obtained. In this way thematic maps can be made.

The styles used for rendering the map have to be specified in a Style Layer Descriptor (SLD) document. SLD is an XML encoding for the definition of the styles of geographic features.

Web Map Context (WMC)

It is possible to combine layers from different WMS’s on the client into one single map view. Requirement is that the layers are requested with the same bounding box and outputsize and are within the same coordinate system.

The OGC developed the WMC specification to store the definition of these map views in a XML document. Using these WMC documents is it possible to retrieve map views on another client and on later moment.

Web 3D Service (W3DS)

The Web 3D Service is a service for displaying three-dimensional data.The service returns 3D geographic elements from a specified area in VRML format which are rendered on the client. Because rendering takes place on the client, realtime navigation through the 3D image is possible.

Web Feature Service (WFS)

The WFS is the service for selecting, inserting, updating and deleting geo-features and enables geographic and attribute filtering of the geo-features. The OGC filter encoding specification has te be used for the filter definition. It describes the XML to be used to define spatial and attribute filters. The selected geo-features are returned in GML format.

Catalogue Service fro Web (CS-W)

A Catalogue service offers functionality to store (harvest) and query metadata of services and geographic datasets in a catalogue.