A Web Feature Service (WFS) is an interface to query and to supply geographical vector data. The communication and supply of the data is done by means of XML. The data is supplied in the form of a specific scheme for geographical data, better known as GML (Geography Markup Language).
The WFS standard is being developed by the Open Geospatial Consortium (OGC) and the International Organization for Standardization (ISO). However, this standard hasn’t made as much progress as the standard for WMS services. This means that ISO still hasn’t an officially published WFS standard (ISO: 19142)
WFS defines 3 operations:
§ GetCapabilities: gives metadata on service level
§ DescribeFeatureType: gives a description of the structure of the data
§ GetFeature: supplies the data
These operations can be started from standard web browsers via Uniform Resource Locators (URLs). Unlike WMS services, only a limited number of applications can use these services. Likewise, the implementation of the standard is less current on server level. WFS is concentrating especially on searching for data, whereas WMS is intended more for visualisation.
SDI-Flanders testbed WFS services:
Temporarily, 3 WFS services only are offered. Each service contains one or more information layers.
Only the last WFS version 1.1.0 is supported. WFS can invoke the transfer of large amounts of data. Therefore, it is very important to use a WFS service correctly, so as to obtain only the required information. Queries can be made by passing xml to the server, containing a directed query, see the example below, inquiring the district of Bruges in the layer RefArr in the Vrbg-service.
http://ogc.beta.agiv.be/ogc/wfs/vrbg/?request=getFeature&typename=vrbg:RefArr&maxfeatures=10&filter=<Filter+xmlns="http://www.opengis.net/ogc"><ogc:PropertyIsEqualTo matchCase="true" xmlns:ogc="http://www.opengis.net/ogc"><ogc:PropertyName>NAAM</ogc:PropertyName><ogc:Literal>Brugge</ogc:Literal></ogc:PropertyIsEqualTo></Filter>
For each service figuring below, there is also a reference to the capabilities. The latter give a description of the service. At the first query on this page, the user name and the web service key (not the password! - see Security below) have to be entered. This is no longer required for the next consultations during the same session.
1. Gwp (Regional plan (= ‘Gewestplan’), vector, situation 01/01/2002) Getcapabilities
2. Vha (Flemish Hydrographical Atlas (= ‘Vlaamse Hydrografische Atlas’)) Getcapabilities
3. Vrbg (Temporarily Reference Database of Administrative Boundaries = ‘Tijdelijk Referentiebestand Gemeentegrenzen’) Getcapabilities
Under the section examples, a WFS test page can be found, allowing two operations to be carried out. The way to implement a third operation, is illustrated above by means of an example.
Security
In order to be able to use the WFS services, except for the Vrbg, you need to have a user name and a web service key.
You can ask for access to the WFS services via http://login.agiv.be/?ROLE=85352F42-FBD0-44b8-9DBA-F65A40064F85. Your web service key can be found at “Mijn profiel”. The security is identical as for WMS via http digest authentication. If an operation is started from a standard web browser, the credentials have to be entered interactively.
In self developed viewers (.NET), the credentials are attributed to the “WebRequest” or the WebClient.
Metadata
In the service metadata (GetCapabilities), you can find, amongst others, the following information:
§ Service information
§ Name
§ Title (descriptive name)
§ Abstract (description of the service)
§ Keyword list
§ Contact Information
§ Capability (support operations)
§ Filter Capability (support filters)
§ Layer information
§ Name
§ Title (descriptive name)
§ Abstract (description of the layer)
§ Projection systems (CRS [SRS in lower versions])
§ Url to the metadatabase for the respective layer (MetadataURL)