Web Feature Service

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Web Feature Service ( WFS ) describes the internet-based access to geospatial data within a distributed GIS . The WFS is limited to vector data as it can be stored in databases .

As part of the specifications of the Open Geospatial Consortium (OGC), a WFS enables access to geographic features in databases and returns the result at least as an independent file format Geography Markup Language (GML).

A feature is understood here as the general abstraction of a real fact (“real world phenomenon”). A feature type is the representation using a name, further attributes and, in the case of geographic feature types, with a geometry. For example, a database contains feature types of the type "flow". An instance of a feature (a feature instance) is then a concrete flow.

Technical background of an OGC-compliant WFS

Overview of three OGC web services with the various request options from WFS

To get an OGC compliant WFS - i.e. H. To use a web feature service that meets the specification of the Open Geospatial Consortium (OGC), the Hypertext Transfer Protocol (HTTP) has been used for communication . The request is sent as an HTTP request from a client to the WFS. The WFS has six operations that can be requested by a user:

  1. GetCapabilities: This asks about the capabilities of the WFS. In response, an XML document is sent back to the user, which contains general information about the provider of the WFS, the queryable feature types and the possible operations.
  2. DescribeFeatureType: This request returns information about the structure of the individual feature types.
  3. GetFeature: With this request, the individual feature instances, i. H. the actual data is returned. It should be possible for the query to specify which properties of the feature are returned and whether these contain spatial information.
  4. GetGmlObject: A WFS query always has a GML file as the result . With this request it is possible to receive individual elements from the GML file via XLink .
  5. Transaction: A WFS can provide requests of the transaction, i. H. the possibility to change the actual features in the database. This includes the creation, updating and deletion of geographic features.
  6. LockFeature: This ensures that the WFS ensures that when an operation is carried out on a feature type, it is not changed by another instance during the transaction.

Taking these operations as a basis, one can distinguish three types of WFS:

  1. Basic WFS: The Basic WFS offers read-only access with the GetCapabilities, DescribeFeatureType and GetFeature operations.
  2. XLink WFS: The XLink WFS offers all functions of the Basic WFS. It also supports the GetGmlObject operation.
  3. Transaction WFS: The Transaction WFS supports all functions of the Basic WFS. In addition, it enables write access to the data with the Transaction operations and, optionally, LockFeature. The GetGmlObject operation is also only optional for the Transaction WFS. To better distinguish non-transaction-capable WFS from Transaction WFS, the latter are also referred to as WFS-T.

A WFS must therefore offer requests with the first three operations; the Transaction and LockFeature request are optional. The latter does not have to be implemented in a WFS with write access either .

The usual sequence of communication between client and a WFS in a distributed system is therefore as follows:

  • With the GetCapabilities Request, the client asks about the capabilities of the WFS and the feature types offered.
  • Building on this, the structure of the individual feature types can be viewed with the DescribeFeatureType request.
  • This means that a special instance of a feature can be requested with the GetFeature Request. Alternatively, with a WFS with write access, the transaction or the lock feature request can be carried out.

Within the OGC context, the connection of a web feature service with a web map service (WMS) is an example of the architecture of a distributed GIS. The WFS is responsible for accessing the geodata, while the WMS is only responsible for the visualization of the data. The WFS is limited to accessing vector data , while another service, the Web Coverage Service (WCS), controls special access to raster data .

Thus, in the sense of the client-server principle, the WMS is the client of the WFS. In addition, as a cascading WFS, this WFS can in turn be a client of one or more other WFS. This architecture can be seen as a type of web GIS .

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