Overview
The Web Feature Service (WFS) is a key complementary technology to GML. It is to GML as HTTP Servers are to HTML. A key objective of GML was to support geospatial transactions and WFS provides the means to realize that objective.
A WFS is intended to serve geographic content in the form of GML features. It is not intended as a mapping engine. The latter is the role of the Web Map Service (WMS).
A WFS can be read-only or support both reading and transactional operations. Some people designate the transactional WFS by using the label WFS-T. All Galdos WFS are transactional.
A WFS responds to XML messages that encode requests for geographic information or which describe geospatial transactions on features (e.g. insert, delete, update a feature). A WFS can thus respond to messages like:
- Find all abandoned water wells within Watkin County which are older than 1984
- Find the owners of properties whose area exceeds 20 hectares in France.
- Modify the road’s centerline geometry to the following …
- Delete the Firth Bridge specified by id = “pog21”
- Insert a new observation feature – the sampling of the water at the dam at 3:00 pm 2007-1-16
Of course these messages are encoded in XML. To express spatial and arithmetic predicates, the WFS uses the OGC Filter grammar.
The WFS is defined by its key interfaces, namely (Version 1.0):
- GetCapabilities - returns a capabilities document describing the feature types supported by the WFS, as well as supported WFS and Filter operations.
- DescribeFeatureType - returns a GML Schema(s) describing the features supported by the WFS – i.e. lists all properties and their value types.
- GetFeature - returns the requested features.
- Transaction - requests the WFS to execute a transaction (e.g. insert, delete, update)
- Lock - locks the specified feature.
These interfaces were extended in Version 1.1, but the above interfaces constitute the heart of the WFS.
WFS must be able to return a GML feature description (DescribeFeature) and GML data (GetFeature) conforming to the schemas returned in response to a WFS DescribeFeature request.
WFS as a specification.
The WFS is a specification of the Open Geospatial Consortium. The current version from the OGC is Version 1.1. Most implementations are based on Version 1.0, but may support GML Version 3.1.1, which is officially required in Version 1.1. Version 1.2 is being developed as a joint project of the OGC and ISO TC/211, and this is anticipated to be completed sometime in 2008. This project also includes the OGC Filter Specification. The WFS and Filter standards will be published by ISO TC/211 as ISO 19142 and 19143 respectively.