Web Feature Service
Encyclopedia
The Open Geospatial Consortium
Web Feature Service Interface Standard (WFS) provides an interface
allowing requests for geographical
features across the web
using platform-independent calls. One can think of geographical features as the "source code
" behind a map, whereas the WMS
interface or online mapping portals like Google Maps
return only an image, which end-users cannot edit or spatially analyze. The XML
-based GML
furnishes the default payload-encoding for transporting the geographic features, but other formats like shapefiles
can also serve for transport. In early 2006, the OGC
members approved the OpenGIS GML Simple Features Profile http://portal.opengeospatial.org/files/?artifact_id=15201. This profile is designed to both increase interoperability between WFS servers and to improve the ease of implementation of the WFS standard.
The OGC membership defined and maintains the WFS specification. There are numerous commercial and open source implementations of the WFS interface standard, including an open source
reference implementation, called GeoServer
. A comprehensive list of WFS implementations can be found at the OGC Implementing Products page http://www.opengeospatial.org/resource/products/implementing.
The basic Web Feature Service allows querying and retrieval of features. A transactional Web Feature Service (WFS-T) allows creation, deletion, and updating of features.
A WFS describes discovery, query, or data transformation operations. The client generates the request and posts it to a web feature server using HTTP. The web feature server then executes the request. The WFS specification uses HTTP as the distributed computing platform, although this is not a hard requirement.
There are two encodings defined for WFS operations:
In the taxonomy of Web Services, WFS is best categorized as a non-RESTful RPC type service.
The Web Notification Service (WNS) is one of the implementation specifications for the Pub/Sub model. Regardless of the model, URL
format is used and specified in the WFS specification. At this time there are no open-standard implementations of WNSs. Vendors plan to release implementations once the standard has been ratified.
passes data back and forth between a Web Feature Server and a client. GML normally communicates geospatial data, but also supports other types of data.
GML
GML is an XML dialect which can be used to model geographic features. The 1.0.0 version of the WFS specification requires the use of GML version 2.1.2, while the 1.1.0 version of the WFS specification requires the use of GML version 3.1.1. For both versions of the WFS specification, an arbitrary number of other encodings can also be defined, in addition to the required GML 2.1.2 or 3.1.1 format (for 1.0.0 and 1.1.1 respectively).
GML 2.1.2 contains encoding support for basic geometric 'primitives': points, lines, polygons, etc.
GML 3.1.1 contains encoding support for more advanced geometric representations: curves, surfaces, multi-dimensions (time, elevation, multi-band imagery). In addition, GML 3.1.1 includes encoding support for topologically integrated datasets.
Web Service model appears in the figure below.
The Transaction and LockFeature operations are also optional.
When writing a WFS, you must implement the following operations:
WFS dynamic interface web notification model
This model uses the OGC Web Notification Service to send update notifications to registered clients.
Open Geospatial Consortium
The Open Geospatial Consortium , an international voluntary consensus standards organization, originated in 1994. In the OGC, more than 400 commercial, governmental, nonprofit and research organizations worldwide collaborate in a consensus process encouraging development and implementation of open...
Web Feature Service Interface Standard (WFS) provides an interface
Interface (computer science)
In the field of computer science, an interface is a tool and concept that refers to a point of interaction between components, and is applicable at the level of both hardware and software...
allowing requests for geographical
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...
features across the web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
using platform-independent calls. One can think of geographical features as the "source code
Source code
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
" behind a map, whereas the WMS
Web Map Service
A Web Map Service is a standard protocol for serving georeferenced map images over the Internet that are generated by a map server using data from a GIS database...
interface or online mapping portals like Google Maps
Google Maps
Google Maps is a web mapping service application and technology provided by Google, free , that powers many map-based services, including the Google Maps website, Google Ride Finder, Google Transit, and maps embedded on third-party websites via the Google Maps API...
return only an image, which end-users cannot edit or spatially analyze. The XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
-based GML
Geography Markup Language
The Geography Markup Language is the XML grammar defined by the Open Geospatial Consortium to express geographical features. GML serves as a modeling language for geographic systems as well as an open interchange format for geographic transactions on the Internet...
furnishes the default payload-encoding for transporting the geographic features, but other formats like shapefiles
Shapefile
The Esri Shapefile or simply a shapefile is a popular geospatial vector data format for geographic information systems software. It is developed and regulated by Esri as a open specification for data interoperability among Esri and other software products.Shapefiles spatially describe geometries:...
can also serve for transport. In early 2006, the OGC
Open Geospatial Consortium
The Open Geospatial Consortium , an international voluntary consensus standards organization, originated in 1994. In the OGC, more than 400 commercial, governmental, nonprofit and research organizations worldwide collaborate in a consensus process encouraging development and implementation of open...
members approved the OpenGIS GML Simple Features Profile http://portal.opengeospatial.org/files/?artifact_id=15201. This profile is designed to both increase interoperability between WFS servers and to improve the ease of implementation of the WFS standard.
The OGC membership defined and maintains the WFS specification. There are numerous commercial and open source implementations of the WFS interface standard, including an open source
Open source
The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...
reference implementation, called GeoServer
GeoServer
In computing, GeoServer - an open-source server written in Java - allows users to share and edit geospatial data. Designed for interoperability, it publishes data from any major spatial data source using open standards...
. A comprehensive list of WFS implementations can be found at the OGC Implementing Products page http://www.opengeospatial.org/resource/products/implementing.
Overview
The WFS specification defines interfaces for describing data manipulation operations of geographic features. Data manipulation operations include the ability to:- get or query features based on spatial and non-spatial constraints
- create a new feature instance
- delete a feature instance
- update a feature instance
The basic Web Feature Service allows querying and retrieval of features. A transactional Web Feature Service (WFS-T) allows creation, deletion, and updating of features.
A WFS describes discovery, query, or data transformation operations. The client generates the request and posts it to a web feature server using HTTP. The web feature server then executes the request. The WFS specification uses HTTP as the distributed computing platform, although this is not a hard requirement.
There are two encodings defined for WFS operations:
- XMLXMLExtensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
(amenable to HTTP POST/SOAPSOAPSOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
) - Keyword-Value pairs (amenable to HTTP GET/Remote procedure callRemote procedure callIn computer science, a remote procedure call is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space without the programmer explicitly coding the details for this remote interaction...
)
In the taxonomy of Web Services, WFS is best categorized as a non-RESTful RPC type service.
Communication models
The WFS Web Feature Services or Web Feature Server specification supports two communication models:- Stateless Request Reply
- Pub/Sub
-
- A messaging system in which clients address messages to a specific node in a content hierarchy, called a topic. Publishers and subscribers are generally anonymous and can dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a node's multiple publishers to its multiple subscribers. Messages are generally not persistent and will only be received by subscribers who are listening at the time the message is sent. A special case known as a “durable subscription” allows subscribers to receive messages sent while the subscribers are not active. (Source: http://java.sun.com/j2ee/1.4/docs/glossary.html)
The Web Notification Service (WNS) is one of the implementation specifications for the Pub/Sub model. Regardless of the model, URL
Uniform Resource Locator
In computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
format is used and specified in the WFS specification. At this time there are no open-standard implementations of WNSs. Vendors plan to release implementations once the standard has been ratified.
Data
The Geography Markup Language (GML)Geography Markup Language
The Geography Markup Language is the XML grammar defined by the Open Geospatial Consortium to express geographical features. GML serves as a modeling language for geographic systems as well as an open interchange format for geographic transactions on the Internet...
passes data back and forth between a Web Feature Server and a client. GML normally communicates geospatial data, but also supports other types of data.
GML
GML is an XML dialect which can be used to model geographic features. The 1.0.0 version of the WFS specification requires the use of GML version 2.1.2, while the 1.1.0 version of the WFS specification requires the use of GML version 3.1.1. For both versions of the WFS specification, an arbitrary number of other encodings can also be defined, in addition to the required GML 2.1.2 or 3.1.1 format (for 1.0.0 and 1.1.1 respectively).
GML 2.1.2 contains encoding support for basic geometric 'primitives': points, lines, polygons, etc.
GML 3.1.1 contains encoding support for more advanced geometric representations: curves, surfaces, multi-dimensions (time, elevation, multi-band imagery). In addition, GML 3.1.1 includes encoding support for topologically integrated datasets.
Static Interfaces
The static interface model for the OGCOGC
OGC may refer to:* Oculogyric crisis, a dystonic reaction to certain drugs and/or medical conditions* Office of Government Commerce, a department of the government of the United Kingdom...
Web Service model appears in the figure below.
The Transaction and LockFeature operations are also optional.
When writing a WFS, you must implement the following operations:
- GetCapabilities - this queries the WFS service to determine available options.
- DescribeFeatureType - this retrieves the XML schema to allow the WFS client to parse the resultsets.
- GetFeature - this performs the actual query - parameters such as bounding box and any other filters should be passed in, as appropriate, and the WFS service then returns a GML resultset containing full geometry and feature attributes.
Dynamic interface updates
The client gets updates by one of two mechanisms:- Notification: Recommended but not mandatory. Depends on the availability of a WMS implementation.
- Polling: Use this method if a WMS implementation is not available.
WFS dynamic interface web notification model
This model uses the OGC Web Notification Service to send update notifications to registered clients.
See also
- Open Source Geospatial FoundationOpen Source Geospatial FoundationThe Open Source Geospatial Foundation , is a non-profit non-governmental organization whose mission is to support and promote the collaborative development of open geospatial technologies and data. The foundation was formed in February 2006 to provide financial, organizational and legal support to...
- Web Coverage ServiceWeb Coverage ServiceThe Open Geospatial Consortium Web Coverage Service Interface Standard provides an interface allowing requests for geographical coverages across the web using platform-independent calls...
(WCS) - Web Map ServiceWeb Map ServiceA Web Map Service is a standard protocol for serving georeferenced map images over the Internet that are generated by a map server using data from a GIS database...
(WMS)