Web Map Service

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A Web Map Service ( WMS ) is an interface for calling up extracts from maps via the World Wide Web . The WMS is a special case of a web service .

The WMS Implementation Specifications

The specification of the web map service was written by the Open Geospatial Consortium (OGC). This describes the parameters that must or can be named in a request. It also defines how the WMS server is to generate the extract and, if necessary, the content of the map from such a request .

According to the specifications of the OGC, a WMS server can visualize the requested maps from raster data or vector data . In terms of a distributed geographic information system (GIS), a WMS only has the ability to provide information on the necessary meta information , to visualize this geodata and to generally query the underlying factual data . The result, i.e. the map, is usually returned by the WMS in a simple raster graphic format . Depending on the structure, various other file formats such as Scalable Vector Graphics ( SVG ) or Web Computer Graphics Metafile ( WebCGM ) can also be created. B. is necessary for temporary data with a time reference as a film . Therefore, the user can basically address a web map service via his web browser , in which the map can then be displayed. The Web Map Service is thus part of a Web GIS .

Technical background of an OGC-compliant WMS

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

An OGC Compliant WMS - i.e. H. a web map service that meets the specification of the Open Geospatial Consortium - has three functions that can be requested by a user. So far, the Hypertext Transfer Protocol ( HTTP ) has been used for communication . The three functions are sent as HTTP requests from the user to the WMS and are in an OGC-compliant WMS:

  1. GetCapabilities: The WMS is asked about the capabilities of the WMS. In response, an XML document with meta information is sent back to the user, which, in addition to general information about the WMS provider, contains the supported output formats of the WMS for the various queries and the queryable layers for the map.
  2. GetMap: This request returns a georeferenced raster image (map) from the WMS. Within the request, u. a. Options for the desired map layers=...layer ( ), the desired display of the layer ( styles=...), the underlying coordinate system ( srs=...), the map section ( bbox=...), the size of the map width=...&height=...output ( format=...) and the output format ( ) can be made.
  3. GetFeatureInfo(optional): A WMS can voluntarily answer inquiries about a position in the map section shown. As output, it provides specified thematic information on the underlying data, usually in XML format.

Embedding a WMS in modern geographic information systems (GIS)

Usually, a WMS is not used as the only service in a GIS , but as a component in a complex of different GIS services, in which it is only necessary for displaying a map. Accordingly, there are a number of other specifications of the OGC that regulate , among other things, access to the underlying geodata ( Web Feature Service for vector data and Web Coverage Service for raster data) as well as various other standards in this context.

A WMS can hardly be used efficiently without an intermediary client that gives the user the opportunity to change the map section (for example by moving around the map or zooming in or out).

Expanded with a tile cache , a WMS becomes a Web Map Tile Service (WMTS), Web Map Service Caching (WMS-C) or Tile Map Service (TMS).

See also

Web links