Trace: Web Map Service

Web Map Service

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An OGC Web Map Service (WMS) produces maps of spatially referenced data dynamically from geographic information. This international standard defines a "map" to be a portrayal of geographic information as a digital image file suitable for display on a computer screen. A map is not the data itself. WMS-produced maps are generally rendered in a pictorial format such as PNG, GIF or JPEG, or occasionally as vector-based graphical elements in Scalable Vector Graphics (SVG) or Web Computer Graphics Metafile (WebCGM) formats. This is in contrast to a Web Feature Service (WFS), which returns actual vector data and a Web Coverage Service (WCS), which returns actual raster data.

This International Standard defines three operations:

returns service-level metadata
returns a map whose geographic and dimensional parameters are well-defined
returns information about particular features shown on a map (optional)

Web Map Service operations can be invoked using a standard web browser by submitting requests in the form of Uniform Resource Locators (URLs). The content of such URLs depends on which operation is requested. In particular, when requesting a map the URL indicates what information is to be shown on the map, what portion of the earth is to be mapped, the desired coordinate reference system, and the output image width and height. When two or more maps are produced with the same geographic parameters and output size, the results can be accurately overlaid to produce a composite map. The use of image formats that support transparent backgrounds (e.g., GIF or PNG) allows underlying maps to be visible. Furthermore, individual maps can be requested from different servers. The Web Map Service thus enables the creation of a network of distributed map servers from which clients can build customized maps.

A Web Map Service is usually not invoked directly. More often, it is invoked by a client application that provides the user with interactive controls. This client application may or may not be web-based.

How does a WMS Work

WMS servers interact with their clients via the HTTP protocol. In most cases, a WMS server is a CGI program. The following text describes how Mapserver is working.

The WMS specification defines a number of request types, and for each of them a set of query parameters and associated behaviors. A WMS-compliant server MUST be able to handle at least the following 2 types of WMS requests:

GetCapabilities: return an XML document with metadata of the Web Map Server's information
GetMap: return an image of a map according to the user's needs.

And support for the following types is optional:

GetFeatureInfo: return info about feature(s) at a query (mouse click) location. MapServer supports 3 types of responses to this request:
- Text/plain output with attribute info.
- Text/html "wms_feature_info_mime_type".
- GML features.
DescribeLayer: return an XML description of one or more map layers. To execute this:
- for vector layers: to have a valid return the user needs to setup wfs_onlineresource (or ows_onlineresource) metadata either at the map level or at the layer level (the layer level metadata is the one which is used if both are defined)
- for raster layers: the metadata is wcs_onlineresource with the same logic as above.
GetLegendGraphic: returns a legend image (icon) for the requested layer, with label(s). More information on this request can be found in the GetLegendGraphic section later in this doc.

With respect to MapServer specifically, it is the "mapserv" CGI program that knows how to handle WMS requests. So setting up a WMS server with MapServer involves installing the mapserv CGI program and a setting up a mapfile with appropriate metadata in it. This is covered in the rest of this document.

WMS-Services

[open subpage]

WMS Sites Visual catalog of OGC WMS Services

Skylab Mobilesystems List of OGC WMS Services (Freie OGC konforme WMS-Server)

Refractions Research List von WMS, WFS und anderen Services, nach Ländern sortierbar.

geometa.info/: geometa.info ist eine Spezialsuchmaschine - eine Art 'GeoGoogle' - zum Suchen von kartografisch aufbereiteten Geoinformationen. (WMS-Dienste)