WMS-C




 * WMS-C (Web Map Service - Cached) is a tile caching service wrapped in WMS request/response. It is a way to optimize the map delivery performance by keeping the pre-rendering tiles in a web cache. The client communicates with the cache instead of the main WMS, so if a tile is not rendered yet or expired, the main WMS is called, otherwise the pre-rendered tile is delivered. OSM-GB uses this service to provide fast delivery over low to medium zoom levels.


 * The URL of the WMS-C (to be used in GIS applications) is  http://www.osmgb.org.uk/ogc/wmsc?version=1.1.1&tiled=true&reaspect=false 


 * The URL of the WMS-C' GetCapabilities request is http://www.osmgb.org.uk/ogc/wmsc?Version=1.1.1&Service=WMS&Request=GetCapabilities


 * WMS-C serves the single flatten layer (called "OSMGB:__all__").


 * The supported CRS is currently set to EPSG:4326 (Latitude-Longitude) and EPSG:27700 (British National Grid).


 * The supported output format is png.

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 * WMS-C has a predefined set of zoom levels, resolutions, pixel sizes and scales. This settings are mostly not needed for desktop mapping, however it may be needed for advanced users or in web map clients coding like in OpenLayers. The resolutions set for WMS-C are shown in the right side.

For QGIS Users

 * When adding the WMS-C layers, use the "Tile Sets" tab (instead of "layers" tab). Use this qgis project as a sample WMS-C in British National Grid. QGIS verion 1.6.0 (Capiapo) and upper shall be used.


 * The server resolutions matrix can be different from the end client window. In such cases GIS applications like QGIS may try to scale the tile to fit to its defined resolution which reduce the graphical quality of the output maps. Snapping to the fixed zoom steps of the GIS applications may help maintaining the quality.

For OpenLayers Users
Web mapping client tools like OpenLayers may need the resolutions to be manually set in JavaScript code. Also the client dpi setting may be different from the default (90 dpi) set on the server, which needs to manually set. A demonstration template is also available which includes the necessary settings in JavaScript.

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Acknowledgement
The contribution from Jonathan Moules (of Warwickshire Council) in establishing this service is highly appreciated.