A GetMap request is a mandatory operation for an OGC Web Map Service (WMS). The request retrieves a map (a georeferenced image) of some underlying data source.

A standard GetMap request requires a number of parameters (for example, using GET/KVP syntax name=value&):

  • A version number (such as version=1.3.0& or version=1.1.1&)
  • The request type (request=GetMap&)
  • One or more layers (LAYERS=GBR_BGS-HPA_625K_RADON_POTENTIAL&)
  • A style, can be blank if the default style is intended (STYLES=&)
  • The coordinate reference system (which must be one of those listed in the service capabilities response
    • CRS=EPSG:4326 (use CRS for a version 1.3.0 request)
    • SRS=EPSG:3857 (use SRS for a version 1.1.1 request)
  • Width and Height of the image in pixels (WIDTH=877& HEIGHT=607&)
  • A bounding box in the coordinate reference system coordinates like:
    • BBOX=50.179445759563,-4.41340973640857,50.9615344710795,-3.2834397199341&
  • An output format for the map image (like: FORMAT=image/png&)

Output formats for a GetMap operation are listed for the whole WMS service in the GetCapabilities response document such as:

<GetMap>
  <Format>image/png</Format>
  <Format>image/jpeg</Format>
  <Format>image/gif</Format>
  <Format>image/png; mode=8bit</Format>
  <Format>application/x-pdf</Format>
  <Format>image/svg+xml</Format>
  <Format>image/tiff</Format>
  <Format>application/vnd.google-earth.kml+xml</Format>
  <Format>application/vnd.google-earth.kmz</Format>

Coordinate reference systems supported by the Layer are normally listed in the layer metadata like:

<Layer queryable="1" opaque="0" cascaded="0">
  <Name>GBR_BGS-HPA_625K_RADON_POTENTIAL</Name>
  ...
  <CRS>CRS:84</CRS>
  <CRS>EPSG:27700</CRS>
  <CRS>EPSG:3031</CRS>
  <CRS>EPSG:3034</CRS>
  <CRS>EPSG:3413</CRS>
  <CRS>EPSG:3857</CRS>
  <CRS>EPSG:4258</CRS>
  <CRS>EPSG:4326</CRS>
  <CRS>EPSG:900913</CRS>
  ...

Sometimes the coordinate reference systems supported by the the layers of the service are listed in the root layer (and apply to all layers in the service).