WMS primer

In this primer we will demonstrate how to obtain a graps of a small set of data from a large datasets with the OGC Web Mapping Service (WMS) protocol. We will show that ordering images through WMS is just as easy as buying an ice cream.

• Find a data web source that hosts a WMS server (go to an ice cream vendor). We will use the Shuttle Radar Topography Mission datasets hosted at the Woods Hole Institute THREDDS OPeNDAP server as example in this primer. THREDDS OPeNDAP server can be configured to serve WMS for orthogonal datasets with proper geographic information.
• Ask for what the server has to offer (see which flavours he has and which kind of cups). Ypu need to add the following mandatory <keyword,value> pairs to the base server url, separated by an &, e.g.: ?service=WMS&request=GetCapabilities.

  keyword	value	source
  service	WMS	Mandatory WMS standard value
  request	GetCapabilities	Mandatory WMS standard value

  This procedure works for all subsequent <keyword,value> pairs in this primer:

  http://geoport.whoi.edu/thredds/wms/bathy/srtm30plus_v6?service=WMS&request=GetCapabilities
  

  This url will return an xml file that contains an inventory of the available datasets. (You can also request availabe datasets for one WMS version only by appending the optional version keyword.)
• Look at what versions of WMS the server has to offer (check whether the ice cream is fresh). For each version there is a
  tag WCS_Capabilities with attribute version, e,.g.

  <WMS_Capabilities version="1.3.0" ... >
  
  <Capability>
  <Request>
  ...
  <GetMap>
  <Format>image/jpeg</Format>
  <Format>image/png</Format>
  <Format>application/vnd.google-earth.kmz</Format>
  <Format>image/gif</Format>
  ...
  </GetMap>
  ...
  </WMS_Capabilities>
  

  For each version a number of datasets is hosted. Select one dataset (ice cream flavour) from the list through the Layer tag. The name of the dataest is the name tag, in this case there is only topo.

  <Layer>
   ...
   <CRS>EPSG:4326</CRS>
   <CRS>CRS:84</CRS>
   <CRS>EPSG:41001</CRS>
   <CRS>EPSG:3857</CRS>
   <CRS>EPSG:27700</CRS>
   <CRS>EPSG:3408</CRS>
   <CRS>EPSG:3409</CRS>
   <CRS>EPSG:32661</CRS>
   <CRS>EPSG:32761</CRS>
   ...
   <Layer>
    <Title>UCSD SRTM30_v6 Global DEM (30 sec)</Title>
    <Layer queryable="1">
    <Name>topo</Name>
    ...
    <Style><Name>boxfill/redblue        </Name>...
    <Style><Name>boxfill/redblue        </Name>...
    <Style><Name>boxfill/alg            </Name>...
    <Style><Name>boxfill/ncview         </Name>...
    <Style><Name>boxfill/greyscale      </Name>...
    <Style><Name>boxfill/alg2           </Name>...
    <Style><Name>boxfill/occam          </Name>...
    <Style><Name>boxfill/rainbow        </Name>...
    <Style><Name>boxfill/sst_36         </Name>...
    <Style><Name>boxfill/ferret         </Name>...
    <Style><Name>boxfill/occam_pastel-30</Name>
    ...
    </Layer>
   </Layer>
  </Layer>
  

  From the above xml extract we will can distill all values necesarry to request an image.
• Now we can actually get a subset from the dataset want by using request=GetMap instead of the request=GetCapabilities we used above to obtain the meta-data (order ice cream). The following <keyword,value> pairs are mandatory.

  keyword	value	source
  service	WMS	Mandatory WMS standard value
  request	GetMap	Mandatory WMS standard value
  version	1.3.0	One of the mandatory WMS standard values returned by returned by the request=GetCapabilities
  BBOX	0,50,10,55	bounding box: min(longitude),min(latitude),max(longitude),max(latitude)
  layers	topo	WMS counterpart of WCS coverage
  format	image/png	One of the mandatory WMS standard values by returned by the request=GetCapabilities tag <Capability><Request><GetCapabilities><Format>. In the example above there are 4 formats.
  CRS	EPSG%3A4326	One of the server-defined values by returned by the request=GetCapabilities tag <Layer><Layer><Layer queryable="1"><CRS>. In the example above there are 9 geographic projections available . Note that the colon in EPSG:4326 has been replaced by by %3A because colons are not allowed in urls. For more info on the projections see spatialreference.org
  width	800	width of the image in number of pixels. The xml file show a max MaxWidth and MaxHeight allowd by the server.
  height	600	height of the image in number of pixels
  styles	boxfill/redblue	One of the server-defined values by returned by the request=GetCapabilities tag <Layer><Layer><Layer queryable="1"><Name>topo</Name><Style><Name>. In the example above there are 10 styles, for screenshots click the gallery below.

Summary: correct WMS request for an available formats is given below. You can copy it into your browser to obtain the image. We chose the same bounding boxes as in the OPeNDAP primer that access these same datasets via netCDF libraries in Matlab, python or R. We discussed the pros and cons of WCS vs OPeNDAP in a FOSS4G paper.

http://geoport.whoi.edu/thredds/wms/bathy/smith_sandwell_v11?service=WMS
&version=1.3.0
&request=GetMap
&bbox=0,50,10,55
&layers=topo
&format=image/png
&crs=EPSG%3A4326
&width=800
&height=600
&styles=boxfill/redblue

Some additional, optional WMS keywords are: