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 WMS server

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.

Request an overview of the content of a WMS server

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.

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.)

Inspect the overview of the content of a WMS server

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.

Request some WMS server content

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 for the Getmap request.

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:

More

WMS offers more than described in this primer. It it also possible to
request a legend for a specific style, and to request the data at one pixel
an image you requested. For these more sophisticated options, please refer to the full WMS specifications.