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.
| 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:
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.
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.
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.
| 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. CRS always consists of namespace:identifier, in this case there are two namespaces.For more info on the projections see spatialreference.org and epsg-registry.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. Unfortunately, it is not easily possible to supply your own colormap and colorlimits. for this you need to obtain the data and plot it yourselves, please refer to our WCS primer. |
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.
Some additional, optional WMS keywords are:
| keyword | value | source |
|---|---|---|
| TRANSPARENT | True | Background transparency of map, valid for encoding format that can handle transparency, e.g. png. |
| time | yyyy-mm-ddTHH:MM:SSZ | ISO time notation. Only when data are time-dependent. When not specified for data that is time-depended, the latest time is returned. Time is not implemented in most GIS-minded software packages yet. For an example on how to specify time requests see MapServer . For an example of time implementation see the ADAGUC WMS server and WMS web client. |
| elevation | number |
elevation of layer desired |
| BGCOLOR | background color of map |
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style boxfill/sst_36
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style boxfill/rainbow
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style boxfill/occam_pastel-30
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style boxfill/alg2
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style boxfill/ferret
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style boxfill/occam
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style boxfill/alg
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style boxfill/redblue
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style boxfill/ncview
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style boxfill/grayscale
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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.