In this primer we will demonstrate how to obtain a graps of a small set of data from a large colection datasets with using the OGC Web Mapping Service (WMS) protocol. We will show that ordering requesting 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 servers can be configured to serve WMS for orthogonal datasets with proper geographic information.
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Ask for what the server has to offer (see which flavours he has and which kind of cups). Ypu You need to add the following mandatory <keyword,value> pairs to the base server url, separated by an &, e.g.: ?service=WMS&request=GetCapabilities.
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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.as shown in the reduced xml file example below:
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<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 WMS 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 dataset is the name tag, in this case there is only topo. The xml file indicates that this dataset can be delivered in four well-known image different formats (this ice cream flavour ice cream is available in a cup, a cone and on a stick): jpg, png, kmz ad gif.
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<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. Note that this dataset is available in 9 coordinate systems (EPSG:4326 is required, this is the WGS84 lat-lon system), and in 11 different color maps.
Request some WMS server content
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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 |
BBOX | 0,50,10,55 | bounding box: min(longitude),min(latitude),max(longitude),max(latitude) |
layers | topo | WMS counterpart of WCS |
format | image/png | One of the mandatory WMS standard values by returned by the |
CRS | EPSG%3A4326 | One of the server-defined values by returned by the |
width | 800 | width of the image in number of pixels. The xml file show a max |
height | 600 | height of the image in number of pixels |
styles | boxfill/redblue | One of the server-defined values by returned by the |
Summary: correct A valid WMS request for an example for one of the available formats is given below. You can copy it into your browser to obtain the image, and see that is works. We chose the same bounding boxes as in the OPeNDAP primer that access these very same datasets via netCDF libraries in Matlab, python or R. We discussed the pros and cons of WCS vs OPeNDAP in a FOSS4G paper paper in Transactions in GIS that was presented at the FOSS4G conference.
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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 |
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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.