A WCS request can be divided into three main components:
All components are concatenated with a "&" into one single URL.
Service endpoint: http://earthserver.ecmwf.int/rasdaman/ows?
Service and version specification: service=WCS&version=2.0.1&
- DescribeCoverage: &coverageId=coverageId
With the add-on &subset a coverage axis can be subsetted. For every subsetting operation a subset request has to be concatenated. If you want to do e.g. a temporal (time axis) and spatial subset (latitude and longitude axes), you have to concatenate three subset requests. If no &subset is specified, the entire coverage is requested, which can be multiple GBs of data</p>
With the add-on &format a custom format can be specified. The supported formats are specified in the GetCapabilites document. The default format is application/gml+xml. If the format parameter is not specified, the result is returned in a xml file. Following formats are currently supported by OGC WCS 2.0.1:
- netCDF: applications/netcdf
- JPEG2000: image/jp2
- GeoTIFF: image/tiff
- XML: application/gml+xml
- PNG: image/png
The following sections show how the concatenated strings per request will look like.
Examples base on ERA-interim 2m air temperature global fields, 6-hourly values from 1 January 1979 to 31 December 2015.
The GetCapabilities request returns the Capabilities document, which is a XML-document that gives general information to the Web Coverage Service Server. If you use the WCS Server for the first time, the GetCapabilities documents is helpful to see what the service offers.
The XML tree contains following child elements:
The DescribeCoverage request returns a CoverageDescription document, that contains the metadata of a dataset encoded in XML.
The example below shows the metadata of the coverage "temp2m", which is ERA-interim 2m air temperature. Important XML tree child elements are:
The GetCoverage operation requests the actual spatial data. The power of the GetCoverage request is the ability to subset domains (spatial and temporal) and to encode the output in different formats. All examples will take 2m air temperature as example coverage.
With the subset parameter, the dimensionality of the coverage can be reduced. From a multi-dimensional data cube, a time series of a specific latitude/longitude grid point or a 2D field for one specific time can be retrieved. Examples are provided below.
By default, the data is returned in a XML file. The XML tree child element rangeSet contains the actual data.
(NOTE: for a coloured output a colour-scheme has to be applied on-the-fly - see Colouring on the fly)
You now know how to access and retrieve data from a WCS server using the WCS core requests. The next tutorial chapter introduces you to Web Coverage Processing Service, which is an extension of the WCS core suite and a powerful tool to access, process and retrieve spatial data on-the-fly.
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