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82 lines
4 KiB
HTML
82 lines
4 KiB
HTML
<html>
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<head>
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<title>WCS -- OGC Web Coverage Service</title>
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</head>
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<body bgcolor="#ffffff">
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<h1>WCS -- OGC Web Coverage Service</h1>
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The optional GDAL WCS driver allows use of a coverage in a WCS server as a
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raster dataset. GDAL acts as a client to the WCS server.<p>
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Accessing a WCS server is accomplished by creating a local service description
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xml file looking something like the following, with the coverage server url,
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and the name of the coverage to access. It is important that there be no
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spaces or other content before the <tt><WCS_GDAL></tt> element.<p>
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<pre>
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<WCS_GDAL>
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<ServiceURL>http://laits.gmu.edu/cgi-bin/NWGISS/NWGISS?</ServiceURL>
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<CoverageName>AUTUMN.hdf</CoverageName>
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</WCS_GDAL>
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</pre>
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When first opened, GDAL will fetch the coverage description, and a small test
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image to establish details about the raster. This information will be cached
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in the service description file to make future opens faster - no server access
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should be required till imagery is read for future opens.<p>
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The WCS driver should support WCS 1.0.0 and 1.1.0 servers, but WCS 0.7 servers
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are not supported. Any return format that is a single file, and is in a format
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supported by GDAL should work. The driver will prefer a format with "tiff"
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in the name, otherwise
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it will fallback to the first offered format. Coordinate systems are read
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from the DescribeCoverage result, and are expected to be in the form of
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<tt>EPSG:n</tt> in the <tt><supportedCRSs></tt> element. <p>
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The service description file has the following additional elements as
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immediate children of the <tt>WCS_GDAL</tt> element that may be optionally
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set.<p>
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<ul>
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<li> <b>PreferredFormat</b>: the format to use for GetCoverage calls.<p>
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<li> <b>BandCount</b>: Number of bands in the dataset, normally captured from the sample request.
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<li> <b>BandType</b>: The pixel data type to use. Normally established from the sample request.
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<li> <b>BlockXSize</b>: The block width to use for block cached remote access.
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<li> <b>BlockYSize</b>: The block height to use for block cached remote access.
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<li> <b>NoDataValue</b>: The nodata value to use for all the bands (blank for none). Normally defaulted from CoverageOffering info.
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<li> <b>Timeout</b>: The timeout to use for remote service requests. If not provided, the libcurl default is used.
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<li> <b>UserPwd</b>: May be supplied with <i>userid:password</i> to pass a
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userid and password to the remote server.
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<li> <b>HttpAuth</b>: May be BASIC, NTLM or ANY to control the authentication scheme to be used.
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<li> <b>OverviewCount</b>: The number of overviews to represent bands as having. Defaults to a number such that the top overview is fairly smaller (less than 1K x 1K or so).
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<li> <b>GetCoverageExtra</b>: An additional set of keywords to add to GetCoverage requests in URL encoded form. eg. "&RESAMPLE=BILINEAR&Band=1"
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<li> <b>DescribeCoverageExtra</b>: An additional set of keywords to add to DescribeCoverage requests in URL encoded form. eg. "&CustNo=775"
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<li> <b>Version</b>: Set a specific WCS version to use. Currently defaults to 1.0.0 and 1.1.0 is also supported.
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<li> <b>FieldName</b>: Name of the field being accessed. Used only with WCS 1.1.0+. Defaults to the first field in the DescribeCoverage result.
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<li> <b>DefaultTime</b>: A timePosition to use by default when accessing
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coverages with a time dimension. Populated with the last offered time
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position by default.
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</ul>
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<h3>Time</h3>
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Starting with GDAL 1.9.0, this driver includes experimental support for time based WCS 1.0.0 servers.
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On initial access the last offered time position will be identified as the
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DefaultTime. Each time position available for the coverage will be treated
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as a subdataset. <p>
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Note that time based subdatasets are not supported when the service description
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is the filename. Currently time support is not available for versions other
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than WCS 1.0.0. <p>
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See Also:<p>
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<ul>
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<li> <a href="http://www.opengeospatial.org/standards/wcs">OGC WCS Standards</a>
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</ul>
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</body>
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</html>
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