# WCS -- OGC Web Coverage Service

The optional GDAL WCS driver allows use of a coverage in a WCS server as a raster dataset. GDAL acts as a client to the WCS server.

Accessing a WCS server is accomplished by creating a local service description file that contains one `<WCS_GDAL>` XML element. It is important that there is no spaces or other content outside that element. Starting at version 2.3 the service description file is meant to be managed by the driver in a cache directory. User should control the contents of the service file using options. The dataset name is `"WCS:<URL>"`, where the <URL> is the URL of the server appended potentially appended with WCS version, coverage, and possibly other parameters. If the WCS version is 2.0.1 further parameters can be given to control how the data model of the coverage is mapped to the GDAL data model.

If the URL does not contain a coverage name, the driver attempts to fetch the capabilities document from the server, parse it, and show the resulting metadata to the user. Coverages are shown as subdatasets. If the URL contains a coverage name as parameter (the key 'coverage' can be used irrespective of the WCS version), the driver attempts to fetch the coverage description document from the server, parse it, and create service description file. A small test GetCoverage request may be done to obtain details of the served data. If the respective server capabilities file is not cached, it will also be fetched.

With service version 2.0.1 (for which support is available starting at GDAL version 2.3), it may be that the coverage has more than two dimensions. In that case, the driver will append the coverage metadata and show zero bands. At that point, the user must use options to further instruct the driver how to deal with extra dimensions and data fields.

The WCS driver supports WCS versions 1.0.0, 1.1.0, 1.1.1, 1.1.2, and 2.0.1 at basic level (version 0.7 is not supported and support for version 2.0.1 is available starting at GDAL 2.3). Any return format that is a single file, and is in a format supported by GDAL should work. The driver will prefer a format with "tiff" in the name, otherwise it will fallback to the first offered format. However, the user may set the preferred format. Coordinate systems are read from the DescribeCoverage result.

The service description file has the following elements as immediate children of the document element. Note that when the "WCS:<URL>" syntax for dataset name is used, the contents of the service description file is meant to be modified by using options.

- ServiceURL: URL of the service without parameters
- Version: The WCS version that is used in the communication. If the dataset name syntax WCS:URL is used the default is 2.0.1 and the server's response may change the user request, otherwise defaults to 1.0.0. Versions 1.0.0, 1.1.0, 1.1.1, 1.1.2, and 2.0.1 are supported.
- CoverageName: The coverage that is used for the dataset.
- Format: The format to use for GetCoverage calls. If not set, selected by the driver. (WCS version 2.0)
- PreferredFormat: The format to use for GetCoverage calls. If not set, selected by the driver. (WCS versions 1.0 and 1.1)
- Interpolation: The interpolation method used when scaling. Should be one of the server supported values. (GDAL 2.3)
- BlockXSize: The block width to use for block cached remote access.
- BlockYSize: The block height to use for block cached remote access.
- OverviewCount: 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).
- NoDataValue: The nodata value to use for all the bands (blank for none). Normally determined by the driver. With version 2.0.1 this may be a comma separated list of values, one for each band.

- Elements for controlling the range and domain:
- Domain: The axes that are used for the spatial dimensions. The default is to use the first two axes given by the server. The axis order swap may apply. Syntax: axis_name,axis_name A field_name:field_name in the list denotes a range of fields. (Used only with version 2.0.1)
- DefaultTime: A timePosition to use by default when accessing coverages with a time dimension. Populated with the last offered time position by default. (Used only with version 1.0.0)
- FieldName: Name of the field being accessed. Used only with version 1.1. Defaults to the first field in the DescribeCoverage. In version 1.1 the range consists of one or more fields, which may be scalar or vector. A vector field contains one or more bands.
- BandCount: Number of bands in the dataset, normally determined by the driver.
- BandType: The pixel data type to use. Normally determined by the driver.

- Elements for controlling the communication:
- Timeout: The timeout to use for remote service requests. If not provided, the libcurl default is used.
- UserPwd: May be supplied with userid:password to pass a userid and password to the remote server.
- HttpAuth: May be BASIC, NTLM or ANY to control the authentication scheme to be used.
- GetCoverageExtra: An additional set of keywords to add to GetCoverage requests in URL encoded form. eg. "&RESAMPLE=BILINEAR&Band=1". Note that the extra parameters should not be known parameters (see below).
- DescribeCoverageExtra: An additional set of keywords to add to DescribeCoverage requests in URL encoded form. eg. "&CustNo=775"

- Elements that may be needed to deal with server quirks (GDAL 2.3):
  Note: The options are not propagated to the subdataset with the switch -sd.

- OriginAtBoundary: Set this flag if the server reports grid origin to be at the pixel boundary instead of the pixel center. (Use for MapServer versions <= 7.0.7 with WCS versions 1.0.0 and 2.0.1)
- OuterExtents: Set to consider WCS 1.1 extents as boundaries of outer pixels instead of centers of outer pixels. (Use for GeoServer).
- BufSizeAdjust: Set to 0.5 in WCS 1.1 if data access fails due to the response not having expected size. (Use for GeoServer).
- OffsetsPositive: Use with MapServer in WCS version 2.0.1 together wwith NrOffsets.
- NrOffsets: Set to 2 if the server requires that there are only two values in the GridOffsets. Use when the server is MapServer or ArcGIS. With MapServer use also OffsetsPositive.
- GridCRSOptional: Let the driver skip Grid* parameters from a WCS 1.1 GetCoverage request if the request is not scaled. Do not use for GeoServer.
- NoGridAxisSwap: Set to tell the driver not to swap axis order. When reading the grid geometry (in WCS 1.1 the origin and offsets, in WCS 2.0.1 the grid envelope, axis labels, and offsets) no axis order swap is done although it would otherwise be done if this flag is set. In 1.1 it would be done if the CRS has inverted axes. In 2.0.1 it would be done if the axisOrder of the sequenceRule in GridFunction defines so. This is needed usually both in 1.1 and 2.0.1 when parsing coverage descriptions from MapServer and GeoServer.
- SubsetAxisSwap Set to tell the driver to swap the axis names in boundedBy.Envelope.axisLabels when making WCS 2.0.1 GetCoverage request. Needed for GeoServer when EPSG dictates axis order swap.
- UseScaleFactor: Set to tell the driver to use scale by factor approach instead of scale to size when making a WCS 2.0.1 GetCoverage request. Required when the server is ArcGIS.

### Range and dimension subsetting

When WCS version 2.0.1 is used, the range (fields/bands) and the dimension can and/or may need to be subsetted. If the data model of the coverage contains dimensions beyond the two geographic or map coordinates, those dimensions must be sliced for GDAL. The coverage may also contain a large number of fields, from which only a subset is wanted in the GDAL dataset.

Range and dimension subsetting must be done via URL parameters since from one coverage it is possible to create more than one different GDAL datasets. In the WCS cache this means that there may be the sets of files related to a GDAL dataset:

1. server Capabilities file and a GDAL dataset metadata file made from it (key = URL with WCS version number)
2. server DescribeCoverage file, a template GDAL service file made from it, and a GDAL dataset metadata file made for it (key = URL with WCS version number and coverage name)
3. the GDAL service file specifically for this dataset, and a GDAL dataset metadata file (key = URL with WCS version number, coverage name, and range and dimension subsetting parameters)

The following URL parameters are used to control the range and dimension subsetting. Note that these can also be set through options into the service file. The ones in URL take precedence.

- RangeSubset: Used to select a subset of coverage fields to the dataset. Syntax: field_name,field_name:field_name,.. (Note: requires that the server implements the range subsetting extension.)
- Subset: Trim or slice a dimension when fetching data from the server. Syntax: axis_name(trim_begin_value,trim_end_value);axis_name(slice_value) Note that trimming the geographic/map coordinates is done by the driver.

### Other WCS parameters

The following WCS (version 2.0.1) parameters are recognized besides what has been described above. These all can be set either through options or directly into the URL. The ones in URL take precedence. Note that it is up to the server to support/recognize these.

- MediaType
- UpdateSequence
- GEOTIFF:COMPRESSION
- GEOTIFF:JPEG_QUALITY
- GEOTIFF:PREDICTOR
- GEOTIFF:INTERLEAVE
- GEOTIFF:TILING
- GEOTIFF:TILEWIDTH

### Open options

When the "WCS:<URL>" dataset name syntax is used, open options are used to control the driver and the contents of the service description file. In the case the URL does not contain coverage name, the service description file is not used and thus in that case the options are not written into it. Open options are given separate to the dataset name, with GDAL utility programs they are given using the -oo switch (`-oo "NAME=VALUE"`). The -oo swicth expects only one option but more options can be given repeating the switch.

In addition to DescribeCoverageExtra and GetCoverageExtra, which are stored in the service description file, there is also GetCapabilitiesExtra, which can be used as an open option when requesting the overall capabilities from the server. The open option SKIP_GETCOVERAGE can be used to prevent the driver making a GetCoverage request to the server, which it usually does if it can't determine the band count and band data type from the capabilities or coverage descriptions. This option may be needed if GetCoverage request fails.

All above listed element names can be given as options to the WCS driver. In the case of flags the option should formally be "Name=TRUE", but only "Name" suffices.

### The cache

When the "WCS:<URL>" dataset name syntax is used, the server responses, the service description file, and the metadata files are stored in a cache. Generally, if the needed resource is in the cache, it will be used and no extra calls to the server are done.

The default location of the cache directory is $HOME/.gdal/wcs_cache

The cache contents can be seen as subdatasets using an empty URL:

    gdalinfo "WCS:"

The cache control options/flags are

- CACHE=path Overrides the default cache location.
- CLEAR_CACHE The cache is completely initialized and all files are deleted.
- REFRESH_CACHE The cache entry, either capabilities or coverage, depending on the call at hand, is deleted.
- DELETE_FROM_CACHE=k The cache entry (subdataset k), is deleted.

### The WCS: dataset name syntax

The URL in the dataset name is not a complete WCS request URL. The request URL, specifically, its query part, for GetCapabilities, DescribeCoverage, and GetCoverage requests is composed by the driver. Typically the user should only need to add to the server address the version and coverage parameters. The string 'coverage' can be used as the coverage parameter key although different WCS versions use different keys. 'coverage' is also always used in the cache key.

The user may add arbitrary standard and non-standard extra parameters to the URL. However, when that is done, it should be noted that the URL is a cache database key and capability documents are linked to coverage documents through the key. Please consider using the Extra open options.

### Time

Starting with GDAL 1.9.0, this driver includes experimental support for time based WCS 1.0.0 servers. On initial access the last offered time position will be identified as the DefaultTime. Each time position available for the coverage will be treated as a subdataset.

Note that time based subdatasets are not supported when the service description is the filename. Currently time support is not available for versions other than WCS 1.0.0.

See Also:

- OGC WCS Standards