3.4. Web Coverage Service (WCS)

The 1.0.0 and 1.1.1 WCS standards implemented in QGIS Server provide a HTTP interface to access raster data, referred to as coverage, coming from a QGIS project.

Specifikacijos

Standartinės QGIS Serverio teikiamos užklausos:

Užklausa	Aprašymas
GetCapabilities	Returns XML metadata with information about the server
DescribeCoverage	Retrieves XML document about additional information about coverages
GetCoverage	Retrieves coverage

3.4.1. GetCapabilities

Standard parameters for the GetCapabilities request according to the OGC WCS 1.1.1 specifications:

Parameter	Required	Aprašymas
SERVICE	Yes	Name of the service (WCS)
REQUEST	Yes	Name of the request (GetCapabilities)
VERSION	No	Version of the service

In addition to the standard ones, QGIS Server supports the following extra parameters:

Parameter	Required	Aprašymas
MAP	Yes	QGIS project file

URL example:

http://localhost/qgisserver?
SERVICE=WCS
&VERSION=1.1.1
&REQUEST=GetCapabilities

XML document example when a single raster layer (named T20QPD_20171123T144719_TCI) is published in the QGIS project for the WCS service:

<WCS_Capabilities xmlns="http://www.opengis.net/wcs" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:gml="http://www.opengis.net/gml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.0.0" updateSequence="0" xsi:schemaLocation="http://www.opengis.net/wcs http://schemas.opengis.net/wcs/1.0.0/wcsCapabilities.xsd">
  <Service>
    ...
  </Service>
  <Capability>
    ...
  </Capability>
  <ContentMetadata>
    <CoverageOfferingBrief>
      <name>T20QPD_20171123T144719_TCI</name>
      <label>T20QPD_20171123T144719_TCI</label>
      <lonLatEnvelope srsName="urn:ogc:def:crs:OGC:1.3:CRS84">
        <gml:pos>-61.585973 16.331189</gml:pos>
        <gml:pos>-61.52537 16.400376</gml:pos>
      </lonLatEnvelope>
    </CoverageOfferingBrief>
  </ContentMetadata>
</WCS_Capabilities>

VERSION

This parameter allows to specify the version of the service to use. Currently, the version values is not internally used and always fallback to 1.1.1.

3.4.2. DescribeCoverage

This request allows to retrieve additional information about coverages like the format of the underlying datasource, the number of bands, … Standard parameters for the DescribeCoverage request according to the OGC WCS 1.1.1 specifications:

Parameter	Required	Aprašymas
SERVICE	Yes	Name of the service (WCS)
REQUEST	Yes	Name of the request (DescribeCoverage)
VERSION	No	Version of the service
COVERAGE	No	Specify coverage layers (WCS 1.0.0)
IDENTIFIER	No	Specify coverage layers (WCS 1.1.1)

In addition to the standard ones, QGIS Server supports the following extra parameters:

Parameter	Required	Aprašymas
MAP	Yes	QGIS project file

The XML document for a 3 bands GeoTIFF raster layer looks like:

<CoverageDescription xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.opengis.net/wcs" xmlns:gml="http://www.opengis.net/gml" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" updateSequence="0" version="1.0.0" xsi:schemaLocation="http://www.opengis.net/wcs http://schemas.opengis.net/wcs/1.0.0/describeCoverage.xsd">
  <CoverageOffering>
    <name>T20QPD_20171123T144719_TCI</name>
    <label>T20QPD_20171123T144719_TCI</label>
    <lonLatEnvelope srsName="urn:ogc:def:crs:OGC:1.3:CRS84">
      ...
    </lonLatEnvelope>
    <domainSet>
      ...
    </domainSet>
    <rangeSet>
      <RangeSet>
        <name>Bands</name>
        <label>Bands</label>
        <axisDescription>
          <AxisDescription>
            <name>bands</name>
            <label>bands</label>
            <values>
              <singleValue>1</singleValue>
              <singleValue>2</singleValue>
              <singleValue>3</singleValue>
            </values>
          </AxisDescription>
        </axisDescription>
      </RangeSet>
    </rangeSet>
    <supportedCRSs>
    ...
    </supportedCRSs>
    <supportedFormats nativeFormat="raw binary">
      <formats>GeoTIFF</formats>
    </supportedFormats>
  </CoverageOffering>
</CoverageDescription>

COVERAGE

This parameter, defined in WCS 1.0.0, allows to specify the layers to query for additional information. Names have to be separated by a comma.

In addition, QGIS Server introduced an option to select layers by its short name. The short name of a layer may be configured through Properties ► Metadata in layer menu. If the short name is defined, then it’s used by default instead of the layer’s name:

http://localhost/qgisserver?
SERVICE=WCS
&REQUEST=DescribeCoverage
&COVERAGE=mylayer1name,mylayer2shortname

Pastaba

COVERAGE is mandatory if IDENTIFIER is not set.

IDENTIFIER

This parameter replaces the COVERAGE parameter in WCS 1.1.1. But QGIS Server does not filter according to the VERSION parameter so IDENTIFIER and COVERAGE have the same effect.

Pastaba

IDENTIFIER is mandatory if COVERAGE is not set. If both IDENTIFIER and COVERAGE parameters are defined, COVERAGE is always used in priority.

3.4.3. GetCoverage

This request allows to retrieve the coverage according to specific constraints like the extent or the CRS. Standard parameters for the DescribeCoverage request according to the OGC WCS 1.1.1 specifications:

Parameter	Required	Aprašymas
SERVICE	Yes	Name of the service (WCS)
REQUEST	Yes	Name of the request (GetCoverage)
VERSION	No	Version of the service
COVERAGE	No	Specify coverage layers (WCS 1.0.0)
IDENTIFIER	No	Specify coverage layers (WCS 1.1.1)
WIDTH	Yes	Width of the response in pixels
HEIGHT	Yes	Height of the response in pixels
BBOX	Yes	Map extent in CRS units
CRS	Yes	Coordinate reference system of the extent
RESPONSE_CRS	No	Coordinate reference system of the response

In addition to the standard ones, QGIS Server supports the following extra parameters:

Parameter	Required	Aprašymas
MAP	Yes	QGIS project file

BBOX

This parameter allows to specify the map extent in the units of the current CRS. Coordinates have to be separated by a comma. The BBOX parameter is formed like minx,miny,maxx,maxy.

URL example:

http://localhost/qgisserver?
SERVICE=WCS
&REQUEST=GetCoverage
&IDENTIFIER=T20QPD_20171123T144719_TCI
&BBOX=647533,1805950,660987,1813940
&CRS=EPSG:32620

CRS

This parameter allows to indicate the Spatial Reference System of the BBOX parameter and has to be formed like EPSG:XXXX.

RESPONSE_CRS

This parameter allows to indicate the output response Spatial Reference System and has to be formed like EPSG:XXXX. The CRS of the corresponding coverage layer is used by default.

WIDTH

This parameter allows to specify the width in pixels of the output image. The resolution of the response image depends on this value.

HEIGHT

This parameter allows to specify the height in pixels of the output image. The resolution of the response image depends on this value.