Links and Resources

1. OGC Coverage

By OGC definition coverages are digital geospatial information representing space/time-varying phenomena, specifically spatio-temporal regular and irregular grids, point clouds, and general meshes, such as satellite imagery, digital elevation models, or digital aerial imagery.

Grid coverages have a domain comprised of regularly spaced locations along 0, 1, 2, or 3 axes of a spatial coordinate reference system. Their domain may also have a time dimension, which may be regularly or irregularly spaced. A coverage defines, at each location in the domain, a set of fields that may be single-valued (such as elevation), or array-valued (such as brightness values in different parts of the electromagnetic spectrum - RS data). These fields with their values are known as the range of the coverage.

GML 3.2.1. application schema for coverages which has been developed with the goal that coverages handled by a WCS can be more easily interchanged with other OGC services.

Example for a rectified grid coverage (sources OGC)  defined in the GML 3.2.1 Application Schema for Coverages

2. OGC WCS

The OGC Web Coverage Service (WCS) defines an interoperable way of sharing regular grids. We will focus on the version of WCS 2.0. WCS 2.0 defines the Core specifications that have to be implemented by every WCS compatible server.

WCS 2.0 uses the coverage model of the GML Application Schema for Coverages. WCS 2.0 supports all coverage types supported
by Application Schema.

The core specifications can be extended to support:

• different data structure (different Format Encoding): GeoTiff, JPEG2000
• protocol binding or service subtypes (client-server communication protocols):

1. GET-KVP (Key-Value Pair): the HTTP Get protocol for sending key-value-pair encoded request and to receive XML metadata and binary coverage data
2. POST-XML: HTTP POST protocol for transferring XML data and binary coverage data
3. SOAP-XML: using the SOAP protocol for transferring XML data and binary coverage data

• WCS extensions: Web Coverage Service – Transactional (WCS -T) defines how to upload or modify a coverage on a server
• Usability extensions: supports multilinguality

3. WCS core operations

The WCS provides three operations to retrieve geospatial coverage data

GetCapabilities: allows a client to request information about the server’s capabilities and coverages offered. It returns an XML document describing the service, with brief descriptions of the coverages available on the server.

DescribeCoverage  allows a client to request detailed metadata (full descriptions) of one or more selected coverages offered by a particular server. It returns an XML document describing the identified coverages. It provides sufficient information for a client to assess the data’s suitability for use, and to formulate GetCoverage requests.

GetCoverage: allows a client to request a coverage comprised of selected range properties at a selected set of spatial-temporal locations, expedited in a well-known coverage encoding format. Its syntax and semantics bear some resemblance to the WMS GetMap and WFS GetFeature requests, but several extensions support the retrieval of coverages rather than static maps or discrete features.

WCS requests and responses frequently include references to a defined Coordinate Reference System (CRS). A defined CRS is required for spatial referencing by coordinates, and is a coordinate system which is related to an object by a datum.

4. WCS Requests

Let´s exlore the WCS of Swiss open Government data. WCS has an URL http://wms.zh.ch/DEMWCS

To initiate a sequence of WCS requests, a client should first issue a GetCapabilities request to the server to obtain an up-to-date listing of available data.

http://wms.zh.ch/DEMWCS?SERVICE=WCS&Request=GetCapabilities

Then a client may issue a DescribeCoverage request to obtain more details about the particular coverages offered. In response to a DescribeCoverage request, a WCS returns an XML document whose top-level element is a CoverageDescription containing CoverageOffering elements for all coverages requested.

http://wms.zh.ch/DEMWCS?service=WCS&request=DescribeCoverage&version=1.1.1&СoverageId=dtm2014

The information obtained from a DescribeCoverage request can then be used in GetCoverage requests to retrieve spatial subsets of the coverage offerings, by requesting intervals along the parameter axis.

To retrieve a coverage, or a part thereof, a client issues a GetCoverage request. A GetCoverage request returns values or properties at regularly spaced locations, bundled in the requested format. The response to a valid GetCoverage request is a coverage extract, with the spatial reference system, bounding box, size, value sets, and format specified in the request. An invalid GetCoverage request generates an error output in the requested Exceptions format.

Use QGIS WCS in order to open getcoverage and add on yoour map.