Web Coverage Processing Service

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The Web Coverage Processing Service (WCPS) defines a language for filtering and processing of multi-dimensional raster coverages, such as sensor, simulation, image, and statistics data. The Web Coverage Processing Service is maintained by the Open Geospatial Consortium (OGC). This raster query language allows clients to obtain original coverage data, or derived information, in a platform-neutral manner over the Web.

A coverage is the digital representation of some spatio-temporal phenomenon. ISO 19123 provides the definition:

Open Geospatial Consortium standards organization

The Open Geospatial Consortium (OGC), an international voluntary consensus standards organization, originated in 1994. In the OGC, more than 500 commercial, governmental, nonprofit and research organizations worldwide collaborate in a consensus process encouraging development and implementation of open standards for geospatial content and services, sensor web and Internet of Things, GIS data processing and data sharing.

Contents

Overview

WCPS allows to generate pictures suitable for displaying to humans and information concise enough for further consumption by programs. In particular, the formally defined syntax and semantics make WCPS amenable to program-generated queries and automatic service chaining.

As the WCPS language is not tied to any particular transmission protocol, the query paradigm can be embedded into any service framework, such as OGC Web Coverage Service (WCS) and OGC Web Processing Service (WPS).

The Open Geospatial Consortium Web Coverage Service Interface Standard (WCS) defines Web-based retrieval of coverages – that is, digital geospatial information representing space/time-varying phenomena.

The OGC Web Processing Service (WPS) Interface Standard provides rules for standardizing how inputs and outputs for invoking geospatial processing services, such as polygon overlay, as a web service. The WPS standard defines how a client can request the execution of a process, and how the output from the process is handled. It defines an interface that facilitates the publishing of geospatial processes and clients’ discovery of and binding to those processes. The data required by the WPS can be delivered across a network or they can be available at the server. WPS can describe any calculation including all of its inputs and outputs, and trigger its execution as a web service. WPS supports simultaneous exposure of processes via HTTP GET, HTTP POST, and SOAP, thus allowing the client to choose the most appropriate interface mechanism. The specific processes served up by a WPS implementation are defined by the owner of that implementation. Although WPS was designed to work with spatially referenced data, it can be used with any kind of data.

The current WCPS version is 1.0. The standards document, [1] available from the OGC WCPS standards page, [2] presents a condensed definition of syntax and semantics. In addition, there is an introduction to the concepts along with design rationales. [3]

Currently, WCPS is constrained to multi-dimensional raster data, but an activity is under work in OGC to extend it to all coverage types, i.e., digital geospatial information representing space-varying phenomena as defined in OGC Abstract Specification Topic 6: Schema for Coverage Geometry and Functions [4] (which is identical to ISO 19123) and refined to a concrete, interoperable model in the OGC GML 3.2.1 Application Schema - Coverages (GMLCOV) Standard. [5]

International Organization for Standardization An international standard-setting body composed of representatives from national standards organizations

The International Organization for Standardization is an international standard-setting body composed of representatives from various national standards organizations.

WCPS Language in a Nutshell

Sample WCPS query results Sample WCPS query results.png
Sample WCPS query results

WCPS establishes a protocol to send a query string to a server and obtain, as a result of the server's processing, a set of coverages. The query string can be expressed in either Abstract Syntax or XML. In the following examples, Abstract Syntax will be used as it is more apt for human consumption.

The WCPS syntax tentatively has been crafted close to the XQuery language – as metadata more and more are established in XML, and OGC heavily relies on XML (such as Geography Markup Language), it is anticipated that eventually a combination of XQuery and WCPS will be established. This will unify data and metadata retrieval.

Geography Markup Language used to describe geographical features

The Geography Markup Language (GML) is the XML grammar defined by the Open Geospatial Consortium (OGC) to express geographical features. GML serves as a modeling language for geographic systems as well as an open interchange format for geographic transactions on the Internet. Key to GML's utility is its ability to integrate all forms of geographic information, including not only conventional "vector" or discrete objects, but coverages and sensor data.

The following example may serve to illustrate these principles. Task is to inspect three coverages M1, M2, and M3; for each one, deliver the pixelwise difference of red and near-infrared (nir) channel; return the result encoded in HDF5:

for$cin(M1,M2,M3)returnencode(abs($c.red-$c.nir),"hdf5")

This will return three coverages, that is: three HDF5 files.

Next, we are interested only in those coverages where nir exceeds 127 somewhere:

for$cin(M1,M2,M3)wheresome($c.nir>127)returnencode(abs($c.red-$c.nir),"hdf5")

The result might be only two coverages that pass the filter.

Finally, we want to constrain the filter predicate through a pixel mask acting as filter:

for$cin(M1,M2,M3),$rin(R)wheresome($c.nir>127and$r)returnencode(abs($c.red-$c.nir),"hdf5")

The evaluation procedure can be thought of as a nested loop. In general, the language allows to express a range of imaging, signal processing, and statistics operations. The limit is given because the language is safe in evaluation, that is: every request is guaranteed to terminate after a finite number of steps. This excludes recursion. Still, algorithms like classification, filter kernels and general convolutions, histograms, and Discrete Fourier Transform are expressible.

See also

A Web Map Service (WMS) is a standard protocol developed by the Open Geospatial Consortium in 1999 for serving georeferenced map images over the Internet. These images are typically produced by a map server from data provided by a GIS database.

Rasdaman

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References

  1. ↑ Baumann, P. (ed.): OGC Web Coverage Processing Service (WCPS) Language, OGC document 08-068r2, 2008
  2. ↑ OGC WCPS standards page
  3. ↑ Baumann, P.: The OGC Web Coverage Processing Service (WCPS) Standard. Geoinformatica, 14(4)2010, pp. 447-479, DOI 10.1007/s10707-009-0087-2
  4. ↑ OGC Abstract Specification Topic 6: Schema for Coverage Geometry and Functions, OGC 08-126
  5. ↑ OGC GML Application Schema - Coverages, OGC 09-146r2
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