Open Geospatial Consortium
Date: 2012-01-25
Reference number of this OpenGIS® Project Document: OGC 09-153r1
Version: 1.0
Category: OpenGIS® Primer
Editor: Peter Baumann

OGC® Web Coverage Service 2.0 Primer:
Core and Extensions Overview

Copyright © 2012 Open Geospatial Consortium
To obtain additional rights of use, visit http://www.opengeospatial.org/legal/.

Warning
This document is not an OGC Standard. This document provides informative information
about a specific standard, best practices, and implementation guidance.

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OpenGIS® Primer

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ContentsPage

1Scope ............................................................................................................................. 12

Compliance .................................................................................................................. 13

Normative references .................................................................................................. 1

4Terms and definitions ................................................................................................. 1

5Conventions .................................................................................................................. 1

5.1

UML notation .............................................................................................................. 1

5.2

Namespace prefix conventions ................................................................................... 1

6WCS Package Overview ............................................................................................. 2

6.1

Core and extensions .................................................................................................... 26.2

Application Profiles ..................................................................................................... 27Possible extensions....................................................................................................... 3

7.1.1

Introduction ................................................................................................................. 3

7.1.2

Identification ................................................................................................................ 4

7.2

Data model extensions ................................................................................................. 4

7.2.1

Purpose ......................................................................................................................... 4

7.2.2

List of existing and planned extensions ..................................................................... 4

7.2.2.1 Nil values ..................................................................................................................... 4
7.2.2.2 Uncertainty .................................................................................................................. 5
7.3

Coverage encoding extensions .................................................................................... 5

7.3.1

Purpose ......................................................................................................................... 5

7.3.2

List of extensions ......................................................................................................... 5

7.4

Service model extensions ............................................................................................ 5

7.4.1

List of extensions ......................................................................................................... 5

7.4.1.1 Scaling and interpolation ............................................................................................ 5
7.4.1.2 CRS, EPSG-based ....................................................................................................... 5
7.4.1.3 CRS, general ................................................................................................................ 6
7.4.1.4 Transactional service (WCS-T) .................................................................................. 6
7.4.1.5 Web Coverage Processing Service (WCPS) .............................................................. 6
7.5

Protocol extensions ...................................................................................................... 6

7.5.1

Purpose ......................................................................................................................... 6

7.5.2ii

List of extensions ......................................................................................................... 6Copyright © 2012 Open Geospatial Consortium

OGC 09-153r1

7.5.2.1 HTTP GET/KVP ......................................................................................................... 6
7.5.2.2 HTTP POST/XML ...................................................................................................... 6
7.5.2.3 SOAP ............................................................................................................................ 7
7.5.2.4 REST ............................................................................................................................ 7
7.6

Usability extensions ..................................................................................................... 7

7.6.1

Purpose ......................................................................................................................... 7

7.6.2

List of extensions ......................................................................................................... 7

7.6.2.1 Multi-lingual support .................................................................................................. 78

Possible application profiles ....................................................................................... 79

Conventions and best practices .................................................................................. 8

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i. Preface
This document provides an overview on the OGC Web Coverage Service (WCS) 2.0 suite bydescribing WCS core and extensions.
Intended target audience are developers intending to implement WCS servers and/or clients.
This document aims at providing an overview and giving useful hints and best practices
beyond the pure standards texts. It is a “living document” which will evolve to reflect new
developments and best practices.
As such, the contents of this document is informative and not of normative nature.

ii. Terms and definitions
This document uses the specification terms defined in Subclause 5.3 of [OGC 06-121r9],
which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting ofInternational Standards. In particular, the word “shall” (not “must”) is the verb form used toindicate a requirement to be strictly followed to conform to this standard.

iii. Submitting organizations
The following organizations have submitted this Implementation Specification to the Open
Geospatial Consortium, Inc.:
Jacobs University Bremen
National Center for AtmosphericResearch (NCAR)
Oracle USA
PCI Geomatics Inc.
ERDAS, Inc.

iv

EOX IT Services GmbH
Spot ImageBAE Systems - C3I Systems
Natural Environment Research Council(NERC)
George Mason University

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iv. Document Contributor Contact Points
NamePeter BaumannOrganization
Jacobs University Bremen,rasdaman GmbHv. Revision history
DateReleaseAuthor
2009-10-30 0.0.1
PB

Paragraph modified
All
CreatedDescription

vi. Changes to the OpenGIS® Abstract Specification
The OpenGIS® Abstract Specification does not require any changes to accommodate thetechnical contents of this document.

vii. Future Work
This document needs to be updated whenever a new extension is added to WCS. It may needan update if and when significant functionality changes are made to existing WCScomponents.

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ForewordSome of the elements of this document may be the subject of patent rights. Open Geospatial
Consortium Inc. shall not be held responsible for identifying any such patent rights.
Attention is drawn to the possibility that some of the elements of this document may be thesubject of patent rights. The Open Geospatial Consortium shall not be held responsible foridentifying any or all such patent rights.
Recipients of this document are requested to submit, with their comments, notification of any
relevant patent claims or other intellectual property rights of which they may be aware thatmight be infringed by any implementation of the standard set forth in this document, and toprovide supporting documentation.

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Introduction
The OGC Web Coverage Service (WCS) supports electronic retrieval of geospatial data as"coverages" – that is, digital geospatial information representing space/time-varying
phenomena.
WCS 2.0 consists of a set of normative specifications, collectively referred to as “the WCSsuite”. These specifications encompass (see normative references):§GML 3.2 Application Schema for Coverages§WCS 2.0 Core§

A set of extensions to the WCS Core.

This document provides an overview of the OGC Web Coverage Service (WCS) 2.0 suite bydescribing WCS core and extensions. As such, the contents of this document is informativeand not of normative nature.

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Core and Extensions Overview
1ScopeThe scope of the informative content in this document is the OGC Web Coverage Service 2.0(WCS) and later versions suite of standards.
2ComplianceThis document does not contain any normative statements. Therefore, there are norequirements or compliance statements defined. In the case where differences exist between
the standard and this document, the normative text in the standard texts shall prevail.
3

Normative references

The following normative documents contain provisions that, through reference in this text,
constitute provisions of this specification. For dated references, subsequent amendments to,or revisions of, any of these publications do not apply. For undated references, the latest
edition of the normative document referred to applies.
OGC 06-121r9, OGC® Web Service Common Interface Standard, version 2.0
OGC 09-110r3, OGC® Web Coverage Service 2.0 Interface Standard Core, version 2.0
OGC 09-146r1, OGC® GML 3.2.1 Application Schema for Coverages, version 1.0
4Terms and definitions

Definitions from GML Application Schema [OGC 09-146r1] and WCS Core [OGC 09110r3] apply.
5Conventions

5.1

UML notation

All the diagrams that appear in this specification are presented using the Unified Modeling
Language (UML) static structure diagram, as described in Subclause 5.2 of OGC Web
Service Common [OGC 06-121r9].
5.2

Namespace prefix conventions

None defined here; definitions from GML Application Schema [OGC 09-146r1] and WCSCore [OGC 09-110r3] apply.

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6

WCS Package Overview

6.1

Core and extensions

The WCS model defined in this document and the WCS “core” standard provide the ability
to define many extension packages that add specific functionality to the WCS data and/orservice model.
The Section 7 contains existing, planned, and possible WCS extensions. The section isgrouped based on what appears reasonable as of the publication of this document. However,this structuring is by no means normative and shall not be used to draw any conclusions on
the functionality a particular extension provides.
WCS Core and each extension specify, as normative requirements, which prerequisite
specifications they require. Frequently, options are possible in some specific group ofextensions; for example, every WCS implementation must support at least one protocol
extension.
This constitutes a dependency graph as shown in Figure 1.
6.2

Application Profiles

An Application Profile (AP) defines WCS functionality for some particular purpose. To thisend, two mechanisms are available:§

The AP can make a choice among the extensions it declares mandatory (note that thecore has to be included in any case).
Note that there may be cross dependencies between some extensions that have to berespected. For example, the core requires at least one protocol extension, and the CRS
extension requires the scaling & interpolation extension.
For example, a SCADA (Supervisory Control and Data Acquisition) AP might allow
only 1-D time coverages and GML encoding, whereas an Earth Observation APmight allow only x/y and x/y/t coverages and GeoTIFF, but not include a GMLencoding. A metocean AP might set out for n_D coverages delivered, e.g., innetCDF.

§Additional restrictions can be imposed on core and extensions.
In the above example, the Earth Observation AP might allow only x/y and x/y/t
coverages and GeoTIFF, but no GML encoding. Schematron 1 offers a convenientway for doing so and iscompatible with the overall WCS standard style. Further, theEO AP might further restrict and define the concrete usage of the metaData
element of AbstractCoverage.

Consequently, a conformance test for an AP will in turn inspect the GML Application
Schema for WCS, the WCS Core, any extension listed, and finally the specific requirementsof the AP on hand.
1 www.schematron.com

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Figure 1 —WCS specification hierarchy graphical overview
7Possible extensions

7.1.1Introduction

In this section, some WCS extensions are listed which appeared meaningful to the WCS
Working Group during the development of WCS 2.0. Some of the extensions are alreadyavailable as adopted standards, some of them not. By no means is this list indicative orCopyright © 2012 Open Geospatial Consortium

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comprehensive; some extensions listed here may never be written, and others not listed may
be developed.
Following the presentation in Section 6, extensions are grouped into§

Data model extensions: they extend or refine coverage-related data structures;§

Format encoding extensions: they describe encodings applicable for the transfer ofcoverages (not for the request encodings!);§Service model extensions: they add further service capabilities;§Protocol binding extensions: they describe client/server communication protocols andthe encoding of parameters shipped using these protocols.

§Usability extensions: they add means to better use the services.

7.1.2

Identification

Every WCS extension is identified by a unique URI that, by convention, is specified in thefirst formal requirement of an extension specification.
All extensions implemented by a WCS server must be reported by that server; this is done bylisting the identifying URN of each extension in the Profile list of its GetCapabilities
response.
WCS extension identifiers follow this schema:
http://www.opengis.net/def/conf-class/OGC-WCS/0/
extension/EXTENSION-CLASS/EXTENSION

whereEXTENSION-CLASS identifies the extension class (such as data model);EXTENSION identifies the particular extension (actually, a conformance class thereof)
7.2

Data model extensions

7.2.1

PurposeThis category of extensions focuses on adding information contents to the coverage model,be it for coverages when offered by a WCS server or for coverages generated for delivery orconsumption by a WCS.
7.2.2

List of existing and planned extensions

7.2.2.1

Nil values

Specific values can be designated to represent “nil” (“null”) values. This extension isexpected to be specified and document in 2012. The GML 3.2 Application Schema for WCS[OGC 09-146r1] is already capable of “holding” null values.
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7.2.2.2

Uncertainty

Coverage range values can be annotated with a degree of uncertainty. This extension is underdiscussion, however, the WCS WG felt that an OGC-wide overarching concept should beresolved first rather, than establishing an ad-hoc solution.
7.3

Coverage encoding extensions

7.3.1

PurposeCoverages can be delivered by GetCoverage requests (or uploaded, via WCS-T Transaction
requests) in different data formats.
In addition and independently from any data format, coverages can be delivered as either
pure GML documents, or as a format-encoded data file only, or as a combination whereGML is used to represent the metadata and the data file holds the range values (“pixels”,“voxels”). This will be described in a separate format extension.
Every WCS, be it client or server, shall support at least one encoding format. This can be aformat defined in an extension or the GML coverage representation defined in [OGC 09110r3] and [OGC 09-146r1].
7.3.2

List of possible extensions

This open-ended list is likely to encompass GML, NetCDF, GeoTIFF, and many more.
Note that not all formats are suitable to transport all kinds of coverages; limitations can begiven by, for example, the dimensions supported (TIFF cannot hold 4-D coverages) or thecell type (BMP cannot hold hyperspectral imagery).
7.4

Service model extensions

This category of extensions describes additional functionality that may be added to a WCS.
7.4.1

List of extensions

7.4.1.1

Scaling and interpolation

With this extension, GetCoverage allows for scaling of coverage results; different
interpolation techniques can be selected for the resampling performed during a scaling
operation.
7.4.1.2

CRS, EPSG-based

This will allow coverages to be requested in different CRSs; to this end, the server needs tobe able to perform a CRS transform of the coverage prior to its delivery. CRSs supported arethose defined by EPSG.
This extension requires extension “Scaling and interpolation”.
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NOTE
This will be a normative requirement stated in the WCS 2.0 CRS Extension (which is not yet
existing, but in planning at the time of this writing).

7.4.1.3

CRS, general

As in the previous Subclause, this will allow coverages to be requested in different CRSs; tothis end, the server needs to be able to perform a CRS transform of the coverage prior to itsdelivery. CRSs supported are all types defined by OGC; including nested ad-hoc defined
CRSs.
This extension requires extension “CRS, EPSG-based”.
NOTE
This will be a normative requirement stated in the WCS 2.0 CRS Extension (which is not yet
existing, but in planning at the time of this writing).

7.4.1.4

Transactional service (WCS-T)

WCS-T [OGC 07-068r4] specifies the ability to insert, update, and delete coverages offeredby a WCS server.
7.4.1.5

Web Coverage Processing Service (WCPS)WCPS defines a query language that specifies how to combine and process coverages fornavigation, extraction (download), aggregation, and ad-hoc analysis. The abstract language isdefined in [OGC 08-068r1], the WCS protocol embedding in [OGC 08-059r3]. The WCS
embedding of WCPS requires implementation of [OGC 08-068r1].
7.5

Protocol extensions

7.5.1

PurposeThis category of extensions describes client / server communication protocols (includingrequest parameter encodings) which a WCS implementation may offer and use. Every WCS,
be it client or server, shall support at least one protocol extension.
NOTE

This does not include result coverage encodings as delivered by GetCoverage requests.

7.5.2

List of extensions

7.5.2.1

HTTP GET/KVP

Requests are sent as HTTP GET parameters. Coverages are encoded in some data format (asspecified by a format encoding extension), other response parameters are encoded in XML.
7.5.2.2

HTTP POST/XML

Requests are sent as HTTP POST using an XML encoding. Coverages are transferred using
some data format (as specified by a format encoding extension).

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7.5.2.3

SOAP

Requests and responses are communicated via SOAP using XML. Coverages are transferredusing some data format (as specified by a format encoding extension).
7.5.2.4

REST

Requests are sent using the RESTful paradigm. Coverages are encoded in some data format(as specified by a format encoding extension).
7.6

Usability extensions

7.6.1

PurposeThis set of extensions focuses on usability of the service overall, rather than on bespoke dataor service functionality.
7.6.2

List of extensions

7.6.2.1

Multi-lingual support

Text messages can be delivered in various languages by a service.
8Possible application profiles

A WCS Application Profile (AP) bundles core plus a set of bespoke extensions into astandard suitable for handling coverages as they occur in some specific application domain.
Currently the following APs are under consideration:§Time series
Coverages are one-dimensional and have only a time dimension. Often thisdimension is named “t” or “time”.

§Earth observation
Coverages are two-dimensional and have only geographically horizontal dimensions.
Often these dimensions are named “x” and “y” or “latitude and “longitude”.
Sometimes coverages have a time dimension in addition, yielding 3-D time series
data cubes.

§Metocean data
These communities require coverages with n dimensions, frequently incorporating
non-spatiotemporal dimensions like pressure, or additional time dimensions.
Coverages go beyond the classical raster imagery (“quadrilateral grid coverages”),
including curvilinear grids, for example.

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9Conventions and best practices

The WCS 2.0 suite remains rather general to be adjustable to many different application
domains. To ensure a maximum of interoperability, the following non-normative conventions
are recommended.
Best Practice 1
The optional ServiceIdentification section in a Capabilities
response should be present.
Best Practice 2
be present.

8The optional Contents section in a Capabilities response should

Copyright © 2012 Open Geospatial Consortium