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OGC Uncertainty Markup Language

OGC Releases Discussion Papers: Uncertainty Markup Language and OGC WCS Extension for netCDF Weather Data

April 23, 2009. Wayland, Massachusetts, USA.

The Open Geospatial Consortium, Inc. (OGC) announces the release of two Discussion Papers: Uncertainty Markup Language (UncertML) ( and the OpenGIS Web Coverage Service Standard (WCS) Extension for CF-netCDF Encoding (


UncertML is a conceptual model and XML encoding designed for encapsulating probabilistic uncertainties and may be used to quantify and exchange complex uncertainties in data. Most data contains uncertainty arising from sources such as measurement error, observation operator error, processing/modeling errors, or corruption. Processing uncertain data propagates and often increases uncertainty. Thus there is a need for a standard way of characterizing uncertainty that is readily interpreted by software systems.

UncertML is based on a number of ISO and OGC standards, such as ISO 19138 Data Quality Measures, and addresses the ISO/IEC guide to the expression of uncertainty in measurement (GUM). UncertML utilizes the OGC Geography Markup Language (GML) Standard and the OGC Sensor Web Enablement Common (SWE) Standard. [Excerpt]

WCS Extension for CF-netCDF

The OGC Web Coverage Service Standard (WCS) Extension for CF-netCDF Encoding provides a way for users of CF-NetCDF ( data to use the OpenGIS Web Coverage Service Interface Standard (WCS). WCS defines a protocol-independent language for the extraction, processing, and analysis of multi-dimensional gridded coverages representing sensor, image, or statistical data.

The netCDF (network Common Data Form) interface, library, and format support the creation, access, and sharing of array-oriented scientific data. The CF (climate and forecast) metadata conventions provide definitive descriptions of what the netCDF data in each variable represent, including the data's spatial and temporal properties. [Excerpt]

The OGC is an international consortium of more than 380 companies, government agencies, research organizations, and universities participating in a consensus process to develop publicly available geospatial standards. OpenGIS Standards support interoperable solutions that "geo-enable" the Web, wireless and location-based services, and mainstream IT. OGC Standards empower technology developers to make geospatial information and services accessible and useful with any application that needs to be geospatially enabled. Visit the OGC website at:


Sam Bacharach
Executive Director, Outreach and Community Adoption
Open Geospatial Consortium, Inc
35 Main Street, Suite 5, Wayland, MA 01778 USA
Tel: +1-703-352-3938

Discussion Paper Excerpts

Uncertainty Markup Language (UnCertML)

Uncertainty Markup Language (UnCertML). OpenGIS Discussion Paper. Edited by Matthew Williams, Dan Cornford, Lucy Bastin, and Edzer Pebesma. Copyright © Open Geospatial Consortium Inc. Date: 2009-04-08. OGC document reference number: 08-122r2. Version: 0.6. 61 pages. This document is not an OGC Standard; it is an OGC Discussion Paper, not an official position of the OGC membership, distributed for review and comment.

Scope: The Uncertainty Markup Language (UncertML) is an XML encoding for the transport and storage of information about uncertain quantities, with emphasis on quantitative representations based on probability theory. This document describes the XML schema syntax and conventions that allow an interoperable description of uncertain data, which we define to be random quantities, in a variety of ways including: (1) probability distributions including both uni- and multi-variate distributions and mixture models; (2) statistics, including means, (co-)variances, standard deviations and quantiles; models; (3) realisations or sampled data.

Introduction: "Most data contains uncertainty, arising from sources which include measurement error, observation operator error, processing/modelling errors, or corruption. Processing this uncertain data (typically through models, which can introduce their own errors), propagates the uncertainty, often unpredictably. The ability to optimally utilise data requires a description of its uncertainty which is as complete and detailed as possible, and in the geospatial context, this characterisation and quantification is particularly crucial when data is used for spatial decision making.

Thus there is a well-recognised need for GIS frameworks which can handle and 'understand' incomplete knowledge in data inputs, in decision rules and in the geometries and attributes modelled. A substantial literature exists on mechanisms for representing and encoding geospatial uncertainty and its propagation. However, no framework yet exists to escribe and communicate uncertainty (either in GI data or more generally) in an interoperable manner.

UncertML is an XML schema for describing uncertain information, which is capable of describing a range of uncertain quantities. Its descriptive capabilities range from summaries, such as simple statistics (e.g., the mean and variance of an observation), to more complex representations such as parametric distributions at each point of a regular grid, or even jointly over the entire grid.

The ISO/IEC guide to the expression of uncertainty in measurement (GUM) outlines the importance of quantifying uncertainty by stating that it is "obligatory that some quantitative indication of the quality of the result be given so that those who use it can assess its relibility" when discussing observations. The guide goes on to state that it is necessary to have a readily implemented and generally accepted procedure for characterizng the quality of a result of a measurement; however, it does not outline a mechanism for describing this information via an exchangeable medium.

The GUM guide aspires to provide a worldwide consensus on the evaluation and expression of uncertainty in measurement, not dissimilar to the International System of Units. With the development of the UncertML standard and accompanying dictionary (UncertML: Dictionary) a small step has been taken toward this vision.

Recent developments for sensor observation modeling within the Open Geospatial Consortium (e.g., the Observations and Measurements standard) have opened opportunities for interoperable, sensor-derived datasets to be exchanged over the Internet. As this Sensor-Web community grows, an increasing volume of data will become available and require processing, and much of this data will be used for decision support.

However, rational decision-making using incomplete knowledge (i.e., sensor measurements) is only possible if we can quantify the uncertainty inherent in those measurements, and the uncertainty that is introduced or increased by subsequent processing. To be truly valuable in the context of 'discoverable' Web Services and datasets (for example, within automatic online risk management chains), this uncertainty must be represented in an interoperable manner. Currently, within the Sensor-Web framework no formal method of quantifying complex uncertainties (e.g. probabilistic representations) exists...

WCS 1.1 Extension

Web Coverage Service (WCS) 1.1 Extension for CF-netCDF 3.0 Encoding. OpenGIS Discussion Paper OGC 09-018. Edited by Ben Domenico (UCAR/Unidata) and Stefano Nativi (CNR/IMAA). Editorial contributions from Ethan Davis (UCAR/Unidata), Dominic Lowe (British Atmospheric Data Centre), and Paolo Mazzetti (CNR/IMAA). Open Geospatial Consortium Inc. Publication Date: 2009-04-08. OGC document reference number: OGC 09-018. Version: 0.2.2. 74 pages. This document is not an OGC Standard; it is an OGC Discussion Paper, not an official position of the OGC membership, distributed for review and comment. The OpenGIS Abstract Specification does not require any changes to accommodate the technical contents of this document. References: ncML (netCDF Markup Language).

The Web Coverage Service (WCS) supports electronic retrieval of geospatial data as "coverages" — that is, digital geospatial information representing space-varying phenomena. A WCS provides client access to potentially detailed and rich sets of geospatial information in forms that are useful for client-side rendering, multi-valued coverages, and input into scientific models and other clients. The WCS is currently limited to quadrilateral grid coverages, providing information at the grid points, usually with interpolation between these grid points. This extension of the WCS standard specifies a CF-netCDF3 encoding format option. This is based on the netCDF (network Common Data Form) ver. 3.0 file format using the CF (Climate and Forecast) conventions version 1.1...

Scope: This extension of the WCS standard specifies an Information Community data model with the related encoding that may optionally be implemented by WCS servers. This extension specification allows clients to evaluate, request and use data encoded in CF-netCDF3 format from a WCS server. This document is an extension of the Web Coverage Service (WCS) 1.1 Corrigendum 2 (version 1.1.2) Implementation Standard (OGC 07-067r5). With small changes, this extension is expected to also apply to WCS 1.2... WCS 1.1 and 1.2 are divided into a base standard plus multiple extensions (formerly called application profiles).

WCS CF-netCDF3 encoding extension Standard overview: This extension Standard specifies a CF-netCDF3 data model with the related binary and XML-based encoding formats in which data may be requested by a WCS client and provided by a WCS server. This extension Standard is an optional implementation by servers. The format is netCDF conforming to the Climate and Forecast (CF) conventions (CF-netCDF3). This standard specifies the CF-netCDF data model mapping onto the WCS data model. In addition, this standard specifies the possible binary and XML-based encoding formats returned by a coverage request expressed through the WCS interface. This document is specific to coverage formats encoded in netCDF 3 using CF 1.1.

NetCDF is a widely-used set of interfaces for array-oriented data access and a freely-distributed collection of data access libraries for C, Fortran, C++, Java, and other languages. The netCDF libraries support a machine-independent, self-documenting binary format for representing scientific data. Together, the interfaces, libraries, and format support the creation, access, and sharing of scientific data. The CF conventions define metadata (internal to a netCDF file) that provide a definitive description of what the data in each variable represents, and of the spatial and temporal properties of the data. This enables the users of data from different sources to decide which quantities are comparable and how they relate to one another in space and time.

The perceived need for a WCS 1.1. standard extension for CF-netCDF3 arose from the experiences of the OGC GALEON (Geo-interface for Air Land Environment Ocean Netcdf) Interoperability Experiment. In this experiment, several WCS 1.0 clients were successful in accessing data from WCS 1.0 servers which encoded the data in CF-compliant netCDF (version 3.0) form. The GALEON experiment has proven that the CF-netCDF3 is a viable and valuable WCS encoding format. However, CF-netCDF3 was not among the list of 5 'required supported formats' in the WCS 1.0 specification.

Based on the OGC GALEON experiment and subsequent discussion in the WCS Revision Working Group (RWG) and subsequent Standards Working Group (SWG), this proposal in conjunction with the later WCS 1.x specifications will establish CF-netCDF3 as a WCS 'supported output format'..."

Prepared by Robin Cover for The XML Cover Pages archive. See also "Geography Markup Language (GML)."

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