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Created: March 11, 2005.
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OGC Launches Initiative to Support GML Metadata Encoding in JPEG 2000 Image Files.


In February 2005 the Open Geospatial Consortium (OGC) launched a new Encoding Interoperability Experiment relating to the use of the Geography Markup Language in JP2 (JPEG 2000) image files. The goal is to support a standardized mechanism for inclusion of geo-referencing information as XML-encoded metadata within the ISO 15444 JPEG 2000 image format.

The Geography Markup Language (GML) is the most widely supported open specification for representation of geographic (spatial and location) information. It defines XML encoding for the transport and storage of geographic information, including both the geometry and properties of geographic features. In keeping with OGC's IPR Policy for royalty-free OGC standards, the GML specification is freely available for use on royalty-free terms. GML provides a variety of kinds of objects for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values.

In March 2004 OGC approved the release of the GML Implementation Specification Version 3.1.0 as a publicly available OGC Recommendation Paper. The GML specification is now being edited jointly in the OGC GML Revision Working Group and in ISO/TC 211/WG 4 (Geographic Information/Geomatics). Standardized as ISO 19136 in the ISO/TC 211 context, GML is "a detailed XML implementation of the General Feature Model (GFM), and most of ISO 19123, 19107, 19108, 19111, along with some utility feature types such as observations, and utility components such as units of measure."

JPEG 2000 is successor to the earlier ISO 10918-1 'JPEG' standard for digital images. JPEG 2000 "uses 'wavelet' technology; as well as being better at compressing images (20 percent plus), it can allow an image to be retained without any distortion or loss." JPEG 2000 as ISO/IEC 15444-1, according to LizardTech, is a high-end alternative to the popular JPEG image format which offers high-quality lossy and lossless image compression in a multi-resolution (pyramidal) format that is internal to the file structure. JPEG 2000 is highly scalable in several dimensions: it supports file sizes into the gigabyte range and beyond, multispectral and hyperspectral datasets with increased bit-depths, and selective decompression of scenes within the image at user-controllable qualities. It provides for a rich set of primitives for transporting compressed image data in a Web services environment: in a network environment, JPEG 2000 images can be streamed from server to client while still in compressed form, allowing viewers to access only the data (pixels) they need and at only the resolution and quality they require."

As with other XML-based or XML-aware image file formats, the JPEG 2000 standard makes provision for several forms of XML-encoded metadata in the JPX file format within reserved 'boxes'. Normative Annex M 'JPX file format extended metadata definition and syntax' defines a comprehensive set of optional metadata elements that may be embedded in a JPX file within XML boxes. Metadata types are documented in four logical groups: image creation metadata, content description metadata, metadata history metadata, and intellectual property rights metadata. Section M.8 supplies the JPX extended metadata document type definition (DTD). The standardized part of the metadata model is based upon the DIG35 Metadata Standard for Digital Images. Information in the XML 'box' is not application specific, however, allowing for arbitrary conforming XML-encoded metadata.

The new Open Geospatial Consortium "GML in JPEG" Interoperability Experiment has been formed to "test and refine a draft implementation specification that defines how Geography Markup Language is to be used within JPEG 2000 data packages for geographic imagery. The Interoperability Experiment will implement several prototype GMLJP2 codecs (data compressor/decompressors) based upon an OGC draft specification 04-045 titled "GML in JPEG 2000 for Geographic Imagery"; this specification proposal was submitted to OGC by Galdos Systems Inc. and LizardTech. The purpose is to confirm that the specification will support the requirements of geospatially related imagery over the Internet, and to improve the specification if it does not support these requirements. The participants will perform several individual experiments of increasing complexity and will demonstrate encoding similar to GeoTIFF."

The initiator organizations are Galdos Systems, Inc. (Canada); LizardTech, Inc. (US); and the European Union Satellite Centre (Spain). Other organizations participating in the initiative include DM Solutions Group (Canada); ITT Industries Space Systems Division (US); SPOT Image (France); US Geological Survey, Astrogeology; US NASA Jet Propulsion Laboratory; and Intergraph (Z/I Imaging) (US).

The OGC project team will produce a GMLJP2 Encoding Interoperability Experiment Report based upon the outline of goals identified in the proposed document, covering specification of the uses of GML within JPEG 2000, packaging mechanisms for including GML within JPEG 2000, and GML application schemas for encoding of OGC coverages within JPEG 2000. A number of experiments will be performed using GML application schemas (Coverage Range; Coverage Range Image Copy; Annotated Image; Embedded CRS; Embedded Units of Measure Definition; Bundled GML Features; Multi-image Case) as well as minimal instance experiments for Image Import/Export and Metadata Read. Other OGC specifications impacted by the study include Web Feature Service (WFS) and Web Coverage Service (WCS).

Initiatives classified as OGC Interoperability Experiments are part of the OGC Interoperability Program, which is "an essential part of OGC's fast, effective, inclusive user-driven process to develop, test, demonstrate, and promote the use of OGC Specifications. OGC Interoperability Experiments are designed as "brief, inexpensive, low-overhead initiatives led and executed by OGC members to achieve specific technical objectives that further the OGC Technical Baseline. They provide an opportunity for OGC members to launch and run an initiative without the more substantial sponsorship that supports OGC's traditional testbeds and pilot projects. These initatives can be for specification development, refinement, or testing or for other purposes approved by the OGC Review Board." Other OGC Interoperability Programs include fast-paced, multi-vendor test beds, OGC Pilot Projects, and OGC Interoperability Support Services.

The Open Geospatial Consortium (OGC) is "an international industry consortium of more than 270 companies, government agencies and universities participating in a consensus process to develop publicly available interface specifications. OpenGIS Specifications support interoperable solutions that 'geo-enable' the Web, wireless and location-based services, and mainstream IT. The specifications empower technology developers to make complex spatial information and services accessible and useful with all kinds of applications."

About the Geography Markup Language (GML)

"Geography Markup Language is an XML grammar written in XML Schema for the modelling, transport, and storage of geographic information. The key concepts used by Geography Markup Language (GML) to model the world are drawn from the OpenGIS Abstract Specification and the ISO 19100 series.

GML provides a variety of kinds of objects for describing geography including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values.

A geographic feature is 'an abstraction of a real world phenomenon; it is a geographic feature if it is associated with a location relative to the Earth'. So a digital representation of the real world can be thought of as a set of features. The state of a feature is defined by a set of properties, where each property can be thought of as a {name, type, value} triple.

The number of properties a feature may have, together with their names and types, are determined by its type definition. Geographic features with geometry are those with properties that may be geometry-valued. A feature collection is a collection of features that can itself be regarded as a feature; as a consequence a feature collection has a feature type and thus may have distinct properties of its own, in addition to the features it contains.

Geographic features in GML include coverages and observations as subtypes. A coverage is a sub-type of feature that has a coverage function with a spatial domain and a value set range of homogeneous 2 to n dimensional tuples. A coverage can represent one feature or a collection of features 'to model and make visible spatial relationships between, and the spatial distribution of, earth phenomena.'

An observation models the act of observing, often with a camera, a person or some form of instrument ('an act of recognizing and noting a fact or occurrence often involving measurement with instruments'). An observation is considered to be a GML feature with a time at which the observation took place, and with a value for the observation. A reference system provides a scale of measurement for assigning values 'to a location, time or other descriptive quantity or quality'.

A coordinate reference system consists of a set of coordinate system axes that is related to the earth through a datum that defines the size and shape of the earth. Geometries in GML indicate the coordinate reference system in which their measurements have been made. The 'parent' geometry element of a geometric complex or geometric aggregate makes this indication for its constituent geometries.

A temporal reference system provides standard units for measuring time and describing temporal length or duration. Following ISO 8601, the Gregorian calendar with UTC is used in GML as the default temporal reference system. A Units of Measure (UOM) dictionary provides definitions of numerical measures of physical quantities, such as length, temperature, and pressure, and of conversions between UOMs..." [from the version 3.0.1 specification Introduction]

"ISO 19136 (issued by Open GIS Consortium as GML 3.1) is a detailed XML implementation of the General Feature Model (GFM), and most of ISO 19123, 19107, 19108, 19111, along with some utility feature types such as observations, and utility components such as units of measure. The GML encoding is described directly using W3C XML Schema (WXS). However, rules for mapping GML to and from UML models are carefully described in Annex E and Annex F of ISO 19136. These rules can be applied providing the UML follows the profile described in ISO 19103. In particular GML provides a pattern for defining domain-specific feature-types. GML uses WXS to define components in the gml namespace. Each domain-specific language based on GML is termed a GML Application Language, and defines components (in particular, Features) in their own namespace, which are derived from or incorporate components drawn from GML. Effectively, the complete WXS representation for the GML Application Language is a formalisation of the Feature Type Catalogue for the application domain, plus some supporting components... [SEEGRID II Conference Wiki]

Overview of the OGC's JPEG 2000 and GML Initiative

LizardTech Inc is one of three initial proposers of the OGC Interoperability Experiment. Karen Morley and Michael P. Geriek of LizardTech published an article in Earth Imaging Journal summarizing some of the goals in making JPEG and GML work together; see JPEG 2000 and GML: Standards that Work Together." Excerpts:

"Within the collaborative standards development processes of Open Geospatial Consortium Inc. (OGC), several vendors are developing a standard for using JPEG 2000 imagery in geographic information system (GIS) workflows. Using the OGC Geography Markup Language (GML) standard to describe coordinate reference systems and accurate geographic positions, users can describe geographic features, handle multiple images and benefit from additional advanced features of JPEG 2000...

By design, JPEG 2000 offers no specific support for any particular application domain, such as georeferencing metadata for geospatial imaging. This means JPEG 2000 doesn't specify mechanisms for georeferencing the image, describing the sensor model used to collect the data, or correlating features within the imagery to other GIS datasets. For good reasons, the standards committee decided not to support such domain-specific requirements. Instead, the committee left room within the JP2 file format for 'boxes' containing arbitrary XML data that can refer to the image data within the file.

The Geography Markup Language (GML) is an XML grammar used to describe geographic data such as coordinate reference systems and positioning, geographic features, sensor models, annotations and styling, etc. Like JPEG 2000, GML version 3.1 will be an ISO standard (ISO 19136) in its own right...

Three complex situations show how the power of JPEG 2000 can be exploited using GML:

  • Sensor Models: Original satellite and aerial imagery are now much more than just three 8-bit RGB bands, and JPEG 2000 supports many of the imagery requirements users now see, such as bit-depths of 16 or higher and hyperspectral bands. These images typically have rich sets of associated metadata. For example: (1) full descriptions of the cameras may include sensor characteristics such as the number and wavelengths of the spectral bands, precision and calibration information, and type of sensor; (2) positioning information may include the usual coordinate system and position of the image relative to Earth; (3) image quality information might include cloud cover estimates, NIIRS rating and air quality at time of collection. Using GML, an application schema can be written that captures this metadata structure; this could be defined and provided by the imagery vendor; within their supplied JP2 imagery, GML instance data would provide the actual metadata content.

  • Multiple Images and Feature Identification: JPEG 2000 permits multiple images to be contained within the same file. Consider a workflow in which images of a particular region are to be captured and analyzed during a period of months: (1) multiple sets of stereopair imagery are to be stored, each pair has associated metadata, including date and time stamps; (2) the images all share a common coordinate system, but all are at slightly different coordinate positions; (3) any individual image may have specific features identified on that image. All of this information may be contained in a single JP2 file. In addition to the data corresponding to the individual images, the file may contain 'shared' GML data describing the coordinate system and feature descriptions for all images and 'private' GML data for each image describing specific feature instances and offsets within the image...

  • The Spatial Web: Increasingly geospatial data are being used in the Spatial Web environment, where Web services are used to transparently perform operations such as: (1) providing catalog access to large archives of geospatial data; (2) exporting the data itself, based on query parameters; (3) performing mosaicking and layering operations; (4) performing simple feature classification, description and extraction; (5) styling the data for presentation. Such workflows are already in use today with vector data; GML is the language of the GeoWeb used to describe regions and extents, define and label features and express queries. As users add raster imagery to this system, label features and express queries. As users add raster imagery to this system, they must be able to use GML for characterizing the imagery and its associated features..."

GML - JPEG 2000: Earlier Discussion 2004

"The OGC proposes to use GML to store geo-locational, feature overlay, annotation and text information within JP2 files. Unfortunately this is likely to be incompatible with the existing GeoJP2 format. Specific metadata proposed by the OGC for storage are: (1) acquisition data/time; (2) imagery type, source and rendering; (3) spatial resolution and geolocation accuracy; (4) SensorML — an XML variant used to hold sensor metadata; (5) quality assessment metadata such as percentage cloud cover and missing pixels; (6) ISO 19115 metadata elements such as title, source and classification.

One of the big issues to be addressed is whether to use JP2 or JPX as the OGC standard. JP2, being more restrictive and inflexible, is less able to hold the proposed set of metadata and be expanded in future. One the other hand, there is a danger that existing JP2 readers would be unable to handle a JPX based geospatial format...

Mark Kyle (LizardTech) gave a presentation which "covered work by Galdos Systems, LizardTech and the EU Satellite Centre to use GML encoding of coverage georeferencing within JP2 (both for geo-rectified and geo-referencable grids). The metadata model incorporates coverage definition, coverage metadata and value metadata. Embedded image annotation consists of text and associated geographic regions. The geographic features are defined through an associated applications schema either in-line or via URL pointers. The styling of features and annotations is based on StyledLayerDescriptors and ignores the drawing primitives provided in the JP2 format specification. This is to avoid mixed styling but has the drawback that some vendors do make use of the JP2 drawing primitives..."

Source: "Joint IES/GML Session on including GML in the JPEG2000 Image Format," Report on the Open Geospatial Consortium Technical Meeting at the Illinois Institute of Technology, Chicago.

About OGC Interoperability Experiments

"Interoperability Experiments, a new kind of OGC Interoperability Initiative, are brief, inexpensive, low-overhead initiatives led and executed by OGC members to achieve specific technical objectives that further the OGC Technical Baseline. They provide an opportunity for three or more OGC members to launch and run an initiative without the more substantial sponsorship that supports OGC's traditional testbeds and pilot projects. These initatives can be for specification development, refinement, or testing or for other purposes approved by the OGC Review Board. Interoperability Experiments operate according to the OGC's 'Interoperability Experiment Policies and Procedures'...

To begin an Interoperability Experiment, interested members submit to the OGC Review Board an Activity Plan and letters of support from at least three voting members of the Open Geospatial Consortium. If the Review Board approves the Interoperability Experiment, the board designates an OGC Staff member to oversee the experiment.

Before the Interoperability Experiment begins, the OGC issues a press release announcing the IE so that organizations have an opportunity to participate or observe.

Upon completion of the Interoperability Experiment, the technical deliverables, draft specifications, are documented as "IE Reports". These may be introduced into the Specification Program for review, possible revision, and possible adoption by the OGC Technical and Planning Committees as Adopted OpenGIS Implementation Specifications. [from the IE Summary]

About 'JPEG' Patents

The JPEG 2000 technical effort seeks freedom from the well-known "patent terrorism" surrounding the earlier 'JPEG' standard. Here are excerpts from the FAQ 'What is the patent situation with JPEG 2000?':

"... Firstly, the JPEG committee have tried to ensure in their standardisation work that many 'baseline' parts of the JPEG 2000 standard should be implementable either in full or in part without payment of either royalty fees (volume related) or license fees (non-volume related). How have they done this?

Many companies participate in the work of JPEG. In all cases where patents exist that may read on the core technology being proposed for what JPEG call the 'Baseline' of the standard (that part of the Standard that all implementations need to implement), the following actions are taken:

(1) Patent holders are requested to confirm that they will offer such patents on licensed a royalty and license fee-free basis, with the only constraint being that of reciprocity. By this we mean that the patent holder is expected to issue a license for this use against the baseline implementation of a JPEG 2000 series standard [i] without charge, [ii] and which only requires the licensee to make a similar grant on any patents that the licensee might hold that they would also claim apply to that part of the standard

(2) If such agreement cannot be obtained, the JPEG committee will then look at alternatives that avoid the use of patented technology, a license for which cannot be obtained on the above basis. In the event that no such option exists, the required feature or specification is removed from the baseline specification defined in the Standard.

In the specific case of JPEG 2000 Part 1 (ISO/IEC IS 15444-1 | ITU-T T.800), as of 17th July 2003 a total of 27 such declarations from 11 companies or individuals were submitted. All patents that WG1 believes to be essential to practice JPEG 2000 Part 1 are available on ITU-T's patent policy 2.1, which is fee-free... Work is ongoing to compile an ITU-T patent database.

The JPEG committee recognise the importance of being able to implement their core standards free of the need to pay patent holders, and continue to strive to achieve this goal wherever possible.

In addition, the JPEG committee recognises that there may be occasions where the use of non fee-free technology is unavoidable, or where it offer significant technical advantages over a fee-free solution. In these cases, JPEG may include such technology in its standards in accordance with ISO and ITU patent policy, which require that a signed declaration be obtained prior to publication stating that such patents are available on 'reasonable and non-discriminatory' terms (RAND). As an example, Part 2 of JPEG 2000 details many extensions to the baseline specification, some of which may require the use of patented technology only available on a RAND basis. As of 17th July 2003, only one such declaration was registered within the ITU-T database..." [from the JPEG 2000 web site]

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