| GIS: Nomenclature for Open GIS
Open GIS specification will enable interoperability within and between image and vector worlds.
By Dr. Marshall Faintich Open GIS
The Open GIS Consortium, Inc. (OGC) is a unique membership organization dedicated to open system approaches to geographic information processing, including the processing of remotely sensed Earth imagery. By means of its consensus building and technology development activities, OGC has had a significant impact on the geodata standards community, and has successfully promoted the vision of "Open GIS" that promises to greatly increase the use of geospatial data of all kinds.
OGC members - more than 70 user agencies, integrators, telcoms, universities, and vendors of software, hardware, and geodata - foresee transparent network access to widely distributed and widely heterogeneous geodata servers and to live geodata sources, with standard interfaces managing real-time communication between dissimilar databases, data formats, middleware environments, and applications.
OGC works to facilitate the use and acceptance of advanced information processing technologies in the geosciences on behalf of both public and private sectors, and promotes the integration of geoprocessing techniques with industry standard productivity tools for the development of more effective decision support capabilities.
OGC manages a formal consensus process, the primary vehicle for which is a formal Technical Committee which is responsible for the development of the Open Geodata Interoperability Specification ("OGIS"). The basis of the OpenGIS Specification is the OpenGIS Abstract Specification, which specifies a platform-neutral way of describing geodata and geoprocessing functions. The specification's Open Geodata Model includes a model for imagery, which is the responsibility of the Technical Committee's Earth Imaging Working Group (EIWG).The consensus process also emphasizes implementation of public/private sector development partnerships and the maintenance of close ties with the standards community through both ANSI and ISO technical committees and working groups. Standard Imagery Levels
The EIWG has proposed a standard nomenclature for the definition of imagery levels for airborne and spaceborne remotely sensed imagery. Various sensor specific level definitions have led to confusion among users, and a standard nomenclature is required for the open system approach to be successful.
OGC is not advocating the re-naming of existing sensor-specific imagery levels, but recommends that all imagery related products and services adopt the OGC nomenclature, or a mapping to the nomenclature, for interoperability as a common interface standard. One goal is to have a nomenclature to which all other significant imagery level schemes can map. Five basic levels of image processing and geospatial products have been developed. These levels are further defined to include different generic functions within each corresponding level.
Level 0 - Systems corrections only and raw metadata extraction from telemetry stream is done. These corrections are applied to all imaging events of a given sensor uniformly independent of where the data were collected.
System Corrections include such functions as missing pixel filling or conversion of raw telemetry to sensor engineering data to be applied uniformly to sensor output. These corrections are usually applied at the sensor's receiving station.
Level 1 - The metadata are refined and additions may be made. Radiometric and geometric calibrations may be undertaken using external information.
Level 1A - Geographic formatting using the sensor system's geographic knowledge. The image pixels values are not adjusted, but based on internal system information, metadata values may change or new metadata is added.
Geographic formatting is the repackaging of an imagery event, which includes "chunking" single image sections out of a long continuous image strip, scene shifting, etc. An imaging event is the sensor collection unit (swath, frame, strip, etc.).
Level 1R - Radiometric adjustment using the sensor system's radiometric calibration data measurements.
Radiometric adjustments refine the radiometric values of an image based on the results of a calibration. Level 1R adjustments include changing the pixel values based upon "dark" currents, post-launch or post-flight laboratory calibration data, etc., but do not include adjustments based upon external and geographically specific measurements such a local atmospheric data.
Level 1G - Photogrammetric geopositioning of the sensor data using external geo-referenced information.
Geopositioning is the photogrammetric adjustment of one or more images relative to each other and to an absolute (fixed) ground reference system. Photogrammetry is the science of mensuration and geometric adjustment of, an aerial photograph or satellite image. Photogrammetry requires a rigorous mathematical model of the image formation process, computation of the internal geometry of an image, and subsequent correction of imagery based upon the ground relationship for every part of the image. Correction of imagery is based on computational algorithms, and measurement of geometrical position in an image.
In comparison, geocoding is the assignment of a geographic value to a pixel of an image/air photo via the placement on a desired datum and map projection, and georeferencing is the assignment of a geographic coordinate to a reference pixel in a geo-coded image. A datum is an absolute reference system that fixes the origin (Earth center) and ellipsoid upon which latitudinal and longitudinal lines are drawn.
Level 1N - A non-mapping product that may be used for many different purposes. In most cases, Level 1N products are not used for cartographic or GIS-mapping purposes, because the accuracy can not be certified.
A non-mapping image is any non-photogrammetric image. This includes any imagery product that has gone through rubber sheeting (also referred to as image warping) or any other imagery processing that does not allow for rigorous measurement of error propagation. Rubber sheeting (image warping) includes any geometric manipulation of an image to control points that does not involve the use of a photogrammetric math model.
For the purposes of the OpenGIS Specification, Level 1N products do not support further OGC defined imagery levels. The lack of a rigorous error propagation model does not allow for the assignment of certified values for the accuracy fields of subsequent product metadata.
Level 1N was the subject of considerable discussion in the EIWG. Much Level 1N data is available, and much more will become available. Indeed, a photo taken of a house from street level could be described, processed, and sold as Level 1N imagery. There are many legitimate uses for Level 1N imagery, and the working group has debated various positive-sounding descriptive names such as "pictorial images" or "illustration images," names which could be used in marketing such imagery. But the feeling is very strong in the working group that the public needs to be made aware of the inability to assign a certifed accuracy to Level 1N imagery, and OGC's geoprocessing standards must be constructed so that software can be written which can prevent false representation of an image's accuracy.
Level 2 - This processing involves image pixel modification, transformation and compilation of products from Level 1 data.
Level 2R - Radiometric modification of Level 1 Data using external information. This level includes atmospheric corrections based upon local and external measurements. Atmospheric correction is the removal of the atmospheric effects from a remote sensing data set such that the data set simulates one that would be obtained if the data were collected in an environment that had no atmosphere (e.g., a vacuum, or a measurement taken in space which did not have to go through the Earth's atmosphere).
Level 2G - Geometric transformation of sensor data based on the results of photogrammetric geopositioning. This level includes rectification and orthorectification.
Geometric transformations are the re-sampling, relocation, or re-sizing of image pixels (the smallest discrete unit of an image). Rectification is the geometric adjustment of image pixels to remove distortions caused by the imaging sensor and the geometry of the sensor to the ground. Orthorectification is the process of the adjustment of an aerial photograph or satellite image to remove geometric distortions caused by the imaging sensor and terrain relief displacement.
Level 2T - Terrain compilation from Level 1G Data.
Compilation is the three dimensional collection of information from many sources. A Digital Terrain Model (DAM) is any representation of the bare surface of the Earth with natural and man-made features removed. The representation may be in the form of elevation posts, surface polynomials, irregular patches (e.g., TIN) or contours. A Digital Elevation Model (DEM) is a representation of the bare surface of the Earth with natural and man-made structures and features removed that is described by a regular or irregular grid of discrete terrain elevation values, usually referenced to the gelid (i.e., Mean Sea Level). A Digital Feature Height Model is a height model of natural and man-made features above the bare surface of the Earth. This model can be derived from stereo pairs of air photos or satellite imagery stereo pairs.
Level 2F - Feature compilation and/or thematic classification from Level 1G Data.
Level 3 - This processing involves the extraction and/or classification of image data. Level 3F - Feature extraction and/or thematic classification from the Level 2 data products. Extraction is the two dimensional collection of information from a processed image.
Level 3T - Terrain extraction from the Level 2T data products. Examples include the generation of a regular grid of elevation posts interpolated from an irregular Level 2T network of elevation posts, and the generation of surface polynomials, tesselated patches, or contours from Level 2T values.
Level 4 - The generation of symbolized products from Level 2 and 3 data.
Level 4F - Feature Symbolization - generation of map symbols or the color assignment of thematic classes/categories. These symbols are generated from Level 2 compiled and Level 3 extracted features.
Level 4T - Terrain symbolization - generation of symbolized terrain displays, such as shaded relief displays, wire frame models, tagged and/or colorized contours. OGC Participation
On Aug. 30, 1996, OGC distributed a public Request for Information (RFI), seeking input from anyone interested in the development of the imagery portion of the OpenGIS Specification's Open Geodata Model. The EIWG strongly encourages all commercial and government entities involved with remote sensing to become members of OGC, but you don't have to join to make the EIWG aware of your specific needs or your ideas. You can keep abreast of current consortium activities via the OGC web page at http://www.opengis.org.
OGC is an increasingly international consortium with most of the major geoprocessing technology providers and many major data providers and users (DMA, USGS, DOA NRCS, NASA, NOAA, etc.) participating. OGC provides a fertile forum for developing public/private sector development partnerships, and OGC also provides opportunities for user organizations such as federal agencies and industry associations to work with the technology developer community to specify agency-specific or industry-specific technologies that ease integration of legacy systems and products from multiple vendors.
It is unlike other imagery standards groups, because the goal is not format standards, transfer standards, or metadata standards, but a geoprocessing interface standard for software that will provide transparent access to geodata and geoprocessing resources in distributed computing environments. The implications for the Earth Observation industry are enormous when you consider the combined potential of high resolution scanners, GPS on a chip, spatially enabled databases, the Internet, and the fertile hybridization of telephones, TVs, cell phones, GPS, computers, video games, and other electronic devices. About the Author:
Dr. Faintich is co-founder and Chief Technical Officer of TRIFID Corp. (St. Louis, Mo.), which serves the geographic information industry. He is also chairman of the Commercial Applications Integrated Product Development Team for the NASA Lewis hyperspectral satellite, a member of the Commercial Products Team for the Earth Observing System (EOS), and a member of the NASA Lunar and Planetary Cartography Working Group. He may be reached at 314-991-3095 (phone) or 314-991-4059 (fax). Back |
