Background of Geocentric Coordinate Systems
The National Imagery and Mapping Agency (NIMA) produces numerous mapping, charting, geodetic, gravimetric, and digital products in support of the Department of Defense (DoD). It is advantageous to refer these products to a single coordinate system for many reasons other than ease of working with a large number and variety of products. Such a system is needed due to accuracy and user interface considerations, the need for a product to support the widest possible range of applications (local, worldwide), the need to relate information from one product to data obtained from another source (e.g., map/chart positions to coordinates obtained from inertial navigation systems in real time), and the need to ensure a smooth transition in product use from one part of the world to another.
The world geodetic system provides the basic reference frame and geometric figure for the Earth, models the Earth gravimetrically, and provides the means for relating positions on various local geodetic systems to an Earth-centered, Earth-fixed (ECEF) coordinate system.
Three such systems, World Geodetic System 1960 (WGS 60), WGS 66, and WGS 72, each successively more accurate, have supported DoD activities. WGS 72 has several shortcomings which negate its continued use.
A new more accurate system, WGS 84, has been developed as a replacement for WGS 72. The new system represents DMA’s modeling of the Earth from a geometric, geodetic, and gravitational standpoint using data, techniques, and technology available through early 1984. It is an improvement over WGS 72 in several respects. New and more extensive data sets and improved computer software were used in the development. A more extensive file of Doppler-derived station coordinates was available, and for many more local geodetic systems improved sets of ground-based Doppler and laser satellite tracking data and surface gravity were available, and geoid heights were deduced from satellite radar altimetry.
WGS 84 Relationships with other Geodetic Systems
One of the principal purposes of a world geodetic system is to allow referencing of local geodetic systems to a single geocentric system. The number of local geodetic systems, or local horizontal datums, requiring such referencing is extensive. Counting island and/or astronomic-based datums, the number exceeds several hundred. To accomplish the conversion, local geodetic system and WGS coordinates are both required at one or more sites within the local datum area so that local geodetic system-to-WGS datum shifts can be formed. Doppler stations positioned within WGS 84 with known local geodetic system coordinates were the basic ingredient in the development of Local Geodetic System-to-WGS 84 Datum Shifts. A total of 1,591 such Doppler stations was available for that purpose.
The most accurate approach for obtaining WGS 84 coordinates is to acquire satellite tracking data at the site of interest and position it directly in WGS 84 using the Satellite Point Positioning technique. However, it is unrealistic to presume that use of this technique will always be possible. If direct positioning is not possible, the transformation from WGS 72 to WGS 84 or from local geodetic system to WGS 84 can be used.
World Geodetic System 1984 is more accurate than WGS 72 and replaces the latter as the geocentric system officially authorized for DoD use.
The reference frame for WGS 84 is more accurately defined than that of its predecessor. Local Geodetic System-to-WGS 84 Datum Shifts of improved accuracy are available, and for many more datums. In addition, the WGS 84 earth gravity model and geoid are considerably more accurate than their WGS 72 counterparts, and minor scale, translation, and orientation errors inherent in WGS 72 are reduced in WGS 84. These improvements translate into the following:
- more accurate geodetic coordinates, ellipsoid heights, heights above the geoid (approximate mean sea level), and distances
- an improved capability for satellite orbit determination and prediction
- the capability to place many more local geodetic systems on a world geodetic system and to do it more accurately
The latter is particularly important for those local geodetic systems affected by large distortions. Placement of such local datums on WGS 84, using the variable datum shifts made possible by a well-dispersed set of Doppler sites, effectively removes these distortions.
The value of WGS 84 will become increasingly evident in the early 1990’s when the Navstar Global Positioning System (GPS) will be fully operational. Since the reference system for GPS is WGS 84, high quality geocentric coordinates can be provided automatically by GPS User Equipment. For those using GPS but still utilizing local geodetic systems and products, the availability of the more accurate WGS 84-to-Local Geodetic System Datum Shifts will lead to an improved recovery of local coordinates.
Further information is given in NIMA Technical Report 8350.2, “Department of Defense World Geodetic System 1984, Its Definition and Relationships with Local Geodetic Systems,” Second Edition, 1 September 1991 and its supplements. The report is available from the NIMA Combat Support Center and its stock number is DMATR83502WGS84. Non-DoD requesters may obtain the report as a public sale item from the U.S. Geological Survey, Box 25286, Denver Federal Center, Denver, Colorado 80225 or by phone at 1-800-USA-MAPS.