WGS-84

In Egypt, a Greek scholar and philosopher, Eratosthenes, set out to make more explicit measurements. He had observed that on the day of the summer solstice, the midday sun shone to the bottom of a well in the town of Syene (Aswan). Figure 1. At the same time, he observed the sun was not directly overhead at Alexandria; instead, it cast a shadow with the vertical equal to 1/50th of a circle (7° 12'). To these observations, Eratosthenes applied certain "known" facts (1) that on the day of the summer solstice, the midday sun was directly over the line of the summer Tropic Zone (Tropic of Cancer)-Syene was therefore concluded to be on this line; (2) the linear distance between Alexandria and Syene was 500 miles; (3) Alexandria and Syene lay on a direct north south line. From these observations and "known" facts, Eratosthenes concluded that, since the angular deviation of the sun from the vertical at Alexandria was also the angle of the subtended arc, the linear distance between Alexandria and Syene was 1/50 of the circumference of the earth or 50 x 500 = 25,000 miles. A currently accepted value for the earth’s circumference at the Equator is 24,901 miles, based upon the equatorial radius of the World Geodetic System.

Source: Geodesy for the Layman, Ch. 1

There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at NGS:

• geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level

Even though we adopt a definition, that does not mean we are perfect in the realization of that definition. For example, altimetry is often used to define "mean sea level" in the oceans, but altimetry is not global (missing the near polar regions). As such, the fit between "global" mean sea level and the geoid is not entirely confirmable.

Source: National Geodetic Survey

• The Department of Defense, in the late 1950’s began to develop the needed world system to which geodetic datums could be referred and compatibility established between the coordinates of widely separated sites of interest. Efforts of the Army, Navy and Air Force were combined leading to the DoD World Geodetic System 1960 (WGS 60).
• In January 1966, a World Geodetic System Committee composed of representatives from the Army, Navy and Air Force, was charged with the responsibility of developing an improved WGS needed to satisfy mapping, charting and geodetic requirements. Additional surface gravity observations, results from the extension of triangulation and trilateration networks, and large amounts of Doppler and optical satellite data had become available since the development of WGS 60. Using the additional data and improved techniques, WGS 66 was produced which served DoD needs for about five years after its implementation in 1967.
• After an extensive effort extending over a period of approximately three years, the Department of Defense World Geodetic System 1972 was completed. Selected satellite, surface gravity and astrogeodetic data available through 1972 from both DoD and non-DoD sources were used in a Unified WGS Solution (a large scale least squares adjustment).

Source: Geodesy for the Layman, Ch. 8

• The World Geodetic System was developed by the U.S. Department of Defense in 1966 as a process for accurately surveying the earth and providing methods for assigning latitude/longitude/altitude coordinates for the purposes of navigation. With the deployment of the GPS constellation, the surveying process was updated in 1984 to incorporate GPS as the primary method of reference. Therefore, the ‘84’ is appended to the name to identify the year that the system was last updated.
• Throughout history, there have been numerous methods to survey the surface of the earth, but the WGS-84 system is the most accurate, having an overall fidelity of less than 1 meter. As an example, the WGS-84 system placed the actual Prime Meridian (0o line of longitude) approximately 100 meters east of where it traditionally lies in Greenwich, UK. This is a prime example of how different methodologies can yield different results.
• In 1989, ICAO officially adopted WGS-84 as the standard geodetic reference system for future navigation with respect to international civil aviation. With this policy, virtually all countries use the WGS-84 standard to publish waypoint coordinates for navigation (e.g., airports, runways, navaids, etc.). This data is compiled and disseminated throughout the world in formats such as navigation charts and is the data contained in most FMS Navigation Databases.

Source: Honeywell Direct-To, pg. 11

The WGS 84 Coordinate System is a Conventional Terrestrial Reference System (CTRS). The definition of this coordinate system follows the criteria outlined in the International Earth Rotation Service (IERS) Technical Note 21 [1]. These criteria are repeated below:

• It is geocentric, the center of mass being defined for the whole Earth including oceans and atmosphere
• Its scale is that of the local Earth frame, in the meaning of a relativistic theory of gravitation
• Its orientation was initially given by the Bureau International de l’Heure (BIH) orientation of 1984.0
• Its time evolution in orientation will create no residual global rotation with regards to the crust

The WGS 84 Coordinate System is a right-handed, Earth-fixed orthogonal coordinate system and is graphically depicted in [the figure].

• Origin = Earth’s center of mass
• Z-Axis = The direction of the IERS Reference Pole (IRP). This direction corresponds to the direction of the BIH Conventional Terrestrial Pole (CTP) (epoch 1984.0) with an uncertainty of 0.005
• X-Axis = Intersection of the IERS Reference Meridian (IRM) and the plane passing through the origin and normal to the Z-axis. The IRM is coincident with the BIH Zero Meridian (epoch 1984.0) with an uncertainty of 0.005
• Y-Axis = Completes a right-handed, Earth-Centered Earth-Fixed (ECEF) orthogonal coordinate system

The WGS 84 Coordinate System origin also serves as the geometric center of the WGS 84 Ellipsoid and the Z-axis serves as the rotational axis of this ellipsoid of revolution.

Source: NIMA, ¶2.1

Navigation data may originate from survey observations, from equipment specifications/settings or from the airspace and procedure design process. Whatever the source, the generation and the subsequent processing of the data must take account of the following:

1. all coordinate data must be referenced to the World Geodetic System — 1984 (WGS-84);
2. all surveys must be based upon the International Terrestrial Reference Frame;
3. all data must be traceable to their source;
4. equipment used for surveys must be adequately calibrated;
5. software tools used for surveys, procedure design or airspace design must be suitably qualified;
6. standard criteria and algorithms must be used in all designs;
7. surveyors and designers must be properly trained;
8. comprehensive verification and validation routines must be used by all data originators;
9. procedures must be subjected to ground validation and, where necessary, flight validation and flight inspection prior to publication. For guidance on the validation process see Doc 9906, Volume 5 — Validation of Instrument Flight Procedures;
10. aeronautical navigation data must be published in a standard format, with an appropriate level of detail and to the required resolution; and
11. all data originators and data processors must implement a quality management process which includes:

i) a requirement to maintain quality records;

ii) a procedure for managing feedback and error reporting from users and other processors in the data chain.

Source: ICAO Doc 9613 ¶3.4