For those pilots who want to avoid all things tech, or understanding all things tech, here is what you need to know about WGS-84 in a nutshell:
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: The Flat Earth Society, from artsgr1e.
Contrary to popular folklore, it has long been obvious that the earth is a sphere of some sort, and as early as 240 BC the chief librarian at the Great Library of Alexandria, Egypt, had come up with an idea just how big the sphere is. . .
[Geodesy for the Layman, Ch. 1] 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.
Figure: Oblate spheroid, from Haskel Library
The earth is basically round because gravity pulls with equal strength in all directions, tending to smooth variations towards a norm. But it isn't perfectly round, the centrifugal effects of its rotation tends to make it wider in the middle than it is tall. Technically, you would call the basic shape an oblate spheroid.
Figure: Schematic diagram, from National Geodetic Survey.
It is helpful to think of the earth's shape as a "geoid," the shape it would most closely resemble figuring the effects of gravity.
[National Geodetic Survey] There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at NGS:
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.
The earth doesn't conform to the geoid because the magnetic field isn't uniform and the earth's surface is filled with varying heights of land as well as a sea that does not maintain the same level throughout.
[Geodesy for the Layman, Ch. 8]
[Honeywell Direct-To, pg. 11]
[NIMA, ¶2.1] 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 . These criteria are repeated below:
The WGS 84 Coordinate System is a right-handed, Earth-fixed orthogonal coordinate system and is graphically depicted in [the figure].
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.
The World Geodetic System 84 is a standard used by most of the world to define exactly where a set of coordinates are on the earth. The issues on using one standard versus another are more than just determining where something is left, right, forward, and aft. Another issue is that the world isn't a perfect sphere, or geoid, and defining where something is can also vary in height above the center of the earth.
[ICAO Doc 9613 ¶3.4] 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:
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.
Geodesy for the Layman, Defense Mapping Agency, Building 56 U.S. Naval Observatory DMA TR 80-003, Washington DC 20305, 16 March 1984
ICAO Doc 9613 - Performance Based Navigation (PBN) Manual, International Civil Aviation Organization, Fourth Edition, 2013
http://www.ngs.noaa.gov/GEOID/geoid_def.htm, National Geodetic Survey
World Geodetic System 1984, Department of Defense, National Imagery and Mapping Agency (NIMA), NSN 7643-01-402-0347, NIMA TR8350.2, Third Edition, Amendment 1, 3 Janaury 2000
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