Geodesy

Fundamentals


Concepts


Technologies


Standards

ED50

European Datum 1950

SAD69

South American Datum 1969

GRS 80

Geodetic Reference System 1980

NAD83

North American Datum 1983

WGS84

World Geodetic System 1984

NAVD88

N. American Vertical Datum 1988

ETRS89

European Terrestrial Reference
System 1989


History



In geodesy, a reference ellipsoid is a mathematically defined surface that approximates the geoid, the truer figure of the Earth, or other planetary body. Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.
Contents

Ellipsoid parameters 1

Coordinates 2

Historical Earth ellipsoids 3

Ellipsoids for other planetary bodies 4

See also 5

Notes 6

References 7

External links 8
Ellipsoid parameters
In 1687 [2]^{[3]} uses the word 'ellipsoid' alone in preference to the full term 'oblate ellipsoid of revolution' or the older term 'oblate spheroid'. In the rare instances (some asteroids and planets) where a more general ellipsoid shape is required as a model the term used is triaxial (or scalene) ellipsoid. A great many ellipsoids have been used with various sizes and centres but modern (post GPS) ellipsoids are centred at the actual center of mass of the Earth or body being modeled.
The shape of an (oblate) ellipsoid (of revolution) is determined by the shape parameters of that ellipse which generates the ellipsoid when it is rotated about its minor axis. The semimajor axis of the ellipse, a, is identified as the equatorial radius of the ellipsoid: the semiminor axis of the ellipse, b, is identified with the polar distances (from the centre). These two lengths completely specify the shape of the ellipsoid but in practice geodesy publications classify reference ellipsoids by giving the semimajor axis and the inverse flattening, 1/f, The flattening, f, is simply a measure of how much the symmetry axis is compressed relative to the equatorial radius:

\begin{align} f&=\frac{ab}{a}. \end{align}
For the Earth, f\,\! is around 1/300 corresponding to a difference of the major and minor semiaxes of approximately 21 km. Some precise values are given in the table below and also in Figure of the Earth. For comparison, Earth's Moon is even less elliptical, with a flattening of less than 1/825, while Jupiter is visibly oblate at about 1/15 and one of Saturn's triaxial moons, Telesto, is nearly 1/3 to 1/2.
A great many other parameters are used in geodesy but they can all be related to one or two of the set a, b and f. They are listed in ellipse.
Coordinates
A primary use of reference ellipsoids is to serve as a basis for a coordinate system of latitude (north/south), longitude (east/west), and elevation (height). For this purpose it is necessary to identify a zero meridian, which for Earth is usually the Prime Meridian. For other bodies a fixed surface feature is usually referenced, which for Mars is the meridian passing through the crater Airy0. It is possible for many different coordinate systems to be defined upon the same reference ellipsoid.
The longitude measures the rotational angle between the zero meridian and the measured point. By convention for the Earth, Moon, and Sun it is expressed as degrees ranging from −180° to +180° For other bodies a range of 0° to 360° is used.
The latitude measures how close to the poles or equator a point is along a meridian, and is represented as angle from −90° to +90°, where 0° is the equator. The common or geodetic latitude is the angle between the equatorial plane and a line that is normal to the reference ellipsoid. Depending on the flattening, it may be slightly different from the geocentric (geographic) latitude, which is the angle between the equatorial plane and a line from the center of the ellipsoid. For nonEarth bodies the terms planetographic and planetocentric are used instead.
The coordinates of a geodetic point are customarily stated as geodetic latitude and longitude, i.e., the direction in space of the geodetic normal containing the point, and the height h of the point over the reference ellipsoid. See Geodetic system for more detail.
Historical Earth ellipsoids
Currently the most common reference ellipsoid used, and that used in the context of the Global Positioning System, is the one defined by WGS 84.
Traditional reference ellipsoids or geodetic datums are defined regionally and therefore nongeocentric, e.g., ED50. Modern geodetic datums are established with the aid of GPS and will therefore be geocentric, e.g., WGS 84.
Ellipsoids for other planetary bodies
Reference ellipsoids are also useful for geodetic mapping of other planetary bodies including planets, their satellites, asteroids and comet nuclei. Some well observed bodies such as the Moon and Mars now have quite precise reference ellipsoids.
For rigidsurface nearlyspherical bodies, which includes all the rocky planets and many moons, ellipsoids are defined in terms of the axis of rotation and the mean surface height excluding any atmosphere. Mars is actually egg shaped, where its north and south polar radii differ by approximately 6 km, however this difference is small enough that the average polar radius is used to define its ellipsoid. The Earth's Moon is effectively spherical, having no bulge at its equator. Where possible a fixed observable surface feature is used when defining a reference meridian.
For gaseous planets like Jupiter, an effective surface for an ellipsoid is chosen as the equalpressure boundary of one bar. Since they have no permanent observable features the choices of prime meridians are made according to mathematical rules.
Small moons, asteroids, and comet nuclei frequently have irregular shapes. For some of these, such as Jupiter's Io, a scalene (triaxial) ellipsoid is a better fit than the oblate spheroid. For highly irregular bodies the concept of a reference ellipsoid may have no useful value, so sometimes a spherical reference is used instead and points identified by planetocentric latitude and longitude. Even that can be problematic for nonconvex bodies, such as Eros, in that latitude and longitude don't always uniquely identify a single surface location.
See also
Notes

^ Isaac Newton:Principia Book III Proposition XIX Problem III, p. 407 in Andrew Motte translation, available on line at [1]

^ Torge, W (2001) Geodesy (3rd edition), published by de Gruyter, isbn=3110170728

^ Snyder, John P. (1993). Flattening the Earth: Two Thousand Years of Map Projections. University of Chicago Press. p. 82.
References

P. K. Seidelmann (Chair), et al. (2005), “Report Of The IAU/IAG Working Group On Cartographic Coordinates And Rotational Elements: 2003,” Celestial Mechanics and Dynamical Astronomy, 91, pp. 203–215.

Web address: http://astrogeology.usgs.gov/Projects/WGCCRE

OpenGIS Implementation Specification for Geographic information  Simple feature access  Part 1: Common architecture, Annex B.4. 20051130

Web address: http://www.opengeospatial.org
External links

Coordinate System Index

Geographic coordinate system

Coordinate systems and transformations (SPENVIS help page)

Coordinate Systems, Frames and Datums
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