The first two are angles and the third is a distance. Three values are recorded (x, y and z), and there are no angles - only distance.Īnother way is to use latitude, longitude and ellipsoidal ⁄ spheroidal height (the height above or below the datum’s ellipsoid ⁄ spheroid surface). This is called an Earth-centred Cartesian Coordinate System. The most common method Geodesists use involves measuring the distance from the centre of the Earth to the point on the surface of the Ellipsoid ⁄ Spheroid. In Geodetic terms every point on the surface of the Earth’s Geoid is defined by 3 values. The second was determined by the US Defence Department and is known as the World Geodetic System 1984, WGS 84.įitting an Ellipsoid ⁄ Spheroid to the Earth.The first was determined by the International Association of Geodesy (IAG) is the Geocentric Reference System 1980, or GRS 80.There are now two Ellipsoids ⁄ Spheroids most commonly used to describe the shape of the Earth: Semi-minor axis = semi-major axis x (1 - flattening)īecause flattening is a small number and usually quoted to numerous decimal places, it is sometime given as 1 ⁄ f. it is the distance from the North Pole to the centre of the EarthĪ Ellipsoid ⁄ Spheroid is usually described by the semi-major axis and a flattening component (f).in Earth’s case this aligned between the North Pole.it is the distance from the equator to the centre of the Earth.in Earth’s case this aligned with the Equator.The Ellipsoid ⁄ Spheroid is a slightly ‘squashed’ sphere shown diagrammatically below: The closest basic mathematical figure that approximates the shape of the Earth is the Ellipsoid ⁄ Spheroid. A sphere would have been a poor third choice but even that isn’t the case. Even a cube would have been easier for map makers. In order to understand what is happening and what it all means, a basic understanding of datum is required.īut first a warning - please read the section which explains the difference between Ellipsoids and Spheroids. Some modern software converting between different projections ‘on–the–fly’ without allowing for differences between datums.This assists in creating confusion between what a projection does with what a datum does.
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