Ellipsoid has a smooth surface and, as opposed to the geoid, it can be described mathematically. When an ellipsoid is used to model the actual shape of the Earth (or shape of geoid), it is referred to as a reference ellipsoid. The terms ellipsoid and oblate spheroid are used interchangeably in geodesy. In this case the x and y axis on horizontal plane are equal and the z axis (on vertical plane) is shorter than the other two. An oblate spheroid is a special case of an ellipsoid obtained by rotating an ellipse about its shorter axis resulting in a squashed sphere. However the Earth bulges at the equator (flattened at the poles) due to the centrifugal force of rotation, therefore a more accurate model for its shape is defined to be an ellipsoid of revolution or an oblate spheroid. As a result, a simpler, more practical model of the earth is employed in order to be able to align a coordinate system grid with a representation of the Earth's shape. Ellipsoid – Oblate SpheroidĪs was seen above, Geoid has a complex irregular shape that is not easily defined mathematically. Over the past decade, satellite measurements such as the GRACE (Gravity Recovery and Climate Experiment - video) and GOCE (Gravity Field and Steady-State Ocean Circulation Explorer - video) satellite missions have enabled global gravity measurements, providing detailed mapping of the geoid with high accuracy and improved geoid models of the Earth. The Earth's gravity field is measured on land (using gravimeters) and from space. As a result the geoid (and the mean sea level) has an irregular shape characterized by an undulated surface with bulges and dips. In terms of the shape, greater density (relative surplus of mass) causes positive undulation, while relative deficit in mass causes a dip in the surface of the geoid. Gravity anomaly maps show the gravitational force variations over the surface of the Earth. The Earth's mass is not distributed evenly therefore parts of the Earth are subjected to stronger gravitational forces than others. The separation between the geoid and mean sea level is referred to as sea surface topography (SST) or ocean surface topography with ranges between -2m to +2m globally.Įarth's Gravity Field Anomalies Map - GRACE © NASA In reality the geoid and mean sea level do not coincide due to such factors as ocean currents, water temperature, salinity and air pressure variations. The geoid is the best global approximation of the mean sea level ( MSL) which is used as a reference in calculating elevations of features. The surface of the oceans would align with the surface of the geoid. This would be the surface if the oceans were to flow freely and cover the entire earth (in the absence of winds, tides and currents). It is an equipotential surface of the gravity field, indicating that the gravity potential is constant and the direction of gravity is perpendicular at every point on this surface. The geoid is shaped by the Earth's gravity field. Geodesists have defined two main models or reference surfaces, namely geoid and ellipsoid, to approximate the Earth's shape. An entire branch of science, called geodesy, is dedicated to the study of the shape and size of the Earth. mountains and valleys) would be an impossible task. Representing the shape of the Earth with all the variation of its complex surface topography (e.g. Consequently, an understanding of the shape of the Earth and different ways of representing it is a prerequisite to any discussion of datums. Different models of the Earth produce different datums, and therefore affect the value of a location's elevation and position (coordinates). Both of these reference frames are dependent on the way the size and shape of the Earth is represented. As an example, the height of a local mountain is determined by measuring from which surface? the base of the mountain, center of the Earth, closest water body, or the town mayor's house? Similarly the position coordinates of a point on the surface of the earth needs to be determined based on some frame of reference, raising the question of how to align coordinate system grids over the Earth's surface.Ĭonsequently two types of datums are defined: vertical datums are used as a reference from which elevations and depths are measured and horizontal datums act as base reference for measuring locations on the surface of the Earth. Naturally, there needs to exist consistent and standard regional or global reference frames for locating features on earth. A datum in general is a reference point, surface, or baseline from which measurements are made.
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