The Clarke 1866 and GRS 1980 ellipsoids are positioned such that the surface of the ellipsoid better matches the North American continent. The position of the ellipsoid with respect to the center of the Earth (indicated by the eccentricity ) also varies among datums. You can see that there are slight differences in the values (indicated by bold type). For example, here are the parameters for several datums that are commonly used in the United States: So, what is the "set of numbers that define the size, shape, and position of the ellipsoid"?Īn ellipsoid can be mathematically described by four parameters: semi-major axis (the equatorial radius), semi-minor axis (the polar radius), the degree of flattening, and the ellipsoid's position with respect to the center of the Earth. The Earth represented as an oblate spheroid The example of the deformation of the spinning bowling ball is exactly what happens to the Earth - its rotation about the polar axis causes an equatorial bulge, with a resultant flattening at the poles. This flattened shape is called an oblate spheroid (basically, a three-dimensional ellipse). The result would be a sphere that appears to be squashed, similar to what would happen to a round balloon if you squeezed it from both sides simultaneously. If you could spin a bowling ball so fast that centrifugal force were able to deform the ball, its equatorial radius would be slightly larger than 4.25" and its polar radius would be slightly less than 4.25". That is, its equatorial radius of 4.25" is the same as its polar radius. Simply put, a datum is a set of numbers that define the shape, size, and position of an ellipsoid which best approximates the true surface of the Earth, either locally or globally.Ĭonsider a bowling ball. To learn more, refer to the ArcGIS Help Library. The discussion below will give you a basic understanding of essential geodesy concepts. When working with the Land Mapper, or any geographic data, it is essential to have a basic understanding of these concepts. The ability to accurately determine locations on the surface of the Earth is fundamental to GIS and GPS, and this is accomplished by using various datums, map projections, and coordinate systems.
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