Geoid models are based on Mean Sea Level - or the elevation of the planet’s sea level surface if the water were completely flat. However, because ellipsoids are still completely smooth, unlike the Earth itself, they are typically referred to as being only a “best approximation” of the planet’s shape.Ī geoid model of the Earth, meanwhile, adds some necessary complexity and gives a fuller representation of the way the Earth’s surface varies in elevation. The specific ellipsoid model that best replicates the Earth is called the earth ellipsoid. However, any cross-section plane of the shape must still be either an ellipse or a circle. Unlike a sphere, however, the length, width and height of an ellipsoid do not all have to be identical, according to Encyclopedia Britannica. An ellipsoid is an imperfect spherical shape. In order to understand the idea of a reference surface and properly find and utilize datums, you first have to know the difference between an ellipsoid and a geoid model of the Earth. Notably, there are multiple reference surfaces, depending on where you are and whether you’re measuring for a horizontal or vertical datum. To be useful in surveying, a geodetic datum must be based on a reference surface, which takes these points from an abstract idea to a set location on the Earth’s surface. There are two kinds of geodetic datum: horizontal and vertical.
While a datum can be any reference used to measure an object - including planes, lines and points - in surveying they are typically points, called geodetic datums. Have you ever wondered where the coordinates on your GPS device come from? What about stats like sea level? These numbers are derived from datums - an essential part of any coordinate reference system and the backbone of so much of surveying.ĭatums are used as the starting reference point for surveying jobs and put your work in a larger context.
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