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Tuesday Term of the Week | Datum

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horizontal reference datum is a mathematical model of the shape of the surface of the earth at mean sea level, used to describe the position of any exact x,y point on the planet.  A datum for the earth’s surface is created from an ellipsoid and a geoid, informed and refined using a set of actual physical points on the earth’s surface known as “control points”.  As we mentioned previously, the earth is not a perfect sphere due to physical and gravitational variations.  A spheroid is used because it is a better way to describe the earth’s general shape.  

There are numerous other types of datums that can be calculated.  A vertical datum is used when measuring elevations above or below sea level.  Datums can actually be constructed for any planet; they are not limited to describing the earth even though that is how they are most typically used in GIS. Understanding the size, shape and position of the Earth is extremely important in achieving accuracy in digital mapping, and datums help us accomplish that.

Next week, we'll discuss the specific datum, WGS84.