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Typically, local coordinate systems were developed and vertical and horizontal control were derived from a local frame of reference or datum.

Field surveying was performed on a local basis using traditional mechanical, optical, and laser equipment. Until recently, a comprehensive knowledge of-or even much interest in-datums was limited to mathematicians, geodesists, and geographers specializing in cartography. As surveying technology improved and data acquisition increased worldwide, the mathematical formulas and the related parameters used to describe the shape of our irregular earth continually improved and were expressed in spheroids such as Everest 1830, Airy 1849, Clarke 1866, Clarke 1880, and Krasovsky 1940. A datum defines the position of the spheroid relative to the center of the earth. Geographic coordinate systems use a spheroid to calculate positions on the earth. Prior to satellite mapping technology, the best approximation of the shape of the earth was the mathematically calculated geoid, which evolved into slightly flattened spheroids or ellipsoids. These are all very real examples caused, in large part, by using data sets lacking a common datum. GPS coordinates acquired by hikers show their position half way up a very steep cliff instead of on the mapped trail below.

Fire hydrant points collected with GPS appear located in parking lots or inside buildings instead of out by the curb. The same features in two street centerline themes or coverages may appear offset by many meters or feet. When was the last time you thought about what datum the data you were using was in? Perform analysis using data sets created in different datums or even display them together and you won't like the results.