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Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition by Michael D. Kennedy

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Scientific Notation, Numerical Significance, Accuracy, and Precision

How Old Is the Dinosaur?

The curator of a natural history museum had a habit, from time to time, of walking around and listening to the guides give their lectures at the various exhibits. One day he arrived just in time to hear that the museum’s Tyrannosaurus rex was sixty-five million and three years old. He went back to his office and told his secretary: “Gladys, tell George I want to see him as soon as he’s done with his tour.” When George appeared, the curator exasperatedly asked him, “What do you mean telling people that the Tyrannosaurus rex fossil is sixty-five million and three years old?” George looked abashed, but said confidentially, “Well, you hired me three years ago and you told me then that the skeleton was sixty-five million years old, so . . .”

A GIS will deal with very precise numbers—that is, numbers that contain many digits. For example, locating the longitude of a point on Earth’s surface within a centimeter (not at all an unrealistic expectation nowadays) requires a number with 10 significant digits—for example, 123.4567890. Because of the number of bits devoted to “ordinary” numbers by most computers, and because fractional decimal numbers may not be represented exactly by fractional binary numbers, GIS frequently use “double-precision” numbers to represent positions.

Also, a GIS may deal with very big and very small numbers. Very large and very small numbers are stored in the computer in ...

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