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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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Precision of the Geographic Coordinates in TIGER Files

Consider that the 90-degree angle from the equator to the North Pole (latitude) is about 10,000 kilometers. So:

A degree of latitude is about 111 kilometers.
A tenth of a degree of latitude is 11.1 kilometers.
A hundredth is 1.11 kilometers or 1110 meters.
A thousandth is 111 meters.
A ten-thousandth is 11.1 meters.
A hundred-thousandth is 1.11 meters.
A millionth is 0.111 meters.

The precision of the recorded number (e.g., 38.047734), where the last digit is in the “millionths” position, might suggest that the position is known to about the nearest tenth of a meter (i.e., 0.111 meters). Here is where your knowledge of the difference between accuracy and precision comes in. TIGER data is rarely good to 10 centimeters accuracy. Ten meters might be more like it. Sometimes the last two digits are zeros in the files, which implies an accuracy of the nearest ten-thousandth of a degree, or about 10 meters—roughly 33 feet.

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