Did you know there are two kinds of feet?

Ed McKay describes some fallout in the conversion from older measuring systems to newer ones in the mapping and survey fields.

First, remember this rule: There is only one meter, BUT, there are two types of feet. The two types of feet are:

1. The U.S. Survey Foot: It is defined as: 1 meter = 39.37 inches. If you divide 39.37 by 12 (12 inches per foot), you get the conversion factor: 1 meter = 3.280833333… U.S. Survey Feet.

2. The International Foot: It is defined as: 1 inch = 2.54 centimeters. If you convert this to meters and feet, you get the conversion factor: 1 International Foot = 0.3048 meters.

These two conversion factors produce results that differ by 2 parts per million; hence for most practical work it does not make any difference to the average surveyor which one is used since they usually do not encounter distances this large. For example, converting a distance of 304,800 meters (about 1,000,000 feet) to feet using the two conversion
factors, these are the results:

304,800 meters = 999,998.000 U.S. Survey Feet
304,800 meters = 1,000,000.000 International Feet

A difference of 2 feet in 1 million feet.

It appears that different states use different feet. Most folks are trending towards metric via the international foot. But the National Geodetic Survey uses survey feet when it specifies elevation.

We publish elevations in meters to the nearest millimeter (3 decimal places) and in feet to hundreds of feet (2 decimal places). For elevations above 5,000 feet (1,524 meters), the conversion factor will change the foot value by one in the second place.

So the heights of passes and mountain peaks on the maps might be off a bit from what you might think they ought to be. Now, the real question is how on earth they can measure things on land to that degree of accuracy?

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