There is only one meter, and it measures 3.280833 U.S. Survey Feet (1893) and 3.280840 International Feet (1959). To understand how they are used, some background and history will be helpful to put it into perspective.
Congress promulgated and legalized the domestic use of the metric system within the United States in 1866.
In 1893, the yard was defined as 3600/3937 meter, setting the standard for the measurement commonly divided into three parts and known as U.S. Survey Feet (1200/3937 meter). This domestic yard, measuring 0.914401828804 meter but later stated as “0.91440183 meter,” would describe an “approximate relation” in a future announcement in 1959. However, these measures conformed to the ones set by Congress in 1866. This domestic yard yields the U.S. Survey foot at 0.304800609601 meter. Dividing the meter (at 39.37 inches) by 12 (using 12 inches per foot) yields 3.280833 U.S. Survey Feet.
A.V. Astin, Director of the National Bureau of Standards, and Rear Admiral H. Arnold Karo, Director of the Coast and Geodetic Survey, jointly announced in 1959 that all United States weights and measures would continue to be based upon metric measurement standards, with an exception made for the U.S. Coast and Geodetic Survey under special circumstance. This announcement was approved by the Secretary of Commerce.
Thus, the International Yard was established, measuring exactly 0.9144 meter.
The Standard Inch, defined as 25.4 millimeters, forms the basis for the International Foot. 304.8 millimeters equals one foot. There are 3.280840 International Feet in the meter.
The exception for published data on United States geodetic surveys held that the U.S. Survey Foot, already defined in 1893, would continue to be used until a future time when the total geodetic survey networks in the United States would be upgraded and readjusted. Upon the new readjustment, the International Yard, equal to 0.9144 meter would apply in survey measure. It would have been horribly inconvenient in 1959 to convert published map measurements to the new standard, so it was decided to keep using the old data as it was.
Because the size and measure of the international meter never changed, the defined International Yard of 1959 came out smaller than the 1893 yard by about two parts in one million. Of course, the two parts in one million were never lost, and resulted in the new numerical measures being increased by that amount.
The readjustment of United States geodetic survey networks envisioned in the 1959 joint announcement finally took place in 1983. The North American Datum of 1983 (NAD 83), published in 1986, ushered in a new National Coordinate Reference System of the United States. The International Foot, while not needed because the survey was published in meters, thus became integrated in NAD 83.
Despite the advancement in metric usage, altitudes and elevations of many geodetic survey products are still rendered using the U.S. Survey foot, possibly to avoid confusion about metric adjustments that would be noticed in higher elevations. Uninitiated people may feel that the mountains are shrinking!
The National Institute of Standards and Technology (NIST) and the National Geodetic Survey (NGS) of today, routinely using the meter, have avoided dictating which foot measure to be used by individual states. Many states made their own laws as to their preference of the U.S. Survey Foot or the International Foot. The interest of the NIST is only in calibration, but the NGS publishes survey maps in the preferred foot measure desired by an individual state. The NGS, while primarily using the meter except for states’ preference, still uses the U.S. Survey foot in publishing elevation data in order to lessen any confusion by users. However, the NGS strictly computes in meters and converts to the desired format for states’ convenience.
