U.S Survey Foot vs. Common Foot vs. Metric
- The Mass State Plane coordinate system is metric but may be converted to U.S. survey feet by multiplying meters by the fraction 3937/1200 (not 3.28084).
- U.S. survey feet may be converted to meters by multiplying U.S. survey feet by the fraction 1200/3937 (not 0.3048).
- The common foot, also known as international or Imperial foot, is a familiar but different unit of measurement, one-third of the Imperial standard yard, which is defined to be 0.9144 meters, exactly. Thus the common foot is defined as 0.3048 meters, exactly. It is not used in Massachusetts survey coordinates.
If you are not working in metric you need to make absolutely sure you and everyone working on a survey's Mass State Plane coordinates are working in U.S survey feet and that conversion to and from metric is done correctly. Errors resulting from bad conversions are on the scale of about 2 meters in the Northampton area.
In 1959, the directors of the National Bureau of Standards and the United States Coast and Geodetic Survey agreed on a redefinition of the inch-centimeter relationship. This redefinition defined 1 inch as equal to 2.54 centimeters, exactly, or 1 foot as equal to 0.3048 meters, exactly. However, their agreement stipulated that the older value for 1 meter equaling 39.37 inches, exactly, be retained for identifying the U.S. survey foot. The State Plane Coordinate Systems, which are derived from the national geodetic control network, are based on the relationship of 1 meter equaling 39.37 inches, exactly.
- The difference between these two values for the foot is very small, two parts per million, but not trivial in coordinate work where the numbers are large.
- Fundamental survey units, such as rods, chains, statute miles, acres, sections and townships are all related to the U.S. survey foot, not the common foot.