U.S. Survey vs. International Feet
The U.S. Survey Foot 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.
The International Foot 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.
The two conversion factors produce results that only differ by 2 parts per million; hence for most practical work it doesn’t make a difference which one is used since we usually don’t encounter distances this large. For example, converting a distance of 304,800 meters 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 only 2 feet in 1 million feet.
Before you start to draw, you decide what one drawing unit will represent based on what you plan to draw. But the format of the Units (set using the AutoCAD “Units” command) are only seen when you enter and display linear units. Therefore, when drawing in feet, you are neither drawing in International Feet, nor US Survey Feet. It doesn’t matter, the display is the same – the foot symbol.
If you start a drawing in one assumed system of measurement (US Survey Foot, International Foot, Metric) and then want to switch to another assumed system (a coordinate transformation), you’ll need to scale the model geometry by the appropriate conversion factor to obtain correct distances and dimensions. But that’s why Land Desktop, Civil 3D, MAP, and Raster Design have tools like NADCON built in.
Do we assume a system in the United States? Do we assume a unit of measure when we set “Units” in AutoCAD? Absolutely. We assume the US Survey Foot, and we define it as “1 Meter = 3.28083333 US Survey Feet,” exactly 3.28083333 (exactly 8 decimal places).
Civil 3D Library
Looking for Survey tutorials? Check out The COGO Branch in the Library. These downloads aren’t software demonstrations. They were created for clients using AutoCAD Civil 3D in the real world. Registration is required.
