Which value is used for Earth’s radius R in the EF calculation example?

• A. 20,906,000'
• B. 20,900,000'
• C. 21,000,000'
• D. 20,910,000'

Answer: (A)

In EF calculations you use the Earth's radius to turn angular measurements into linear distances with the arc length formula s = R × θ (θ in radians). In the given example, the radius chosen is 20,906,000 feet. This value represents the mean Earth radius expressed in feet, rounded to the nearest thousand as used by that material. It corresponds to about 6,370,000 meters, a standard reference you’ll see in surveying texts. The exact figure is a matter of rounding conventions in the example, but the idea is to stay consistent with that R so the computed EF values align with the example.