Which of the following values is most commonly used as the inverse flattening?

• A. 6378206.4
• B. 6356583.8
• C. 294.97
• D. 1/294.97

Answer: (C)

Inverse flattening describes how much an ellipsoid is flattened compared to a perfect sphere, and it is the reciprocal of the flattening: f' = a/(a − b). In practice, a common historical ellipsoid used in many geographic and surveying systems is Clarke 1866, which has an equatorial radius of 6,378,206.4 meters and a polar radius of 6,356,583.8 meters. From these values, the inverse flattening works out to about 294.97, which is why this number is frequently cited as the inverse flattening. The other numbers shown are different parameters: 6378206.4 meters is the equatorial radius, 6356583.8 meters is the polar radius, and 1/294.97 would be the actual flattening, not the inverse.