This method is a simpler version of the first set of instructions presented in this article, and applies only in feet and miles.
 The formula you will be using is d = 1.2246* SQRT(h)

, (R + h)2 = R2 + d2. Solving for h (making the assumption that R>>h and expressing the radius of the earth in miles, approx. 3959) yields the expression:
d = SQRT(2*R*h)
Assume a flat plane or the ocean.
Solve for the distance in miles by plugging in your height of eye in feet (h) into the formula.
Derive the formula from the Pythagorean theorem.
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