Estimating the Distance Off

In clear weather, one can distinguish the shapes of tall houses, trees, lighthouses, etc. from about 8 miles offshore. The distance to the horizon however can be quite small if, in a small boat, your eye is 5 feet above the water level, the distance to the horizon is only 2.5 miles away. Although the distance can be more accurately estimated using the formula

1.17 times the square root of your height of eye = Distance to the horizon in nautical miles

Some rules of thumb you can use are as follows:

  • A light-colored beach can be seen at approximately 4 miles offshore from the deck of a typical small boat.
  • Individual windows in buildings can be distinguished by day or night at about 2 miles off.
  • A large buoy is visible at 2 miles.
  • A small buoy is visible at 1.5 miles but color and shape will not be clear.
  • The shape of a small buoy can be distinguished at about 1 mile.
  • The color of a large buoy can be distinguished at about 1 mile.
  • A person is seen as a moving black dot without limbs at 1 mile.
  • Movement of a person’s legs or arms can be distinguished at about 400 yards.
  • Faces can be seen, but not necessarily recognizable, at 250 to 300 yards.
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