As a result of the curvature of the earth, there is a maximum distance at which an object of a given height can be seen before it disappears beneath the horizon. The chart below shows these distances for structures of heights from 5 feet through 1,000 feet.

This distance is increased when the observer is located at a point above the surface of the earth, and since most aids to navigation are viewed from the deck of a vessel, the maximum distance of visibility is increased. This is represented in the "Plus  15 foot observer" column.