U.S. COAST GUARD LIGHT LIST, gives the approximate geographic range of visibility for an object which may be seen by an observer at sea level. You should add to the distance for the height of any object the distance to the height of the observer's eye above sea level.
Example: Determine the geographic visibility of an object, with a height above water of 65 feet, for an observer with a height of eye of 35 feet.
Height of object / Light '' 65 feet '' = 9.4 miles
Height of observer ''35 feet'' = 6.9 miles
Computed geographic visibility= 16.3 miles
The formula for which the Geographic Range Table is based is:
Distance to the horizon = square root height x 1.17
By knowing the formula, it saves time in having to look the distances up in the table. Since the table does not tabulate distances for each foot. I use a calculator and running the formula to get the distances. Using the same heights; the formula; and this format:
Ht of Lt / Object = 65 ft = (8.0622577) x 1.17 = 9.43 or 9.4 miles
Ht of eye = 35 ft = (5.9160797) x 1.17 = 6.92 or 6.9 miles
Geographic Range = 16.3miles
Except for range and some directional lights, the nominal range and height of the light is listed in the U.S. Coast Guard Light List and on the chart.
I also have a form I use which you can see on my link.
NAME ________________________
VIS ________ HT. OF LT. ________
NR ________ HT. OF EYE ________
LR _________ GR ________
The luminous Range Diagram shown in the Light List is used to convert the nominal range to the luminous range. When using this diagram, it must be remembered that the ranges obtained are approximate, the atmosphere may vary between the observer and the light, and glare from background lighting will reduce the range at which lights are sighted. The Luminous Range Diagram is entered with the nominal range at the bottom" nominal range scale" a vertical line is followed up until it intersects the visibility curve or reaches the region on the diagram representing the meteorological visibility; from this point ( you take 90° turn to starboard) a horizontal line is followed until it intersects the luminous range scale on the right.
