The Geographic range of a light is the maximum distance at which a light can be seen when the observer’s eye is at sea level.

The Computed range of a light is the maximum distance at which a light can be seen, taking into consideration the height of both the light and the observer.

The distance at which a light can be seen increases as the elevation of the light and the observer’s height increase.

To calculate the Computed range of the light “x”, you need to first know the elevation of the light, which can be obtained either from the the nautical chart or a Yachtsmans Almanac

You must also know the observer’s height of eye above sea level.