It is possible to use something as simple as a pencil to determine approximate distance off. In order to use pencil radar:
- Locate an object that can be seen onshore which is shown on your chart with a charted height.
- Hold a pencil up at arm’s length and site the top of the object with the tip of the pencil with one eye.
- Switch eyes.
- You should estimate the horizontal distance (HD) that the tip appears to have jumped relative to the known height of the object. For example, if you are spotting the top of a spire on a mountain (as shown on your chart at 600 feet), when you shift eyes the pencil appears to shift twice the horizontal distance as the height, you would have an estimated horizontal distance of 1200 feet.
- Now you simply use the formula D (Distance Off in feet) = HD (in feet) X 10. In our example, if the spire was 600 feet and the horizontal distance (HD) is 1200 feet, the distance off the spire = 1200′ X 10 = 12,000′ or 2 approximately miles.