clutch skill Master the Rule of Thumb
Even if you can see your destination across a broad lake or bay, gauging its distance is tough. That info is critical, however, in the face of changing tides, incoming storms, or impending darkness. You can estimate the mileage to a landmark—such as a cliff, island, or the mouth of a bay—using your thumb.
1. Determine the landmark’s width using your map (for example, a 600-foot-wide island). Now, with your right eye closed, stretch out your arm and hold your thumb upright so its right edge lines up with the right side of the landmark. Then close your left eye and open your right one; note how far the right edge of your thumb moved to the left. All the way across the landmark? Halfway? Twice as far?
2. The distance to that landmark is about 10 times the distance that your thumb shifted—thus, the phrase "rule of thumb." (The reason: Most people’s arms are 10 times longer than the distance between their eyes, which, when you work out the geometry, yields the rule of thumb.) If your outstretched thumb moved halfway across the 600-foot-wide island, your distance away is about 3,000 feet—600 divided by two, times 10. Depending on the individual, the factor may be closer to 11 or 9, but your estimate will still be in the ballpark.
Triangulate Your Position
With a map and compass, you can pinpoint your position on a lake or bay. First, grab a true bearing on a landmark you can positively identify on the map—say, a lighthouse at 300 degrees. Draw a long, 300-degree line through the landmark. (Notice the line could also be considered a 120-degree line. This is called the back bearing—300 minus 180 equals 120.) Repeat this with another landmark, ideally about 90 degrees apart from the first. Where the two lines intersect is your location.