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Measuring distance at sea
Seven Distance-Finding Tips when Cruising
Published: 7/7/2017
Author John Vigor came up with a super useful table to show how to estimate your distance off by eye. Did you know that if you sit in the cockpit and the distance from the water surface to your eye measures 5 feet, the horizon that you see will be just 2.5 miles away? 

That's because of the curvature of the earth. Of course, other factors increase or decrease the distance you can see an object on the horizon. 
If it's hazy or foggy, you may need radar or some other range finder instrument to help determine distance. But for now, let's look at a clear day and take it from there. Here's how far you can expect to see certain objects from your boat:

1.  Light colored sandy beach: 4 miles

2.  Individual windows in a house or building: 2 miles.
3.  Large navigation buoy: 2 miles.
4.  Small navigation buoy: 1 mile.
5.  Shape and color of a small navigation buoy: 1 mile.
6.  Person (shows as a black dot): 1 mile.
7.  Faces (some detail): 250 to 300 yards

* table based on a 5' height of eye above sea level.