A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2017 Feb 18, 07:36 -0800
Tony, Just to reiterate what Bob Goethe has said, you may be looking at a true horizon with the land as a backdrop, in which case you can use the horizon in front of that distant land without any difficulty.
You can test for this by calculation and chart by noting the distance to the true horizon. It's given by approximately by taking the square root of the height of eye in feet. For better accuracy add 15% to the result. So if your height of eye on the beach is, for example, 7.5 meters, you can convert that to feet by multiplying by 3.3; the result is 25. The square root of that is 5. Add 15% and you get 5.7 nautical miles (and since 54 nautical miles is 100 km, with near perfect accuracy, that distance is just about 10km). There are shorter ways to do this: take square root of height of eye in meters and multiply by four to get distance to the horizon in kilometers directly. In any case, if the opposite shore is further away than that, then you have nothing to worry about except possibly contrast problems. As long as you can see it clearly, the horizon you see is the true sea horizon in front of a land backdrop.
You can also often test for this visually. With binoculars, start at the clear sea horizon and scan to the left and right. If the apparent dividing line of sea and distant land remains perfectly straight, in line with the clear sea horizon, then it is almost certainly in front of a land background. And again you can then use it for sights.