You can use similar triangles for a clearing distance. Suppose you wish to stay one mile off a light 41 metres high.
AC, which is your hand to eye distance = 0.57 metre.
AE, which is the distance off = 1 nautical mile = 1852 metres. DE, which is the height of the feature = 41 metres.
You want to know BC, which is the height on your ruler. By re-arranging the equation:
you get:
BC = DE x AC/AE BC = 41 x .57/1852 Clearing Distance = BC = 0.013 = 0.013m = 1.3 centimetres.
Now you know that if the reading is less than1.3 centimetres, then you are too close.
12.4 Distance Off by Vertical Angle Distance Off by Vertical Angle
Alternatively you can find the distance off by vertical angle between the feature and the shore. Without a sextant to measure the angle hold a ruler 57cm from your eye (Figure 12.4) and your measurement in centimetres and millimetres is in degrees and decimals of a degree. The distance off is found using the equation:
Distance off = Height of feature/tangent of vertical angle. Distance Off by Horizontal Angle
Measuring horizontal angles using a ruler (see Chapter 13) is easier than using a sextant and probably just as accurate.
Distance off = Distance apart/ (2 x Tan half the subtended angle)
If you have two features that are a known distance apart, it is simple to calculate your distance off (Figure 12.5). The features could be the markers for a measured mile - two correctly identified charted features whose distance you know or can measure from the chart.
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