Friday, 20 July 2018

Distance of Visible Sea Horizon

Distance of visible sea horizon (Nautical Miles)

(v [square root of] Height of eye in metres) x 2.09
Example: eye height of 3 metres above sea level,
v [square root of] 3 = 1.74
1.74 x 2.09 = 3.63
Visible horizon = 3.63 Nautical Miles

Distance of a visible feature on the horizon, if the height of the feature is known (dipping light)
((v [square root of] Height of eye in metres) x 2.09) + ((v [square root of] height of feature in metres) x 2.09) = distance of feature in Nautical Miles

Abnormal refraction may cause substantial errors in this calculation, be weary of figures obtained near sun rise and sun set.  Remember to adjust the height of the feature to the height of the tide, charted height of features are mostly referenced to mean high water springs (MHWS).  Light houses, the height marked on the chart is referenced to the centre of the light.


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