On a large ship it is possible to make use of objects aboard the ship for estimation when the
visibility is very low, but it should be recognized that these estimates are likely to be in error
since the ship may affect the air. For the higher ranges, the appearance of the land when
coasting is a useful guide, and the distance of landmarks, just as they are appearing or
disappearing, may be measured from the chart. Similarly, in the open sea, when other ships
are sighted and their distances known, e.g. by radar, the visibility can be obtained. In the
absence of other objects, the appearance of the horizon, as observed from different levels,
may be used as a basis of the estimation. Although abnormal refraction may introduce errors
into such methods of estimation, they are the only ones available in some circumstances. At
night, the appearance of navigation lights can give a useful indication of the visibility.
Note
- When the visibility is
not uniform in all directions it should be estimated or measured in the
direction of least visibility (excluding reduction of visibility due to ship's smoke).
The sea horizon is clearly visible at the extreme right of the picture, but elsewhere it is completely
blotted out by dense fog banks, the nearest of which is on the left at a distance of 150 m
(photo by A.J. Aalders)
Reduced visibility by sea smoke (photo by P.W.J. van Hurck, m.v. Statengracht)
