coastal navigation

coastal navigation. The difference between coastal navigation and pilotage is narrow, but a general definition of the former would be the safe conduct of a ship where the navigator has the land on one side of his course and the open sea on the other, even though he is in fact navigating in what are known as pilotage waters. When a ship is proceeding in sight of a coastline, its navigator need not be in doubt as to his position, for the largest-scale chart of the area will show all landmarks, lighthouses, lightships, buoys, etc., and by taking bearings of objects on shore, and laying them off on the chart, he can fix the ship's position. Where the chart shows only one conspicuous object in a long coastline, a position can be obtained by a running fix. A compass bearing of the object is transferred to the chart and later, when the bearing of the object has altered sufficiently to give an adequate angle of cut, which should preferably not be less than 45°, a second bearing is taken and laid off on the chart. The first bearing is then transferred by parallel rulers by the distance the ship has run in the interval between the bearings along the course steered, making due allowance for the distance and direction the ship has been carried by the tide. The point of intersection of the transferred bearing with the second bearing will be the ship's position.

Other methods of fixing the ship's position when in sight of land are by a vertical sextant angle of, for instance, a lighthouse when its height above sea level is known. Simple mathematics, a calculator, or distance-off tables, available in most nautical almanacs, will give the distance off which can then be laid down on the chart on the correct bearing. A horizontal sextant angle between two objects ashore will give a position line which is the arc of a circle on which the ship and the two objects both lie. A second horizontal sextant angle between two other objects gives a similar position line, and again the ship's position is at the point of intersection of the two arcs.

Satellite navigation systems such as GPS, with its high accuracy and flexibility, will solve many of the navigator's problems. Radar, too, will enable him to take bearings and ranges in poor visibility (keeping in mind that the bearings will be less accurate than the ranges).

The cautious navigator will not place complete reliance on buoys, which can occasionally drag their moorings. For the history of navigation see navigation.

Mike Richey