Following on from previous posts concerning traditional navigation methods, this post looks at some traditional methods used for fixing a vessel’s position at sea, within sight of land. Electronic fixes using chart plotters are very straightforward to record, but if for some reason a vessel’s electronics are faulty it is essential that a skipper knows how to use traditional methods.
Traditional methods of navigation use a hand bearing compass, paper charts, plus plotting equipment including a course plotter, dividers and pencils (remember to use soft pencils rather than hard pencils, i.e. 2B or 3B rather than HB).
Position lines
A hand bearing compass is used to take visual bearings of ships to check whether a risk of collision exists. It is also used to take visual bearings of charted objects such as buoys and land based charted objects such as lighthouses, towers, churches and other tall buildings. When a bearing of a known object is taken, then a pencil line from that object can be drawn on the chart using the plotter. The navigator then knows that their vessel lies somewhere along that line; this line is known as a position line (Figure 1).
In order to gain a better idea of their position, the navigator needs to draw at least two more position lines from other known or charted objects that are visible from the vessel.
For example, let’s assume that a navigator who is coastal sailing can clearly see a tall factory chimney, a church spire and a radio mast. Then the navigator checks to see if these landmarks are marked on the chart. Once they are identified, then the navigator or a crew member takes bearings of each object. A note of the time and log reading is recorded. The magnetic bearings need to be converted to true according to the magnetic variation given on the chart’s compass rose.
Position lines can then be plotted on the chart and the vessel’s position can be marked where the three lines intersect (Figure 2). While this method might seem to be clear and logical, it does rely on the accuracy of the bearings, so a wise navigator would allow for a margin of error, otherwise known as a sector or area of uncertainty.
Area of uncertainty
It is more than like that the three position lines made using hand bearing compasses will not intersect precisely, leaving an area of uncertainty, resulting in an approximate position fix only being made on the chart (Figure 3). Added to this, the chosen features may not be as obvious as a church spire or a radio mast; for example the navigator might have chosen a large building or perhaps a breakwater or a pier head. Worse still, the visibility may not be good or the sea state could prevent accurate bearings from being made.
Recording the depth of water at the time of the fix will help provide back up information. In tidal waters, however, allowance will need to be made for height of tide to improve accuracy. The seasoned navigator will always err on the side of caution and make allowances for error when navigating near potential hazards. When in doubt, they will often take another fix to double check whether they are where they think they are, especially when navigating through hazardous waters.
Summary
In summary, navigation is not about relying on the chart plotter, it is about maintaining your vessel’s estimated position on a paper chart and dead reckoning (time-speed-distance and heading) and cross checking with electronic aids. If a vessel suffers electronic failure, the skipper will have no idea where they are unless they have kept an accurate log and DR plot of their position.
