Plotting Positions

Although you may not know your exact course and speed you have a good idea of their range. You have been sailing due east and know you have made four to five knots and steered between 085° and 095°. There is a choice how you plot this information on a chart. The simplest is take the mean of course and speed, draw a course of 090° for 4.5 nautical miles and put a circle of error around your DR position. Its diameter reflects your confidence. If you are 90% confident it would be a small circle, say half a mile across; at 80% it would be a mile (see Figure 21.1).

Circle of Error

Your actual position is somewhere inside this circle. Unless there Is a hazard nearby accept your DR plot.

090 for 4.5nm

If there Is a danger nearby ALWAYS select a position that puts you closest to the danger.

21.1 Circles of Error

You could make two plots. One uses the minimum values of course, speed, tide, current and leeway; the other the maximum values. Each plot has its own circle of error. Hopefully they will overlap. If there is no nearby hazard, your assumed position will be in the middle of the overlap and can be plotted by taking the mean of your values (see Figure 21.2).

Finally, you can reject the circle of error in favour of the Quad of Uncertainty. This accepts that you stay within the maximum and minimum of the estimated courses steered and the distances sailed, and the result places you inside a box (see Figure 21.3).

On average

Whenever possible, take the average of several measurements. If one measurement is wildly different from the others then it is probably wrong, and should be discarded.

One Step at a Time

It helps to break a passage into as many legs as possible, the more the better. If possible each leg should end at a position that gives you a fix. This way you do not make one long voyage but a series of shorter passages, each starting from a known position.

21.2 Circles of Er
Quad of Uncertainty

Cross Checking

When crossing the North Sea or sailing back to England from northern France there may seem to be no place for celestial navigation. Wrong. A latitude from the noon sun or Polaris should pass through your DR position (Figure 21.4). If it does not, then it is back to the drawing board.

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