Contour-Based Position Lines

Extract

Contour charts are of great value ‘providing’, as Hughes says, ‘the navigator with a quick and sure means of comparing the echo sound record he has just made with the variations in contour and so enabling him actually to plot his line of position by soundings’. But it is evident that the most useful method of using contour lines would be if the position lines based on them are plotted in the same way as every position line, i.e. by plotting the intersection of two or more of these lines. The object of this paper is to propose such a method.

It is possible to plot a contour-based position line by means of the sight line to one point of a contour line and its range, because if we draw (Fig. 1) from every point of contour line A, parallels of the same length to the sight line, all points at the other end of contour line define such a position line, which may be called a range-bearing (RB) position line. Better accuracy can be obtained by sextant, using a second sight line to the same or another contour line (Fig. 2), measuring the angle α between them—then we have a position line which may be called a two-range position line. When it is possible to measure the minimum range to a contour, the angular datum can be omitted, using the position line which may be called a mini-range position line.