Methods of Position Plotting.

This article offers an overview of common methods of coastal position plotting. A navigation manual should be consulted for in depth analysis of the procedures.

Any coastal feature that can be seen by the navigator, can be used for position finding. The conscientious seafarer should practice these methods whenever opportunity arises, as the value of any method depends on being able to determine position promptly. The Cross Bearing Fix is probably the most common for position finding at greater distances from shore, when smaller coastal features are indiscernable. Other methods will be more appropriate under varying circumstances.     Click on each for details.


Cross Bearing Fix. The small craft fixed magnetic compass is essential for navigation, and while it can be used to get an approximate line of sight bearing on a fixed object, the hand held magnetic compass, or hand held electronic type is essential for accuracy. Bearing sights on at least 3 fixed objects should be taken over the widest arc and over the shortest period of time to obtain a position fix. Try to take the bearing most abeam of the vessel, last.
Write down the sights taken (compass bearings) and convert to true bearings by applying variation and deviation. Remember that because we are converting from compass to true, the mathematics will be opposite to converting from true to compass. If these are East, then add the corrections. If West, subtract the corrections. Apply the true sightings to the chart as follows. Remember that deviation is for the vessels heading, not for the bearing of the object.

a. For each true bearing obtained, set the parallel rule on a compass rose, step the 'sight' to pass through the observed point, and lay off a line at the approximate vessel position. Repeat for the other 'sights'.
b. Alternatively use the square protractor, centering it on each observed point, to lay off each line of sight.
The vessel is on all lines of sight and therefore must be at the point of intersection of the three lines. Due to unavoidable errors the resulting diagram will be in the form of a triangle or ‘cocked hat’. If the cocked hat is small, its centre may be taken as being the position of the vessel. However, to err on the side of caution, the point of the triangle closest to danger should be taken as the position of the vessel. Often, a consistently large triangle indicates an error in the hand bearing compass, possibly due to unsuspected deviation. Take bearings well away from any possible ship board magnetic influence.

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Bearing and Loom. This accurate fix can be used only when an object is on the horizon. Most bearings of this type would be taken of the light of a lighthouse breaking the horizon line. Land just visible on the horizon could be just as useful during the day, although not as accurate.


Position Lines. Fixes involve the use of position lines. A position line may be defined as a line on some point of which, a vessel can be presumed to be located, as a result of measurement or observation. e.g. If a vessel is approaching the Gladstone Fairway beacon at night, on the leads, it is holding a course of 224 deg. 15' true, and is on a position line. Another position line could be plotted by compass bearing on the adjacent Clews Point or East Point lights. The intersection of these lines ' fixes' the position of the vessel.

Aligning two objects on shore with the vessel gives an accurate position line called a transit.
If a single compass bearing is taken on another object as the vessel transits, the intersection of the line and the transit line gives the vessel position. In all cases, plot the position and note the time.
Position lines can be obtained from :-

Running Fix. The most widely used method of obtaining a fix, where there is only one prominent object in sight.


Running Fix on Two Objects. Sometimes it is not possible to take two bearings on the same navigation mark. The running fix can be adapted by transfering the first bearing taken of an object, to a second bearing taken of another object, to obtain a fix.


Doubling the Angle on the Bow. Similar to the running fix, but the geometry involves a simple isosceles triangle, i.e. the distance run will equal the distance from the object. It is a handy way of determining distance off when it is difficult to consult a chart.

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Bearing and Depth. Although not a particularly accurate fix, it can be useful when shore objects are scarce, or visibility is poor. Soundings and contour lines need to be clearly marked on the chart.