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HSH Yacht Articles

Your position from three bearings

By

Jeremy R. Hood

With the approach of fall many readers may well be anticipating chartering in the Caribbean where there will be fair winds, pretty anchorages and an abundance of rum! But to get to some of the better locations does require some knowledge of navigation. OK, so the charter companies do usually provide pretty good briefings but don't forget Loran C doesn't work in the islands and few boats will be equipped with GPS. So how can you find where you are?

Not infrequently when I was working in Sint Maarten did we here a plaintive cry on the radio from some vessel asking where they were despite the fact that they were surrounded by islands from which they could fix their position. Approaching Sint Maarten just before dawn on my first visit to the island, I was a little anxious because of the number of unlit rocks between the French island of St. Barts and the southern (Dutch) coast of Sint Maarten. But by taking several bearings with my hand bearing compass I was able to fix my position on the chart and confirm that I would remain well clear of the rocks until after first light. I have used this method of position fixing time and again in the islands to make sure I was not being set down on to a reef by the current or to simply fix a position so as to calculate a course to my destination. Heading out of Union Island in the Grenadines I was able to stay clear of the Grand de Coi reef using three bearings from my hand bearing compass. The buoy marking the reef was missing as many are. But when they are there it is always good practice not to rely on them. Often it takes ages for them to be reset on station once they have drifted off position. Instead of using the buoys, establish your own position independently.

Though it is possible to fix ones position from two bearings of known objects, normally three such bearings are taken as a check on accuracy. If after plotting all three bearing lines on the chart they form a small triangle (known as a "cocked hat") you can be fairly confident of the position. Should the triangle be large then one or more of the bearings is inaccurate and then all bearings will have to be taken again and the lines re-plotted on the chart.

Figure 1 shows the islands of Sint Maarten, St Barts and the offshore rocks. In this case bearings were taken of the peak on Ile Forche, the Right Hand Edge (RHE) of Tintamarre, and the Right Hand Edge of St Barts as follows:

Ile Forche (peak) 057  (M)

RHE Tintamarre 009  (M)

RHE St Barts 121  (M)

Converting these to true bearings and plotting them on the chart we can see that our vessel is well to the south of all the dangers.

[Figure 1]

It takes practice to obtain a good fix from three bearings so why not start when you are next out sailing. Most bays or inland lakes have an abundance of features which can be used to obtain a three-position fix. Normally I will look around for some obvious places such as tall chimneys, tanks, headlands or similar and then make sure I can identify the same features on the chart. One error that can easily occur is in assuming that the object you can see and the one on the chart are the same. There may be two similar antennas close to each other, two chimneys or two similar headlands. Before I take the sights I need to be sure that what I am looking at is really the same as the one I have identified on the chart. Then with my hand-bearing compass I will stand, usually in the middle of the cockpit, take a bearing and note the magnetic heading before proceeding to the next. As a matter of routine I always then convert these to true headings before plotting on the chart. Remember the rules for magnetic variation:

VARIATION EAST, MAGNETIC LEAST

(ie the Magnetic heading is a smaller number than the True heading)

VARIATION WEST, MAGNETIC BEST

(ie the Magnetic heading is a larger number than the True heading)

Being able to fix ones position without the aid of electronics will give you a lot more confidence in your ability as a navigator but don't forget the warning on all NOAA charts: The prudent mariner will not rely solely on any single aid to navigation... And don't start on the rum until you are safely anchored!


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