Seamanship: fix your position without a GPS

Jackie & Noel Parry
Posted September 2 2015

It is good seamanship (and sometimes imperative) to be able to fix your position without using a GPS, experienced sailor and author Jackie Parry explains.

Bearings - Three bearing fix

This is a great way to double check your position.

On your chart select three conspicuous landmarks to use, to avoid large errors when underway. Take a bearing with your hand compass of the landmark closest to your stern first, then closest to your bow, and finally, the one that is abeam (90° from your bow).

You must be 100% sure that you visually identify the correct landmarks and, ideally, the landmarks should be around sixty degrees apart.

You will need to apply Compass error (Variation), but no Deviation if using a hand held compass (Deviation is only applied if you are using the ship’s compass and you know the Deviation).

Compass errors (Variation and Deviation)

The difference between True North and Magnetic North is called Variation. The degree of Variation and its annual rate of change is indicated on nautical charts within the Compass Rose. Deviation is the deflection of the compass from its proper orientation. It is usually caused by magnetic materials on the boat (or indeed the boat itself). Deviation can be east or west, or zero, depending on the magnetic conditions on the vessel. The value will change with the boat’s heading. (See ‘Compass: True to Compass’ further on in this section.)

Once you have converted the Compass bearing to a True bearing, plot the bearings on your chart. Where the three bearing lines cross, is your fix position. The time of the bearings taken is noted on the chart next to your fix position. This is very important, especially when using DR. (See ‘Position - DR’ in this section - Ded Reckoning is actually ‘Deduced Reckoning’.)

Do not use navigation buoys for bearings, as they may have moved from the charted position. You can take just two bearings, but they will not show any errors that may have occurred, as three bearings will.

With three bearings, you may end up with a cocked hat where the three bearing lines meet (usually looks like a witch’s hat, see picture below). If it is not too big, mark your position in the cocked hat at the closest point to danger and have another go (in the example below, that position would be nearest to land). Cocked hats occur with errors (plotting, wrong identification of object, compass error incorrectly applied or unknown compass error), or this could occur with the imprecise reading of the compass or an unsteady hand when at sea.

2 people found this helpful. Do you?Thank Jackie & Noel Parry


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