Definition of a bearing
The bearing is the angle between magnetic north and a characteristic point on land (a landmark). The bearing is unique and independent of the boat's course. To take a bearing, a bearing compass is usually used. It's often a small, portable compass that aligns an alidade with the landmark and reads the bearing.
How to choose your bitter ?
A landmark is a characteristic point on land that is plotted on the map. They are often lighthouses, churches, water towers, antennas... Landmarks are characteristic sites that are often distinguished from the coast by their height or color.
Wearing a bearing
Plotting a bearing on a map consists of drawing a line that passes through this bitter and that respects the angle with north.
Caution: You should not directly transfer the angle measured with the compass. You must first correct it for deviation and declination (see our topic: " Understanding and using magnetic variation ")
Know your position
To know its position - or to take stock - we cross the results of at least 3 bearings. The 3 plots will form a triangle in which we are at the time of the bearing. The size of the triangle depends on the accuracy of the bearings. As the boat moves forward, we must try to make these bearings with as few intervals as possible. The triangle drawn is called the "hat" in which the boat is at that moment.
Exercise in the Rade d'Hyères
We begin by choosing 3 landmarks in the distance that we locate then on the map. Here we have chosen :
- 1 - the Yellow Guard turret
- 2 - the Chateau Fort Sainte Agne above the village of Porquerolles
- 3 - The Cape of Medes
For each of these landmarks, bearings are taken with a compass, which is then corrected with the Deviation (here +1°).
With the help of a ruler, we trace the different bearings (see our topics on the use of Cras or protractor rulers).
Our position at the time of the survey (14:58) is located in the green hatched triangle. The 3 lines do not intersect at a single point because of the inaccuracies of the bearings and plots.
