Bearings are a way of expressing the angle between two objects, and there is a specific set of rules about how bearings should be calculated and expressed. The reason for having these rules is probably because sailors needed a way to communicate their locations to each other, but that can get confusing when you’re in the middle of the ocean and every direction looks the same. One way to fix this issue was to say that all angles should be measured from the North line going clockwise.
For example, suppose we have two points, A and B, and we are asked for the bearing of B from A.
Note: the terminology “B from A” is always used, as opposed to “A to B”. This is one of those rules that we mentioned, and it just ensures everyone is speaking the same language. What it means for our calculations is that we want to find direction of B if we were looking from A, so A is where we choose to measure our angle from.
So, we have our two points and a North line coming off both. The bearing of B from A is measured from the North line going clockwise until we hit the straight line which joins points A and B. The angle is measured to be 110 degrees, and that is the bearing of B from A.
Another rule is that whenever we are expressing a bearing, it should be three digits. This is natural for 110 degrees, but if the resulting angle was 6 degrees or 52 degrees, then the bearings should be expressed like 006\degree and 052\degree respectively. For this reason, you will often hear them referred to as three-figure bearings.
In general, you will either be asked to use an existing diagram to determine the bearing between two points (using a protractor or facts given in the question) or, given the size of a bearing and the distance between two points, construct a picture like the one above.
In summary, the rules are:
1. Always measure bearings from the North line going clockwise;
2. Always express your answers as three-figure bearings;
3. The bearing of B from A refers to the angle which is measured from the point A looking at B.
Example: Two boats A and B are 5km apart, and the bearing of B from A is 256\degree. Using the scale 1cm:1km, construct a diagram showing the relative positions of points A and B.
As we’re finding the bearing of B from A, we’re going to measuring our angle at the point A, so we’ll start by drawing the point A with a North line and measuring an angle of 256\degree going clockwise from it.