Navigation at sea is a tricky business and one that requires constant vigilance as at any given time things can & do go wrong. At one time to do so you would send a man aloft to spot a landmark or some known point, or shoot the sun or stars to get a bearing on your position. Then came the invention of radio and everything became simpler, but to a point. D.F. bearings are used in navigation and are commonly called R.D.F fixes (radio directional findings) using radio waves in combination with a gyro compass. The pic here is an accurate depiction of how it works and why. Down the right side you will see the degrees marked off from 10 to 180 and on the left from 180 to 350. The compass rose is marked off in degrees from 0 to 360. On this particular drawing you will see that inside the right & left margins there are numbers with + & - signs to them. This represent the mean deviation of the compass bearings ergo at 70 degrees you will see the number +3 degrees telling you that there is a + factor of 3 degrees of 70 degrees true. One can adjust compass bearings in accordance to this. Compass bearings deviate about 2 degrees a year give or take, so you need to make the allowances shown on the chart. To each ship this will be different irregardless of size or shape. This particular diagram was taken from the steamer " Meldrum Bay " hull # 34 & built in 1949 and scrapped in 1986. Sailing is enjoyable and something you need to understand fully and respect greatly as if not then chances are you will find your self in peril at sea, which can never be good.....thanks for the peek