Distances at sea are measured in nautical miles. Each mile is considered to be 1852 metres long – about 15% longer than the English statute mile. But a nautical mile started life as an angular measurement in exactly the same way as latitude was calculated – in fact, one nautical mile is equal to one minute of latitude and a ‘knot’ is a speed of 1 nautical mile per hour. The practical significance of this is that degrees and minutes of latitude are shown on the side margins of all charts, and therefore distances can be measured by comparing them with the latitude scale – but never the horizontal longitude scale which we know to be distorted. Incidentally, you will sometimes hear people refer to ‘knots per hour’ – clearly nonsensical since it would mean nautical miles per hour per hour!
9
1 minute of latitude equals 1 nautical mile
This position is 46° 58’ N 4° 03’W
Never attempt to measure distances from the top or bottom scales of a chart
