Distances at sea are measured in nautical miles
- 1’ (minute) of latitude = 1 nautical mile
- 1∘ degree of Latitude = 60’ (minute) or 60 nautical miles
- Land mile = 1609m
- Nautical mile = 1852m / 1.15 land miles
Some charts (i.e. Imray) have a ‘Scale’ on the chart but if this is not available then you will need to measure the distance from the latitude scale (side) of the chart:
Why do we measure from the side (Latitude Scale) of the Chart?
Distances are always measuring from the Latitude Scale on the Chart NOT the Longitude Scale. As detailed in the image below, the distances between each latitude line are equal, whereas the distances between each Longitude line gets smaller as the lines approach the poles. Only at the Equator are the lines of latitude and longitude at the same distance.
How to measure distance?
Measure the distance with your dividers, parallel rules, a regular ruler or even a piece of paper!
Take that distance to the latitude scale on the chart to gather the distance in minutes.
Therefore in the example above:
3.2 n/m (Distance) ÷ 10knots (Speed) = 0.32 x 60 = 19.2 minutes (Time)
D ÷ S = T (in decimals of an hour) x 60 (to convert the decimals into minutes)