The chart: position and distance

How it works

Every point on Earth is described by two numbers. Latitude (parallels) is how far you are north or south of the equator, from 0° to 90°. Longitude (meridians) is how far you are east or west of Greenwich, from 0° to 180°. On the chart, latitude is read from the scale at the sides and longitude from the scale at the top and bottom. Latitude is always written first: 50°08'.6 N 004°43'.2 W.

At sea we write degrees, minutes and tenths of a minute, never seconds. One degree has 60 minutes (not 100): 50°30' is fifty and a half degrees, but 50°30' is not 50.30°. Watch the dot: 08'.6 means 8 minutes and 6 tenths, that is 8.6 minutes. Longitude is always written with three figures (004°, not 4°) so it is not mistaken for latitude.

The useful fact on the chart is that one minute of latitude is one nautical mile (1 NM = 1852 m), so one degree of latitude is 60 miles. You ALWAYS measure the nautical mile on the latitude scale, level with the point you are interested in. Never on the longitude scale: one minute of longitude is a mile long only at the equator, and it shrinks going north. At 50° N one minute of longitude is about 0.64 miles, at 60° N only 0.5.

To measure the distance between two points: set the dividers on the two points (the actual points, not the coastline), then, without changing the spread, lay them against the latitude scale level with the middle of the leg, and count the minutes. For a long leg, step it off: set the dividers to 5 miles on the scale, walk them along the line counting the steps, then add the remainder. Example: from 50°08'.6 N to 50°20'.6 N on the same meridian the difference of latitude is 12'.0, that is 12 NM.

To plot a given position (lat/long): with the parallel rules carry the edge to the latitude on the side scale and draw a light horizontal line; then take the longitude from the top or bottom scale and draw the vertical. Where they cross is the position. To read a position, do the reverse: from the point go across to the side scale (latitude) and up or down to the top or bottom scale (longitude). Another way to give a position is by bearing and distance from a known point: '246°(T) from Skerries Point 1.6M' means you are 1.6 miles from Skerries Point, in the direction 246° true.