Most of us are used to miles or kilometres, but these are not the measurements used at sea or in the air. The problem is that these measurements are not related to the geometry of the Earth and so the user has to have a scale of distance to use whenever the distance must be measured.

The Nautical Mile

The nautical mile is directly related to the circumference of the Earth. One degree (60 minutes) of latitude is equal to 60 nautical miles. Thus,

• One nautical mile equals one minute of latitude.

Distance can be measured directly from the latitude graticule of the chart. The distance from the Equator to 60 degrees North is 60 X 60 = 3600 nautical miles.

If you look carefully at a chart, you will see that the latitude graticule doesn't have constant spacing. This is especially true on a Mercator projection, where the distance between each 10 degrees of latitude gets progressively greater. Where a chart covers a large portion of the Earth's surface, this is especially critical when measuring distance.

So, when measuring distance, we should use the region of the latitude graticule at almost the same latitude as the distance to be measured. If the distance between two points is longer than the 'open' distance of the dividers, measure a convenient length on the latitude graticule and then 'walk' the dividers along the line, counting each step. Measure the remaining small distance and add this to the number of steps and you have the total distance.

Under no circumstances should you use the longitude graticule to measure distance. Only at the Equator it does give approximately the distance, and as you move further away the error increases. At 30 degrees North, 1 minute of longitude equals 0.866 nautical mile and at 60 degrees North (or South) it's only half a mile. At the poles, it is of course 0 nautical mile.

When you change chart scales, check the latitude graticule carefully to ensure that you know what each coloured part

represents: 1 mile, 5 miles, 10 miles, etc. It is really easy to make a mistake, especially when one chart is lying on top of another and you can see both latitude graticules.

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