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6.3  Charts

Maps, or charts as they are called at sea, are the navigator’s prime tool. They provide muchof the information you require, serve as a work sheet for calculations, and as a temporary record of what has happened. First and foremost, however, a chart is a representation of part of the real world, so it makes sense to start by looking at the earth as a whole. Our earth is an uneven, slightly flattened ball of rock spinning through space. To simplify the job of defining directions, distances and positions on its surface, it is divided up by a grid, or graticule, of imaginary lines of latitude and longitude. It is rather like the grid on a street map, but with the important difference that the grid of latitude and longitude is not purely arbitrary.

Latitude and Longtitude

The fact that the earth is spinning gives us two natural reference points, at the ends of the axis of spin, called the North and South Poles. Exactly midway between them, and at right angles to the axis, is the equator, running round the fattest part of the earth. Latitude can be defined as angular distance from the equator measured at the centre of the earth. Of course, lots of places are exactly the same distance north of the equator. If you were to join together all the points that are, say, 50° north, the result would be a circle running round the earth, parallel to the equator; so it is called a parallel of latitude (photo 6.3.1). Parallels of latitude are equivalent to the horizontal lines in the grid on a street plan, and appear as horizontal lines on most navigational charts.
The corresponding vertical lines on the chart are called meridians. They run from Pole to Pole. The equator was a reasonably obvious baseline from which to measure latitude, but you could draw any number of meridians between the Poles, none of which has any particularly strong case for being singled out as a starting point for measurements of longitude.

Photo 6.3.1

Photo 6.3.2

For historic reasons, though, the meridian that passes through the Greenwich Observatory in London is internationally accepted as the prime meridian. So the longitude of somewhere can be defined as the angular distance between its meridian and the prime meridian, measured at the centre of the earth (photo 6.3.2).

 

Latitude and longitude are both angles, so they are normally expressed in degrees. Latitude is measured as 0° at the equator and
increases until it becomes 90° north or south at each Pole. Longitude is 0° at Greenwich increasing to 180° east and west. The earth is so big, however, that one degree measured at its centre corresponds to up to 60 miles at its surface. For this reason each degree is usually broken down into 60 minutes, while for even greater precision, each minute can be furthersubdivided – either into 60 seconds, or into decimal parts.


Nowadays decimal parts are much more common, so you are likely to find the position of Portland Bill lighthouse, for instance, given as 50° 30’.82N 2° 27’.32W. Note that, by convention, latitude is always given first followed by longitude and that their directions (north or south, and east or west) are always included. They are important, because 50° 30’N 2° 27’E is a small town in northern France about fifty miles from the sea, while 50° 30’S 2°27’W is a remote and inhospitable spot in the southern ocean, some 2000 miles from Cape Town.

Nautical Mile

The standard unit of distance used at sea is the nautical mile, now internationally defined as 1852 metres, making it about 15% longer than an English statute mile. It is not a purely arbitrary figure, but is based on another older unit of distance called the sea mile – which is the length of one minute of latitude at the surface of the earth. Unfortunately, because the earth is not a perfect sphere, the length of a sea mile varies slightly from place to place, ranging from 1843 metres at the equator to 1862 metres at the Poles. The discrepancy between these two and between the international nautical mile is so small that for most practical navigation purposes it can be ignored, and a minute of latitude taken to be a nautical mile. A minute of longitude is useless as a measure of distance because it varies from 1855 metres at the equator to zero at the Poles. So to sum up: when measuring distance in nautical miles, use the latitude scale down the side of your chart: eg 6 miles equals 6 minutes of latitude. Distances less than a mile are nowadays often given in metres or sometimes yards, but you are still quite likely to come across distances given in cables. A cable is one tenth of a nautical mile or about 200 yards.

Direction
The grid of latitude and longitude also gives us a reference for our measurement of direction – north being the direction of a meridian heading towards the North Pole, whilst south is the direction of a meridian heading towards the South Pole. East and west are at right angles to these two, with east being the direction of the earth’s rotation, and west the opposite.

Distance
The standard unit of distance used at sea is the
nautical mile, now internationally defined as
1852 metres, making it about 15% longer than
an English statute mile. It is not a purely
arbitrary figure, but is based on another older
unit of distance called the sea mile – which is
the length of one minute of latitude at the
surface of the earth. Unfortunately, because the
earth is not a perfect sphere, the length of a sea
mile varies slightly from place to place, ranging
from 1843 metres at the equator to 1862
metres at the Poles. The discrepancy between
these two and between the international
nautical mile is so small that for most practical
navigation purposes it can be ignored, and a
minute of latitude taken to be a nautical mile.
A minute of longitude is useless as a measure
of distance because it varies from 1855 metres
at the equator to zero at the Poles. So to sum
up: when measuring distance in nautical miles,
use the latitude scale down the side of your
chart: eg 6 miles equals 6 minutes of latitude.
Distances less than a mile are nowadays
often given in metres or sometimes yards, but
you are still quite likely to come across
distances given in cables. A cable is one tenth
of a nautical mile or about 200 yards.