navigation - page 3
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 LONGITUDE
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 (photo 6.3.3) down the side of your chart: eg 2 miles equals 2 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.
Photo 6.3.3
Speed
Speed, of course, is distance covered divided by the time taken, so at sea the most common unit of speed is a nautical mile per hour, known as a knot. The name, incidentally, comes from the days when speed was measured by throwing a ‘log chip’ overboard. The log chip was a small, flat piece of wood, ballasted so that it floated upright in the water to serve as a miniature sea anchor. Attached to it was a long piece of string with knots at 100 foot intervals. The number of knots that were dragged overboard in one minute as the ship sailed away from its log chip gave its speed in nautical miles per hour.
​
NAVIGATIONAL TERMINOLOGY
Direction is so important in navigation that navigators have many different words for it, just as Eskimos have many different words for snow. Each has a precise meaning, so they are not interchangeable:
Bearing is the direction of one object from another, eg ‘the lighthouse is on a bearing of 270°’, or ‘the lighthouse bears 270°’ means ‘the lighthouse is to the west of us’.
​
Course is the direction in which the boat is being steered, and is ideally (but rarely) the same as...
Heading which is the direction the boat is pointing at any given moment. So if the navigator asks the helmsman ‘what is you heading?’ he means ‘what direction is the boat actually pointing now?’ rather than ‘in what direction is it supposed to be pointing?’
Track angle is the direction in which the boat is moving – as opposed to the direction in which it is pointing. The word ‘angle’ is often omitted. For some purposes it is useful to differentiate between the water track angle sometimes called the wake course, meaning the direction in which the boat is moving through the water, and the ground track angle – the direction in which it is moving over the seabed.The ground track angle is sometimes called the course made good (CMG) or the course over ground (CoG).
​
CHART PROJECTIONS
A chart is intended to be an accurate representation of part of the earth. Unfortunately, despite the best efforts of surveyors and cartographers, it can never be absolutely perfect, because the earth’s surface is curved while a chart is flat. There are many different ways of projecting a curved surface on to a flat one, but they all introduce distortions of one kind or another, so which projection the cartographer decides to use is determined by which distortions are acceptable and which are intolerable. In other words, it is determined by the chart’s intended purpose. On a political map of the world in a school atlas, for instance, the main requirement may be for the whole map to be at the same scale, so that all countries appear to be the right size compared to each other.
For navigation, the most important requirement is usually that direction should be undistorted, so that north appears to be in the
same direction everywhere on the chart, and that a straight line (such as a bearing or a constant course) appears to be straight when it is drawn on the chart. Although, strictly speaking, projections are defined mathematically it can be quite useful to visualize them as the picture that would be cast on a sheet of paper wrapped round a transparent globe with a light somewhere in the middle.
MERCATOR CHARTS
One of the most useful projections for navigation is the Mercator projection, which – using the globe and paper analogy – would be the result of rolling the paper into a cylinder, centred on the earth’s axis so that it touched the globe only at the equator, while the globe is lit internally by an all-round light at its centre (photo 6.3.4). The effect is to make meridians appear on the chart as vertical parallel lines, and the parallels of latitude as horizontal parallel lines.
​
From the coastal navigator’s point of view, this meets the main requirement of making a straight course appear as a straight line on the chart. Its relatively minor disadvantage is that distances are distorted. On the real world the meridians converge towards the Poles, so making them parallel on the chart involves ‘stretching’ land masses near the Poles. Having stretched east–west distances to account for the distorted meridians, north–south distances have to be stretched as well, to preserve the shape of land masses, so the parallels of latitude are not evenly spaced, but are moved further apart towards the Poles. On a chart covering an area the size of the English Channel or the North Sea this change of scale is just large enough to be apparent with normal chartwork instruments, but for most coastal navigation it can be almost entirely ignored.
​
SELECTION OF CHART SYMBOLS
Dangerous rocks (+ with dotted circle) plus a small island 10 m high. Blue area is 3 m deep. Bottom is rock (Rk).
Underwater cables. All the numbers are the depth in meters.
