Navigation

C

Compass

The magnetic compass is one of the oldest and most reliable navigation instruments still in use today. It always points north, providing navigators with a fixed reference point. Using a compass and a map or chart, a skilled, careful navigator can direct a craft from one destination to another, even in fog or at night.

The visible part of a magnetic compass includes a magnetized, round metal dial called the compass card, which is marked with a prominent N for north. Contained within a clear plastic dome filled with fluid, the compass card swings freely on a sharp point called the pivot. The magnetized compass card points north under the influence of Earth’s magnetic field (see Earth: The Core and Earth's Magnetism). Also rimming the compass card are letters for east, south, and west (called the cardinal points) and finer ticks marking each of the 360 degrees of a circle. These are encircled by as many as eight small posts, called lubber’s lines, which make the compass easier to read when a craft is underway.

Magnetic compasses do not point to true north—that is, the top of the axis around which Earth rotates, also called the North Pole. Instead, compass arrows point to magnetic north, the point where Earth’s magnetic field is the most concentrated (see Magnetic Pole). This point lies in northern Canada at approximately latitude 74° north, longitude 101° west—about 1,600 km (about 1,000 mi) from true north. The difference in direction between true north and magnetic north, called variation, differs from place to place and year to year because Earth’s magnetic field shifts over time. Most nautical charts graphically represent the directions to true and magnetic north, plus the local variation, in the form of a printed compass dial called a compass rose.

Any magnetic substance—for instance, the steel in a ship—can interfere with a magnetic compass and cause errors. Magnetic compasses must be regularly tested and compensated by experts called compass adjustors.

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Large vessels and commercial craft usually use a gyrocompass, which incorporates a gyroscope, a spherical or disk-shaped device that rotates in one direction. A gyrocompass provides a fixed reference point because it rotates continuously in one direction, regardless of the movement of the craft. Gyrocompasses are not subject to magnetic errors because they point to true north and are unaffected by magnetism.

D

Plotting Tools

Navigators use plotting tools to draw and record navigation information on paper charts. Every navigator needs a straightedge, such as a ruler, to draw a straight line between points. Dividers, two short metal legs joined at a hinge, help measure distances on the chart. A plotter enables the navigator to calculate and record compass courses on the chart. There are several types of plotters. Parallel rulers consist of two wooden or plastic straightedges linked by short, hinged arms. Parallel rulers are designed to transfer compass directions from a chart’s compass rose to other sections of the chart. Navigators move the parallel rulers—one straightedge at a time to ensure accuracy—across the chart to transfer courses and compass bearings from one place to another. Another type of plotter, called a course protractor, consists of a straightedge and an angle-measuring device called a protractor. Instead of the compass rose, course protractors use the chart’s longitude-latitude grid as a directional reference.

Today many navigators choose to work with electronic charts on computers instead of paper charts, and with electronic plotting tools. This computer software essentially mimics the function of traditional charts and plotting tools.

III

Plotting and Position Finding

Navigators use several different techniques to track their progress and locate their position. When traveling within sight of land, pilots and marine navigators rely on landmarks and aids to navigation to pinpoint their location. In the absence of visual clues, navigators use other clues, such as the depth of the water or the time it takes sounds to bounce off nearby cliffs.

A

Dead Reckoning

Dead reckoning is estimating position by calculating the distance, direction, and amount of time that a craft has traveled. Dead reckoning provides navigators with a way to quickly and efficiently calculate their craft’s position with reasonable accuracy as they travel. Boaters, pilots, and other navigators use dead reckoning position when they are offshore—that is, out of sight of land and aids to navigation—or near shore when fog or darkness obscures visual position clues. Sailors and pilots of small aircraft also use dead reckoning to estimate the time they will arrive at their destination.

A ship, airplane, or submarine following a compass course travels on an imaginary line called a track. If a navigator knows the track, the coordinates of the starting position, and the distance covered, the navigator can estimate the craft’s position. To calculate the distance-run, or the distance covered, a navigator needs to know only the speed and the amount of time traveled. For example, a boat sailing across a harbor at a speed of approximately 2 knots (2 nautical miles per hour) will have traveled approximately one nautical mile after 30 minutes. Dead reckoning does not consider wind, steering errors, or water current. If a strong wind slowed the sailboat to 1.9 knots, the estimate of position derived from dead reckoning would be slightly inaccurate, but still far better than no estimate at all.

Some simple rules of thumb help navigators figure distance-run. One is the six-minute rule. Because one-tenth of an hour equals six minutes, the distance covered in six minutes equals 0.10 times the speed. If a craft covers one kilometer in six minutes, it is going ten kilometers per hour. Another handy rule is the rule of threes. In three minutes, a vessel making three knots will travel about 300 m (about 1,000 ft); at six knots, about 600 m (about 2,000 ft); and at nine knots, about 900 m (about 3,000 ft).

B

Coastal Piloting

Coastal piloting means navigating within sight of land. In coastal piloting, navigators determine their position more accurately by taking compass bearings—angular measurements of the line of sight between the craft and nearby landmarks or aids to navigation. To take a compass bearing, the navigator looks across the compass toward a charted landmark and reads the degree mark on the dial. The navigator then plots the compass bearing on the chart, drawing a line along the compass bearing that extends from the charted object toward the vessel’s location. This line is called a line of position. The navigator knows the craft lies somewhere on the line of position because that line on the chart duplicates the line of sight between the navigator and the object.

The point where one line of position intersects the dead reckoning track is the estimated position. An estimated position is more reliable than a dead reckoning position. The most reliable position is a fix. To obtain a fix, a navigator takes two or more compass bearings, then plots the resulting lines of position on a chart. The fix lies on the point where two or more lines of position intersect.

A navigator can also calculate an estimated position or a fix using the craft’s distance from a mapped or charted object. Instead of a line of position, this method produces a circle of position around the charted object. Every point on the circle of position is the same distance from the charted object, so the navigator knows that the craft’s position lies somewhere on the circle of position. The navigator obtains an estimated position by determining where the dead reckoning track (or a compass bearing, in the case of a fix) crosses the circle of position.

The distance between a craft and a charted object is called the distance-off. It can be measured by taking two compass bearings on the same object several minutes or more apart. The navigator takes the first bearing when the object lies at an angle of 45° off the front of the craft, then takes another compass bearing when the object reaches an angle of 90° off the front of the craft. The navigator uses the compass bearings to calculate the distance traveled between the two bearings. According to the laws of geometry, the distance traveled between the two bearings equals the distance between the craft and the object.

Navigators use several shortcuts to roughly estimate distance-off. The eye blink method uses a map, the length of the navigator’s arm, and the distance between the navigator’s eyes to estimate distance-off. To try the eye blink method, stretch out your right arm and raise a finger (or hold up a pencil or stick). Then, closing your left eye, align your finger with a tree, house, or other landmark. Now open your left eye and close your right eye and note how your finger appears to have moved. Use the map to estimate the distance between the location of the landmark (where your finger was with your left eye open) and the point to where your finger moved (where your finger was with your right eye open). Then multiply that number by 10 to determine the distance from you to the landmark. If your finger seemed to move 1 km (0.6 mi), you’re 10 km (6 mi) away from the landmark. If it appears to have moved 185 m (about 600 ft), you’re 1,850 meters (about 6,000 ft) or one nautical mile away. The eye blink method works because almost everybody’s arm is ten times longer than the span between the two eyes.

In clear weather navigators can use a simple, general rule called counting trees to roughly estimate distance-off. Navigators who use the counting trees method know that if they can count individual trees on a hill, they are about one nautical mile away from the hill. If they can count windows on houses, their distance-off measures about two nautical miles. If navigators can see the line where the land and the shore meet, the distance between the shore and the craft measures about three nautical miles.

Navigators sometimes use sound to estimate distance because sound travels one nautical mile in five seconds. Old-time sailors used this simple formula when sailing near a hill or cliff. When the ship’s dog barked, they timed the echo and divided the time interval by ten (because the sound had to go out and then return). This calculation gave them a rough estimate of the distance to the hill in nautical miles. Modern navigators sometimes blow a horn or ring a bell for the same reason.