Direction and bearings

Directions fixed to the Earth

In Lesson 2 you described where things are relative to the boat: ahead, abeam, astern, fine on the port bow. That is perfect for telling the crew where to look, but it changes every time the boat turns. To draw on a chart, plan a passage or tell another vessel where you are going, you need directions that are fixed to the Earth and mean the same thing to everyone.

Navigators measure these as an angle in degrees, starting at north and counting clockwise all the way round. East is 090°, south is 180°, west is 270°, and after 359° you are back at north, which is written 000° (you will sometimes see 360°, which means the same).

A direction is an angle measured clockwise from north, from 000° to 359°.

Three-figure notation

Courses and bearings are always written with three figures, adding zeros at the front where needed: 5° is written 005°, 45° is 045°, and 270° stays 270°. There are good reasons for the habit:

• A slip stands out. Every direction has the same shape, so a missing or extra figure is obvious. A scribbled “45” could be a careless 145 or 245; 045 can’t.
• It can’t be mistaken for another number. The chart table is full of speeds, distances, depths and times. Three figures and a degree sign say “direction”.
• It is spoken clearly. Say each figure separately — “zero four five”, never “forty-five” — whether you are calling a course to the helm or speaking on the radio.

A letter after the degree sign says which north the direction is measured from. In this lesson it is always T, for true: 045°T. In Module 2 you will meet M (magnetic) and C (compass).

Points of the compass

Long before degrees, sailors named directions. The names are still used every day — for winds, in weather forecasts, and for rough directions such as “the anchorage is north of the island”.

• The four cardinal points: north (N), east (E), south (S) and west (W).
• The four intercardinal points, halfway between: north-east (NE, 045°), south-east (SE, 135°), south-west (SW, 225°) and north-west (NW, 315°).
• Halve the gaps again for the eight three-letter points, such as north-north-east (NNE, 022½°). Their first letter is the nearer cardinal point: NNE lies between N and NE, while ENE lies between E and NE.

That gives 16 points, each 22½° from the next:

Traditional compass cards go further, to 32 points 11¼° apart, but the 16 above are all you need.

True north and the compass rose

True north is the direction of the geographic North Pole — along the meridians you met in Lesson 3. On a Mercator chart every meridian is a straight line running up and down the chart, so true north is straight up, and every parallel of latitude runs exactly east–west (090° and 270°).

The compass rose printed on the chart (Lesson 4) has an outer ring marked in degrees from true north, with a star at 000°. Its inner ring is magnetic: a ship’s compass points to magnetic north, which on the Isle of Kerran practice chart is 3° west of true. Converting between the two is the first lesson of Module 2. Until then, every direction in this course is true, written °T.

Course, heading and bearing

Three words describe directions afloat. They are easy to blur, so keep them distinct from the start:

With nothing pushing the boat sideways, it goes where the bow points and your heading matches your course. Tide and wind make them differ; Modules 4 and 5 show you how to allow for that.

Reciprocal bearings

Every direction has a reciprocal: the direction exactly opposite, 180° away. If a lighthouse bears 045°T from you, then you bear 225°T from the lighthouse. Same line, opposite ends.

You need reciprocals more often than you might think:

• The bearing of you from an object, when you know the bearing of the object from you (the basis of position lines in Module 2).
• The course home: the return leg of a passage is the reciprocal of the outward course.
• Checking your measuring: a plotter reading that is the reciprocal of your estimate means something is upside down.

Measuring a direction with a plotter

A plotter — Portland and Breton are the common styles — is a transparent rectangular base with a rotating protractor, the dial, in the middle. The dial is marked 0 to 360 and carries a grid of lines parallel and square to its north–south line. Arrows on the base run parallel to its long edges, and an index mark on the base shows where to read the dial. To measure the direction of a line:

1. Estimate. Look at the line and decide roughly which way it runs — “about north-east”.
2. Lay the edge along the line, with the arrows pointing the way you are going: from the start towards the destination, or from you towards the object.
3. Turn the dial (not the base) until its grid lines are parallel to the meridians and parallels on the chart, with the dial’s N towards the top of the chart — true north.
4. Read at the index, and check the answer against your estimate.

Without a plotter: parallel rule and compass rose

A parallel rule is two straight rules joined by hinged links, so one can move while staying parallel to the other.

1. Lay one edge of the rule along your line.
2. “Walk” it across the chart — hold one half firmly, swing the other, then swap — until an edge passes through the centre of the compass rose.
3. Read where the edge cuts the outer (true) ring. It cuts the ring twice, 180° apart: take the reading on the side you are travelling towards, the one that matches your estimate.

Parallel rules can slip over long distances, which is why most small-boat navigators prefer a plotter.

Drawing a line on a given bearing

Often you know the direction and need the line — for example, to lay off a course or plot a position a given distance and direction from a buoy. Work the other way round:

1. Set the dial first: turn it until the required bearing is at the index.
2. Put the edge on your starting point, arrows pointing the way you want the line to go.
3. Turn the whole plotter (the dial stays set) about that point until the grid is square to the meridians, N to the top.
4. Draw along the edge, then measure the distance along it with dividers against the latitude scale (Lesson 5).

Measure it yourself

On screen, the chart’s dividers do the plotter’s job: drag point A to where you start (or where you are) and B to the destination (or the object), and the readout gives the true bearing from A to B.

Common errors and how to catch them

• 180° out. The dial was turned with its N to the bottom of the chart, or you measured from the destination back to the start. The grid still lines up perfectly, so only your estimate catches it.
• Grid not square to the chart. If the dial’s grid is a few degrees off the meridians, every reading is a few degrees off. Check it against a meridian and a parallel.
• The plotter slipped off the line while you turned the dial. Hold the base down firmly with one hand.
• The wrong ring. Reading the inner, magnetic ring of the compass rose puts you out by the variation — 3° on the practice chart.
• Two figures. Writing 45 instead of 045 invites someone to misread it.

Estimate, measure, check. If the plotter disagrees with your estimate, don’t use either figure until you have found out why.