Royal Army College

Lesson Overview

The map shows the ground and the contours show its shape, but neither holds a direction on a featureless hillside in mist. That is the compass's work. This lesson teaches the instrument and the job each part does, how it finds north, and the three norths a soldier must keep straight. It explains what a bearing is, how to take one from the map and one on the ground, and how to convert between them using the magnetic variation printed in the margin. It then covers the compass's uses: setting and following a bearing from mark to mark, checking a course with a back bearing, fixing your own position by resection, and getting round an obstacle without losing your line. Lesson 02 gave you the map's language and the three norths in the margin, Lesson 03 taught you to read the shape of the ground, and this lesson gives you the means to follow a line across country when nothing on the ground will guide you. It feeds straight into Lesson 05, where map and compass are joined into a complete route.

This is the knowledge layer. Holding a compass steady and clear of metal, marching an accurate bearing across rough country, and resecting a position under pressure are skills built on the ground with a real compass, under instruction, and certified in person. Learn here how the compass works and what a bearing is, so that when you take one in your hand you already understand what you are doing.

By the end you will be able to name the parts of a baseplate compass and say what each does, distinguish True, Grid, and Magnetic North and explain magnetic variation, take a grid bearing from the map and convert it to a magnetic bearing using the stated variation in the correct direction, set and follow a magnetic bearing on the ground from mark to mark, work out and use a back bearing, explain what resection and intersection do, box an obstacle while keeping your line, and recognise the metal and electrical hazards that pull a needle off true.

Key Terms

Baseplate compass: the protractor type of compass used for marching and for working on the map, built around a rotating housing set into a clear plastic baseplate.
Bearing: a direction expressed as an angle measured clockwise from north, from 000 to 360 degrees.
Magnetic North: the direction the compass needle points, towards the Earth's magnetic pole; it differs from place to place and drifts slowly over time.
Grid North: the direction of the vertical grid lines on the map, the north to which a grid bearing is measured.
True North: the direction of the geographic North Pole, the north of the lines of longitude.
Grid-magnetic angle (magnetic variation): the angle between Grid North and Magnetic North at a place, given in the map margin with its direction (east or west), applied when converting between a grid bearing and a magnetic bearing.
Orienting arrow (the shed): the outline arrow marked on the floor of the housing, into which the north end of the needle is settled to read or set a magnetic bearing.
Back bearing: the reverse of a bearing, 180 degrees from it, used to check a course, retrace a line, and plot a resection.
Resection: fixing your own position by taking bearings to two or three known features and plotting them on the map.

The compass and the job of each part

The compass works on one fact: a small magnetised needle, hung so it can swing freely, turns until it lies along the Earth's magnetic field and points to Magnetic North. Everything else on the instrument is built around reading and using that needle. The standard soldier's compass for this lesson is the baseplate or protractor type, the kind you both march on and lay on the map. Its parts are few. Learn each by the job it does, because every later step is just those parts used in order.

The baseplate is the flat, clear plastic body. Its long, straight side is the edge you lay along a route line on the map, and being clear, it lets you see the map through it.
The direction-of-travel arrow is printed on the baseplate, pointing away from the housing along the line of march. When you follow a bearing, this arrow points the way you walk.
The rotating housing, or bezel, is the round dial set into the baseplate, marked round its rim from 000 to 360 degrees. You turn it to set or read a bearing, and the figure that comes to the index is the bearing.
The index line, sometimes called the read-here mark, is the fixed mark on the baseplate at the front of the housing where the dial figure is read. The bearing is always the number against the index.
The magnetic needle floats in fluid inside the housing, one end coloured (usually red) to show north. It points to Magnetic North and nothing else.
The orienting lines are the parallel lines on the floor of the housing. You turn the housing until these lie along the north-south grid lines on the map; that is how the dial is squared to Grid North.
The orienting arrow, the outline arrow on the housing floor between the orienting lines, is what soldiers call the shed. Settle the north end of the needle inside this outline, "putting the needle in the shed", and the housing is aligned to Magnetic North so the dial reads a magnetic bearing.
The romer scales are the small graduated scales on the edge or corner of the baseplate, used to read off the tenths of a six- or eight-figure grid reference, as Lesson 02 taught. A good baseplate also carries a millimetre and a ground-distance ruler along the edge for measuring on the map, and many carry a small magnifier over the romer.

Hold those nine things in mind and the compass has no mysteries. The baseplate and direction-of-travel arrow point you and lie on the map; the housing, index, and orienting lines do the map work; the needle and the shed do the ground work; the romer reads the grid.

The needle obeys magnetism and nothing else, which means it obeys nearby metal or electrical fields as readily as the Earth's. Held close to a rifle, a vehicle, a radio, or even a steel watch or belt buckle, the needle is pulled off true by several degrees and the reading is quietly wrong, with no sign on the dial that anything is amiss. So the first habit, drilled until it is automatic, is to step well clear of all metal and electronics, hold the compass level so the needle swings free, steady it at chest height, and read it squarely from directly above. The clearances are given at the end of this lesson, and they are larger than most soldiers expect.

   THE BASEPLATE (PROTRACTOR) COMPASS: the parts

   baseplate ............. the clear ruler; carries the romer scales
   direction-of-travel ... the arrow you point the way you walk
   housing / bezel ....... the rotating ring marked 0 to 360 degrees
   index line ............ where you read the bearing off the housing
   magnetic needle ....... swings to magnetic north (red end north)
   orienting lines ....... lines in the housing; align them to grid north
   orienting arrow ....... the outline ("the shed") you settle the needle into

The three norths, and why you walk on magnetic

A soldier must hold three norths clearly in mind, because a bearing means nothing until you know which north it is measured from, and confusing them puts you on the wrong line. Lesson 02 introduced them from the map margin; here is the working recap.

True North is the direction of the geographic North Pole, the north of the lines of longitude. It is the truest north but not the one a soldier works with directly. Grid North is the direction of the vertical grid lines on the map. Because the flat grid cannot perfectly match the curved Earth, Grid North differs very slightly from True North, but for the soldier's purposes that difference is small, and Grid North is the north of all map work. Magnetic North is where the needle actually points, towards the Earth's magnetic pole, which is not the geographic pole and lies in a different direction again.

Two of these matter in practice: Grid North, which you read on the map, and Magnetic North, which you read on the compass. You walk on a magnetic bearing because the needle is a magnet and can only show Magnetic North; it has no way of knowing where the grid lines on your paper run. The map knows nothing of magnetism: every bearing you measure on it runs from Grid North. The map speaks grid and the compass speaks magnetic, the two do not agree, and the angle between them is what you must account for every time you move between map and compass.

The grid-magnetic angle: converting between grid and magnetic

The angle between Grid North and Magnetic North at a place is the grid-magnetic angle, also called the magnetic variation, and it bridges the map and the compass. A bearing measured on the map is a grid bearing, measured from Grid North. A bearing followed on the compass is a magnetic bearing, measured from Magnetic North. To turn one into the other you apply the variation, and you must apply it in the right direction or you will be confidently wrong.

Three facts govern the work. First, the variation differs from place to place: it is not one fixed number but a local value, larger in some regions than others, and lying east of Grid North in some places and west of it in others. Second, it changes slowly over time as the magnetic pole drifts, so even for one place the figure is not permanent. For both reasons the rule is the same: take the current local variation, and its direction, from the margin of the map you are using, where it is printed beside the north points and dated, and never carry a remembered figure from another place or year. Third, and this catches the careless, whether you add or subtract depends on which way the variation lies, east or west, and on which way you are converting.

Here is the rule, complete and correct, for both signs of variation. Learn it as two short lines and know which line applies by reading the margin first.

When the variation is WESTERLY (magnetic north lies west of grid north):

Grid to magnetic: ADD the variation. (Grid bearing + variation = magnetic bearing.)
Magnetic to grid: SUBTRACT the variation. (Magnetic bearing minus variation = grid bearing.)

The memory line for the westerly case is the one most soldiers first meet: "grid to mag, add; mag to grid, subtract." Worked example, variation 8 degrees west: a route measured on the map as a grid bearing of 049 becomes, to follow on the compass, 049 + 8 = 057 magnetic. A feature shot on the ground at 065 magnetic becomes, to plot on the map, 065 minus 8 = 057 grid.

When the variation is EASTERLY (magnetic north lies east of grid north):

Grid to magnetic: SUBTRACT the variation. (Grid bearing minus variation = magnetic bearing.)
Magnetic to grid: ADD the variation. (Magnetic bearing + variation = grid bearing.)

This is the exact reverse, because magnetic north now sits to the other side of grid north. Worked example, variation 12 degrees east: a grid bearing of 049 to follow on the compass becomes 049 minus 12 = 037 magnetic. A feature shot at 065 magnetic to plot on the map becomes 065 + 12 = 077 grid.

Now you see why a single fixed rhyme is dangerous. "Always add going to magnetic" is true only where the variation is westerly and will send you several degrees wrong wherever it is easterly, and the other way round for the easterly rhyme. The safe drill is therefore: read the margin, note the figure and whether it says east or west, then apply the matching line above. Two further safeguards make it foolproof. Keep a rough picture in your head of where magnetic north sits relative to grid north for your sheet, so a wrong conversion looks obviously wrong before you walk it. And if a sum runs below 000 or above 360, add or subtract 360 to bring it back into range, since 360 and 000 are the same direction. The amount of variation may be small, but over a long leg even a few degrees open into hundreds of metres off course, so the conversion is never skipped and never guessed.

What a bearing is, and taking one from the map

A bearing is a direction written as an angle, measured clockwise from north, running from 000 degrees round to 360. North is 000, east is 090, south is 180, west is 270. Any direction can be named by its bearing, and that lets a soldier record a course, pass it over the radio, and follow it precisely on the ground.

To take a grid bearing from the map with a baseplate compass, work through these steps in order. Practise them until they need no thought.

Find your start and your finish on the map and lay the long straight edge of the baseplate so it joins them, with the direction-of-travel arrow pointing the way you mean to go, from start towards finish. If the two points are further apart than the edge is long, line the edge up on the route and slide it along, keeping the direction true. This is the commonest blunder to guard against: the edge laid the wrong way round gives a bearing 180 degrees out, and you walk away from your objective.
Hold the baseplate still and turn the housing until its orienting lines lie parallel with the north-south grid lines on the map, and the orienting arrow points to grid north, the top of the map. The orienting lines must run the same way as the grid lines, not across them.
Read the figure at the index line. That number is the grid bearing of your route. At this stage the needle is ignored completely; taking a grid bearing off the map is pure protractor work and the magnetism of the needle plays no part.
Convert the grid bearing to a magnetic bearing by applying the variation from the margin in the correct direction, as taught above, before you try to follow it. The figure you march on is the magnetic one.

That sequence, edge along the route the right way, housing turned to the grid, read the grid bearing, convert to magnetic, is the heart of map-to-ground navigation, and you will use it on every leg of every route.

Setting and following a bearing on the ground

Once you have a magnetic bearing to follow, you set it and walk it, but never by strolling along with your eyes on the dial, which only wanders you off line. You follow a bearing from one fixed mark to the next. Here is the drill.

Set the bearing. Turn the housing until your magnetic bearing stands against the index line. Leave it there; the housing now holds your line.
Orient the compass to yourself. Hold the compass level in front of you at about chest height, the direction-of-travel arrow pointing away from your body, and turn your whole body, compass and all, until the north end of the needle settles inside the orienting arrow, the shed. Needle in the shed means you are facing along your bearing.
Look up along the direction-of-travel arrow and pick out a distinct feature standing on that exact line ahead: a lone tree, a boulder, a fence post, a particular bend of ground, a building corner. This is your steering mark. Choose one as far along the line as you can still see clearly, and one you will not lose against its background.
Walk to that mark without watching the compass. With the mark chosen, the compass has done its job for this stretch; staring at the dial as you walk is how soldiers drift. Just go to the mark.
On reaching it, repeat. Take up the compass again, settle the needle in the shed to confirm you are still on the bearing, pick the next steering mark further along the same line, and walk on. Leg by leg, mark by mark, the course stays straight even across ground that gives no other guide.

In mist, in darkness, or on bare ground where no distant feature can be seen, you cannot pick a far steering mark, so keep the section closed up and move on the bearing held carefully in the hand, sending one soldier a short way ahead to stand on the line as a near mark and going to them, then repeating. You also count your paces, taught in Lesson 05, so that you know distance as well as direction. A bearing tells you which way, the pace count tells you how far, and you need both to know where you are.

The back bearing

The back bearing is the reverse of a bearing, exactly 180 degrees from it, and it is the navigator's cheap and constant check. To work one out, use the same arithmetic every time:

If your bearing is 180 degrees or less, add 180.
If your bearing is more than 180 degrees, subtract 180.

So a course out of 042 has a back bearing of 042 + 180 = 222; a course of 290 has a back bearing of 290 minus 180 = 110. (A bearing of exactly 180 may be called 000 or 360.) Take this arithmetic carefully, because a slip of the 180 sends you in precisely the wrong direction.

The back bearing has three uses. First, to check your line: having walked a leg, turn and sight back along the back bearing to the point you started from. If it lines up you have held your course; if the back bearing has crept off the reciprocal, you have drifted, and by how much, and can correct before the error grows. Second, to retrace a line exactly, walking out the reverse of the bearing that brought you in, which is how a patrol returns by the route it came. Third, and most important here, the back bearing is the tool that makes resection work, as the next section shows. Used at the natural halts along a leg, it is a small habit that guards against the slow drift that wrecks a long march.

Resection and intersection

These two methods answer the opposite questions a navigator asks of known features: "where am I?" and "where is that?" Both are named and explained here, and both are built and certified on the ground.

Resection fixes your own position when you are unsure where on the map you stand but can see and identify two or three features that are also on the map, such as a hilltop, a mast, a church, or a road junction. The drill is:

Set the map to the ground with the compass, so map and country agree.
Choose two well-separated known features, three for a better fix, and find each on the map.
Take the magnetic bearing to the first feature with the compass.
Convert it to a grid bearing using the margin variation.
Turn that grid bearing into its back bearing, and draw that back bearing as a line on the map, running from the known feature back towards you.
Repeat for the second feature, and the third if you have one.
Where the lines cross is your position. Read off its grid reference.

Two lines give a fix; three are better and should meet in a small triangle. The smaller the triangle the better the fix; a large one warns that a feature was misidentified or the variation was applied wrongly. You draw the back bearing because you measured the angle from yourself to the feature, but the line must be drawn from the feature, the known end, back to the unknown point, you. If you stand on a road, stream, or other line on the map, a single feature is enough: take and convert one bearing, draw its back bearing from the feature, and your position is where that line cuts the road or stream you are standing on.

Intersection is the mirror image: it fixes the position of a distant feature you cannot reach, such as a fire seen across a valley or a casualty spotted on a far slope, from two known points where you do stand. From the first known point, take the magnetic bearing to the distant feature, convert it to grid, and draw that bearing forward across the map from your known point towards the feature. Move to a second known point well to one side and do the same. Where the two forward lines cross is the feature's position, which you can then report by grid reference. Resection draws the lines back from the features to you; intersection draws them forward from you to the feature. Keep the difference clear and you will never plot the wrong one.

Boxing an obstacle while keeping your line

Sometimes a marsh, a steep gully, a fenced enclosure, or a stretch of deep water sits squarely across your bearing, too big to push through. You get round it without losing your line by boxing it: making a detour of right-angle legs whose sideways steps cancel out, so you return to your original line beyond the obstacle, on bearing and with your distance still known. The method depends on equal turns and counted paces.

At the edge of the obstacle, turn 90 degrees to whichever side is clear, and set that new bearing. (If your line was 040, a right turn gives 130; a left turn gives 310. Ninety degrees is a quarter of the dial.) Count your paces along this leg until you are clear of the side of the obstacle, and remember that pace count.
Turn back onto your original bearing (040 in the example) and walk past the length of the obstacle, again counting these paces and adding them to your running total for the leg, since this part is real progress along your route.
Turn 90 degrees the other way, back towards your original line (from 040, this is now 310 if your first turn was to 130, or 130 if your first turn was to 310), and walk exactly the same number of paces you counted in step 1. Walking the same distance back puts you squarely on your original line again.
Turn back onto your original bearing and carry on. Add only the step-2 paces to your leg distance; the two sideways legs were equal and opposite and so cancel, leaving your along-route distance correct.

By night, or against an unexpected small obstacle, the turns can be made without resetting the housing at all on a needle compass by using the cardinal marks, but the principle is identical: equal sideways legs, the same paces out and back, the obstacle's length added to your count. Box an obstacle correctly and you emerge beyond it still on your bearing and still knowing how far you have come, which is the whole point.

The magnetic hazards that deflect a needle

A bearing is only as good as the needle that gives it, and the needle answers to any magnet or electrical field near it, not only the Earth's. A compass read too close to metal or to a current is wrong by several degrees with nothing on the dial to warn you. The clearances below are the minimum separation to keep between the compass and common sources, and they are larger than instinct suggests:

   Source of interference                         Stand at least
   High-tension power lines                       55 metres clear
   A vehicle, field gun, or heavy plant           18 metres clear
   Telephone or telegraph wires, barbed wire      10 metres clear
   Smaller weapons and ironwork                    2 metres clear
   A steel helmet, a rifle, a belt buckle, a watch half a metre clear

The drill that follows is simple and must be automatic: before taking any bearing, step away from the section, the vehicles, and the wires; hold the compass clear of your own helmet and weapon; lay it flat so the needle swings free; and read it from directly above. A bearing taken carelessly beside a vehicle or under a power line can be ten or fifteen degrees in error, which over a single leg is the difference between arriving and being lost. Non-magnetic metals such as aluminium and most alloys do not affect the needle, but you will rarely have time to judge which is which in the field, so treat all metal and electrics as suspect and stand clear.

In Practice: The Bearing That Held in the Mist

A section on high open ground is moving to a rendezvous when low cloud rolls in within minutes and closes visibility to fifty metres. The path they were following fades into featureless ground, and every direction looks the same. This is the moment the compass exists for. The section commander does not guess and does not push on hopefully. With the map set to the ground, the commander lays the compass edge along the line to the rendezvous, with the direction-of-travel arrow pointing the way of travel, turns the housing until its orienting lines run with the grid, and reads the grid bearing at the index. Then, reading the variation and its direction from the margin, the commander converts it to a magnetic bearing the right way, because applying the variation backwards here would put the section out by twice its value and lose the rendezvous entirely.

With the magnetic bearing set on the dial, the section moves on it: the compass held level and well clear of rifles and radio, the commander turning until the needle sits in the shed, then sending a soldier forward a short way to stand on the line as a near steering mark in the murk, and the section closing up so no one is lost. At each halt a soldier works out the back bearing and shoots it to the last mark to confirm the line has held, and the paces are counted so distance is known as well as direction. Where the route crosses a fenced enclosure too large to climb, the commander boxes it, ninety degrees off, the obstacle's length walked on bearing, the same paces back the other way, and on again, the count kept true throughout. The rendezvous is made, in cloud, on bare ground, because the section trusted a correctly converted bearing and sound handling over the urge to wander towards what looked like the way. A section without that skill would have applied the variation the wrong way or drifted off line, missed the rendezvous, and become the next thing to be searched for. The compass held the direction the ground would not give.

Check Your Understanding

Name the parts of a baseplate compass and say what each does. In one sentence, explain how the instrument finds north, and give two of the magnetic hazards and the clearance you must keep from each.
The margin of your map states the variation as a westerly angle. You measure a grid bearing of 050 on the map. What do you do to it to get the magnetic bearing you will march on, and why? How would your answer change if the margin said the variation was easterly, and why is a single fixed "always add" rule dangerous?
Lay out the steps for setting and following a magnetic bearing on the ground, including the steering mark. Then explain what a back bearing is, how you work it out from a bearing of 200 degrees, and how the back bearing is used in resection.

Reflection (write a short paragraph): The vignette turns on a section that converted a bearing in the correct direction and followed it in cloud rather than wandering towards what looked like the way. Think about an RKA task on open or coastal ground where visibility could fail suddenly, and describe how sound handling of the compass, the right conversion of the variation, and a steady steering-mark drill would decide whether the section arrived or was lost. Given that whether you add or subtract the variation depends on the east-or-west figure read from the margin, what does that tell you about why this knowledge must be drilled on the ground until it is exact and automatic?

Summary

The compass works because a free-swinging magnetised needle settles on Magnetic North. On a baseplate compass, learn each part by its job: the baseplate and direction-of-travel arrow point you and lie on the map; the rotating housing, index line, and orienting lines do the map work; the needle and the orienting arrow, the shed, do the ground work; the romer reads the grid. Hold it level and clear of metal, or the reading is quietly wrong.
Keep three norths straight: True North (the geographic pole), Grid North (the map's grid lines, the north of all map work), and Magnetic North (where the needle points). You walk on a magnetic bearing because the needle can only show Magnetic North, while the map only ever measures from Grid North.
The grid-magnetic angle, the variation, bridges the two. Read its size and direction from the margin. If westerly: grid to magnetic add, magnetic to grid subtract. If easterly: grid to magnetic subtract, magnetic to grid add. Never trust a single fixed rhyme, and bring any answer back into the range 000 to 360.
A bearing is an angle measured clockwise from north. Take a grid bearing from the map with the edge along the route, the housing turned to the grid, read at the index, then convert to magnetic. Set and follow it on the ground by putting the needle in the shed, sighting a steering mark, walking to it, and repeating. Check with the back bearing, 180 degrees away (add 180 up to 180, subtract 180 above it).
Resection fixes your own position from back bearings to two or three known features, the lines crossing in a small triangle; intersection fixes a distant feature from forward bearings at two known points. Box an obstacle with equal, opposite right-angle legs and counted paces to stay on line and keep your distance. All these are knowledge foundations, mastered on the ground with a real compass and certified in person, ready for Lesson 05, where map and compass are joined into a route.

The Compass and Bearings