Royal Army College

Lesson Overview

Lesson 02 taught the flat language of the map: margin, scale, signs, and grid. But the ground is not flat, and the skill that separates a map-reader from a navigator is seeing the shape of the country in the lines on the paper. This lesson teaches relief, the rise and fall of the ground, and how the map shows it through contour lines: what a contour is, how to read a height from it, how the spacing tells you whether a slope is gentle or steep, and how the patterns reveal real landforms, the hill, ridge, spur, re-entrant, saddle, escarpment, plateau, and depression. It then teaches what all of this is for: visualising the real ground from the map, and working out what can be seen from where and where the hidden ground lies.

This is the knowledge layer. The eye that reads a slope at a glance and the judgement that picks a covered route are built by walking real ground with a real map under instruction, and certified in person. Learn here how contours work, so that when you stand on a hillside with the sheet open you already understand what the brown lines are telling you.

By the end you will be able to explain what a contour line is and what the vertical interval means, read a height up or down from a numbered contour, read steepness from contour spacing, identify the main landforms from their contour patterns, and work out from the contours whether one point can be seen from another and where the dead ground lies, including what convex and concave slopes hide.

Key Terms

Relief: the shape and height of the ground, its hills, valleys, slopes, and flats.
Contour line: a line on the map joining points of equal height above sea level.
Vertical interval: the difference in height between one contour and the next, stated in the margin. Often shortened to VI.
Index contour: a heavier, numbered contour drawn at every fifth line, from which the unlabelled contours are read.
Intermediate contour: the finer, unnumbered contours between the index contours, four to every gap.
Spot height and trig point: a marked point of known height; a trig point is a surveyed station, often on high ground.
Knoll: a small, distinct hill, a single low rise of high ground.
Re-entrant: a fold or shallow valley running up into higher ground, usually carrying water; the Commonwealth term for what some manuals call a draw or gully.
Saddle or col: the low neck of ground along a ridge between two high points.
Convex slope: a slope that bulges outward and rolls over a brow, gentle at the top and steepening towards the foot.
Concave slope: a slope hollowed like the inside of a bowl, steep at the top and easing towards the foot.
Intervisibility: whether two points can see one another, given the ground that lies between them.
Dead ground: ground hidden from view from a given position because higher ground or the curve of a slope lies between.

Contours: how the map shows height

A flat drawing must somehow carry the third dimension, height, and the contour line is how it is done. A contour joins all the points on the ground that stand at the same height above sea level. Imagine the sea rising in fixed steps and, at each level, drawing a line around the land exactly at the water's edge; those lines, seen from above, are the contours. Cross from one contour to the next and you have gained or lost a fixed amount of height. A contour can never split, and two contours of different heights can never cross, because no point of ground can be at two heights at once. A single contour closing on itself in a loop means a top or a hollow: higher ground inside is a hill, lower ground inside is a hollow.

That fixed amount of height between one line and the next is the vertical interval, and it is the first thing to find. It is stated in the margin, in words such as "contour interval 10 metres", and on the maps a soldier most often uses it is ten metres. Find it before you read the relief, because the same pattern of lines means a gentle swell at a five-metre interval and a serious climb at a twenty-metre one. Confirm the unit too: almost all modern military sheets are in metres, but an old or foreign map may be in feet, and a height read in the wrong unit is a height read wrong.

Not all contours are drawn the same. To save you counting from the bottom of the sheet every time, every fifth contour is printed as a heavier line and labelled with its height; this is the index contour. The finer lines between them, four to a gap, are the intermediate contours, and they carry no number. You read an unnumbered line by finding the nearest index contour, reading its printed height, and counting intermediate lines up or down from it, adding or subtracting one interval for each. Read the number the right way up: it is printed so that its base sits on the downhill side, so the top of the figures points uphill, a quick check on which way the ground falls. Some maps add a third kind, the supplementary contour, drawn as a broken line at half the interval, used only on very flat ground where the full-interval contours are too far apart to show the shape.

A worked reading makes it plain. Suppose the interval is ten metres and you want the height of a point lying between the lines. The nearest index contour below it reads 300. Count up: the point sits on the second intermediate line above the 300 line, so add two intervals, two tens, and it stands at 320 metres. Had it lain just below the 300 line, on the first intermediate line down, it would be 290. For a point not on a line but between two, take the lower contour and judge by eye: barely above it, call it the lower height; about halfway, add half the interval; nearly up to the next line, call it the upper height. Most ground work needs no more precision than that. The contour is the single most informative feature on the map: once you can read it you can read the shape of country you have never seen.

Spot heights and trig points

Contours carry most of the relief, but the map adds other marks of height to fix it precisely. Spot heights are points printed with their exact height in metres, scattered where the surveyor measured a definite figure, useful for pinning down a summit, a road junction, or a col where the contours alone leave the highest or lowest point in doubt. Trig points, short for trigonometrical stations, are surveyed stations marked with their own symbol, a small triangle with a dot, often standing on commanding high ground. A trig point gives you two things at once: a precise height, and a fine landmark, a built pillar or marker you can recognise on the ground and use to fix your position later, in the resection taught in Lesson 04. Some maps add layer tinting, colouring the ground in bands by height so high and low country read at a glance, or hill shading, a printed shadow as though the light fell across the land from one side, which makes the shape leap off the sheet. These aids never replace the contours; they confirm and sharpen the picture the contours draw.

Steepness: reading slope from spacing

The spacing of the contours tells you how steep the ground is, and this reading must become automatic. Because each gap stands for the same gain in height, the horizontal distance in which that height is gained shows the gradient. Where the contours are close together, a large height change is packed into a short distance, and the slope is steep. Where they are far apart, the same change is spread over a long distance, and the slope is gentle. Contours touching, or merging into a single thick band, mark a face too steep to read line by line, a cliff or crag. Wide-spaced contours, or none at all, mark flat or nearly flat ground.

To put a rough number on it, use the interval and the ground distance the spacing represents. If contours ten metres apart in height are also about a hundred metres apart on the ground, you climb ten metres in a hundred, a gentle rise of about one in ten that a loaded soldier walks without trouble. If those same ten-metre contours crowd to twenty metres apart on the ground, you climb ten in twenty, one in two, a hard scramble where you may need your hands. You do not work this out for every slope; you learn the feel of it, so a glance tells you "easy", "hard going", or "not with a bergan". The rule reverses, too: contours that stay the same distance apart down a whole slope mean a uniform gradient, and contours that bunch and then spread mean the slope changes its angle, which is the next thing to read.

Read this and you can judge a route before walking a step of it. A leg that crosses many contours close together will be slow, tiring, and perhaps impassable with kit; a leg that runs along a single contour, keeping to one height, is easy going, which is why a path traversing a hillside so often follows a contour rather than charging straight up or down. The pattern also tells you where time must be added for climbing, which Lesson 05 turns into timings. See steepness in the spacing and the map shows you not just the shape of the ground but the cost of crossing it.

The shape of a slope: uniform, convex, and concave

Spacing tells you how steep a slope is on average; the way the spacing changes down the slope tells you its shape, and the shape decides what the slope hides. Three shapes matter, and each reads as a clear pattern in the contours.

A uniform slope climbs at the same angle the whole way. Its contours are evenly spaced from foot to crest, like the rungs of a ladder. Every part of it is in view from the top and from the bottom alike: stand at the crest and you see the whole face down to the foot, with nothing hidden.

A concave slope is hollowed like the inside of a bowl, steep where it begins and easing as it falls away. In contours it shows lines crowded at the top and spreading wider towards the bottom. From the crest the whole face falls away in full view, because each lower stretch is gentler than the one above and nothing rises to block the eye. A concave slope hides almost nothing from above, but for the same reason offers little cover to anyone moving on it.

A convex slope is the dangerous one. It bulges outward, gentle at the top where it rolls over a brow and steepening towards the foot. In contours it shows the reverse of the concave: lines spaced wider near the top and crowding together towards the bottom. Stand at the top of a convex slope and you cannot see its foot. The ground curves away beneath you, and everything below the brow, often the lower third of the slope and whatever lies at the bottom, a stream, a track, a fold of ground, is hidden from you. This hidden band is dead ground, and reading it from the contours before you ever reach the crest is one of the most useful things a navigator does. The convex slope looks empty from the top precisely because it conceals its lower part; the contours warn you that the emptiness is a lie, and that you must go forward to the brow, or down onto the slope, to see what it holds.

Reading the landforms

Contours do more than show slope; their patterns draw the actual shapes of the land, and the navigator learns to recognise them as a reader recognises words. Learn each one as a shape on the paper and a shape on the ground at the same time, because the whole purpose is to make the two the same thing. The main landforms are these.

A hill or summit shows as roughly circular or oval contours closing in on a high point, the innermost ring the highest ground, sloping away on every side. A small, distinct hill, a single low rise, is a knoll, the same pattern in miniature, often a useful landmark and a place that offers a short field of view.

A ridge is a long band of high ground, a continuous line of crest with the ground falling away on both long sides. Its contours run as two roughly parallel lines along the crest, the U or V bends in them pointing away from the high ground, downhill. A ridge is a natural high road, often the easiest line to move along because you keep your height and cross no re-entrants, but it is also a skyline on which a figure stands out, a thing the fieldcraft of Lesson 06 teaches you to respect.

A spur is a finger of high ground running out and down from a hill or ridge, the ground falling away on three sides, the spur's own end and its two flanks, and rising only where it joins the higher ground behind. Its contours form a tongue or nose, the points of the U or V aiming away from the high ground, downhill, the same sense as a ridge because a spur is in effect a short, descending ridge. A spur gives you a covered line to climb or descend, in dead ground from the re-entrants on either side.

A re-entrant, also called a gully, is the opposite of a spur: a fold or shallow valley cutting into the high ground, with higher ground on three sides and the open, lower ground in only one direction. Its contours form a U or V with the points aiming towards the high ground, uphill, that is, upstream, and a watercourse, a stream or its dry bed, will usually sit in its floor, for water cuts the re-entrant in the first place. A larger, more open version of the same form, broad-floored and usually with a river, is a valley.

A saddle or col is the dip along a ridge between two summits, the low neck where the ground falls away on two opposite sides into the valleys below and rises on the other two towards the high points. Its contours pinch in from both sides into a rough hourglass or waist. The saddle is often the easiest place to cross from one valley to the next, because it lets you pass the ridge without climbing either summit.

A cliff or escarpment is a face so steep that the contours crowd into one another. A sheer drop shows as contours touching or running together into a single line; a steep but climbable face shows as contours merely very close. An escarpment is the long, steep edge of higher ground, gentle above and below but a sharp step between, its contours bunched along the line of the step. Some cliffs are drawn with a single carrying contour bearing small tick marks pointing to the low ground, so the drop is not missed.

A plateau is high ground that is flat or gently rolling on top, a cap of widely spaced or absent contours ringed by the close-spaced contours of its edges. A depression is a hollow in the ground, a closed loop of contours with the lower values inside, the reverse of a hill, and to make sure it is not read as a hill it usually carries small tick marks, sometimes called hachures, on the inner side of the contour, pointing down into the hollow. Wherever you see those tick marks, the ground falls in the direction they point.

   HOW THE LANDFORMS APPEAR AS CONTOURS

   HILL / KNOLL     SPUR (high ground       RE-ENTRANT (low ground
   closed rings,    running out)            running in, often a stream)
   high in the      contour "V" or "U"      contour "V" or "U"
   middle           points DOWNHILL         points UPHILL (upstream)

   SADDLE  a dip between two heights      RIDGE   a long line of high ground
   VALLEY  contours following a stream    CLIFF   contours touching or merged

   The rule of the Vs: the V points UPHILL in a re-entrant and
   DOWNHILL along a spur. Trace the blue watercourse to be sure.

Telling a spur from a re-entrant: the rule of the Vs

Of all these readings, the one most often got wrong, and the one that matters most on the ground, is telling a spur from a re-entrant. They are mirror images: a spur is high ground running down, a re-entrant is low ground running up. Confuse the two and you will plan to move along a covered finger of high ground and instead find yourself in the bottom of a wet gully, or the reverse. There is a simple, reliable rule.

Where the contours bend into a U or a V as they cross a piece of ground, look at which way the point of the V aims. On a spur, the Vs point downhill, away from the high ground. In a re-entrant, the Vs point uphill, towards the high ground, which is to say upstream. To fix it for good, remember what runs in each: water runs down a re-entrant, never along a spur, so the V in a re-entrant points the way a stream would climb if it ran backwards, up into the hill. Many a soldier reads it the short way: the V points to the high ground in a re-entrant, away from it on a spur.

The surest check of all is to trace the blue. A watercourse always sits in a re-entrant or valley and never on a spur, so follow any stream printed on the map and the ground it runs in is a re-entrant; the high fingers between the streams are the spurs. Where no stream is printed, the rule of the Vs decides it. Heights help too: read the contour values on either side, and the ground higher than its surroundings is the spur, the ground lower than its surroundings is the re-entrant. This single distinction, read correctly, is the difference between a covered, dry, easy line and a slow, exposed, or impassable one, and it is worth more deliberate practice than any other reading in this lesson.

Seeing the ground in the map

Here is the skill all of this serves. A soldier who has learned contours does not see brown lines; they see country. Build it deliberately by reading the map in layers. First trace the high ground: pick out the index contours that close along the tops and run your eye, or a pencil, along each ridge and spur so the skeleton of high ground stands out. Then trace the low ground: follow every stream and the re-entrants and valleys that feed it, so the drainage and the hollows stand out as clearly as the heights. With the high lines and the low lines both picked out, the shape of the country rises off the paper between them: this loop is a hill you would have to climb, this tongue is a spur you could move along in cover, this fold is a re-entrant with a stream in its floor, this pinch is a saddle you could cross without gaining the summit, these crowded lines are a face too steep to take with a bergan.

Then turn the skill the other way, from ground to map. Standing on real ground, set the map so its features line up with the country in front of you, then name what you see in the language of contours: that long crest is the ridge running across the top of the sheet, that nose of high ground falling towards you is the spur marked here, that notch on the skyline is the saddle, that dark fold is the re-entrant carrying the stream. Reading map to ground and ground to map until each confirms the other is how the printed relief and the real hillside become the same thing. The map becomes a model you walk in the mind's eye before you walk it on your feet, judging where the going is hard, where the cover lies, where a tired walker might shelter, where the danger is.

This is built by deliberate practice on the ground, matching map to country until the translation is automatic, and it cannot be learned from paper alone. But the understanding starts here: every contour is a real edge of real ground, and reading them is reading the land. The navigator who can do it arrives with the country already in their head and is rarely surprised by what they meet; the one who cannot is forever discovering the ground the hard way, by walking into it. It is also the foundation the Patrolling and Tactical Movement course builds on, where choosing a route by the ground, a covered spur to climb, a re-entrant to follow, a crest to stay below, is a tactical skill before it is a navigational one.

Intervisibility and dead ground

The last and most operational use of contours is working out what can be seen from where. Intervisibility is the plain question of whether two points can see one another, and contours answer it. Lay a straight edge between the two grid references and read the contour heights along the line. If any ground between them stands higher than the straight line of sight from one point to the other, the view is blocked and the points are not intervisible; a soldier on one cannot see the other, and a radio relying on line of sight may not reach. If no higher ground intervenes, the two can see each other. The quick way to check is to find the highest contour the line crosses between the two points and ask whether it rises above a line drawn from eye height at one end to the object at the other; if it does, the view is cut. Knowing this lets you choose an observation point that overlooks the ground you must watch, or a route that stays out of sight of ground you wish to avoid.

The other side of the same coin is dead ground: ground hidden from a given position because higher ground or the curve of a slope lies between the watcher and it. Every viewpoint has dead ground, the folds and reverse slopes and lower convex faces it cannot see into, and reading where that hidden ground lies is a soldier's skill with two faces. For search and rescue it warns you that a casualty may be lying exactly where you cannot see, in the dead ground below a convex slope or in the floor of a re-entrant, so that ground must be physically entered and searched, not merely scanned from a distance. For fieldcraft it offers the reverse gift: dead ground is cover from view, a route or a position the watching eye cannot reach, and Lesson 06 builds the use of it.

The shape of a slope decides what it hides, and the contours give it away before you arrive: a convex slope, its contours wider at the top and crowding to the bottom, hides its foot and is a standing warning of dead ground, while a concave slope, crowded at the top and spreading below, falls away in full view. Read these from the contours before you reach the ground and you know in advance whether a stretch will reveal what lies below or conceal it. For a searcher this is the difference between scanning empty-looking ground and knowing to walk down into the dead space where the lost may lie.

In Practice: The Slope That Hid the Walker

A section searching for an overdue walker on the moor reaches the crest of a long rise and looks down a broad, smooth slope that falls away towards a stream. From the crest the slope appears empty, and the temptation is to scan it, see nothing, and move on. But one soldier has read the contours. The lines on the map are spaced wide just below the crest and crowd together lower down: the slope is convex, it rolls over a brow, and the ground beyond that brow, the lower third of the slope and the stream in its floor, is dead ground from where the section stands. Nothing in that hidden band can be seen from the crest, however hard they look.

So the section does not move on. They descend the convex face to the brow, and as they cross it the dead ground opens below them, the lower slope and the re-entrant carrying the stream, and there, sheltering in the fold where the contours bunched, is the walker, cold but alive, exactly in the ground the crest could not see. Had the section trusted the empty look from the top, they would have passed a casualty lying a few hundred metres below them. The contours told the truth the eye could not: this slope hides its foot, so walk down and look. Reading relief is not an exercise on paper; here it is the difference between finding the lost and leaving them.

Check Your Understanding

What is a contour line, and what does the vertical interval tell you? How does the spacing of contours show whether a slope is steep or gentle?
Describe how a spur and a re-entrant each appear in contours, and give the surest way to tell them apart. Why does this distinction matter to a soldier choosing a route?
Explain intervisibility and dead ground in your own words. How do a convex and a concave slope differ in what they hide, and why does that difference matter both to a searcher and to a soldier seeking cover?

Reflection (write a short paragraph): The vignette turns on one soldier reading a convex slope and knowing that its foot was dead ground from the crest. Think about a search task on broken ground the RKA might face, and describe how the ability, or the failure, to picture the real ground from its contours would decide whether a casualty was found or passed by. What does that tell you about why "seeing the ground in the map" must be practised on real terrain until it is automatic?

Summary

Relief is the shape and height of the ground, and the map shows it through contour lines, each joining points of equal height; the vertical interval, stated in the margin, is the height between one contour and the next. Every fifth line is a heavier, numbered index contour, and you read an unnumbered line by counting intermediate contours up or down from it.
Contour spacing shows steepness: close together means steep, far apart means gentle, touching or merging means a cliff. Reading spacing lets a soldier judge the going and the cost of a route before walking it.
The way spacing changes down a slope shows its shape. A uniform slope is evenly spaced and fully in view; a concave slope is crowded at the top, spread at the bottom, and reveals its whole face; a convex slope is wide at the top and crowded at the bottom, and hides its foot in dead ground.
The contour patterns draw the landforms: hill and knoll, ridge, spur (Vs pointing downhill, away from high ground), re-entrant or valley (Vs pointing uphill, towards high ground, usually with a watercourse), saddle or col, cliff or escarpment (contours touching or very close), plateau, and depression (a closed loop with lower ground inside and tick marks pointing down). By the rule of the Vs, and by tracing any stream, tell a re-entrant from a spur.
Spot heights, trig points, and any layer tinting or hill shading confirm and sharpen the picture; the central skill is reading the high ground and the low ground in layers to visualise the real ground from its contours, working map to ground and ground to map until the two agree.
Contours reveal intervisibility and dead ground: lay a straight edge to see whether higher ground blocks a line of sight. A convex slope hides its foot; a concave slope reveals it. This matters for finding casualties in unseen ground and for using cover, and is mastered on real terrain and certified in person.

Relief and Reading the Ground