Introduction to Contours: Contours 3.1:

Chris Cowdrey
Jun 11, 2020
11 min read

Updated: May 24

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Welcome to series three in my online navigation course. In this series we will focus on contours, what they are and how we can use them in navigation.

Contours are extremely helpful when navigating as they help us visualise the shape of the ground around us. We will begin by learning about contours and what they represent and then later, look at large and small contour features and how we use them to navigate.

What are contours?

Contour lines indicate height above sea level and link areas of equal height. Imagine draining the bath water after a long soak. The scum line that forms around the bath as the water drains is similar to what contour lines are: a line that links equal height.

We can categorise contour lines into three types:

Normal (thin) contour lines
Index (thicker) contour lines
Auxiliary/Form (broken) contour lines

Normal contour lines are the most common line you will see on the map. They are usually thin and coloured brown (On Harvey Maps, grey contour lines are used to indicate rocky ground).

Index contour lines are thicker lines and spaced as every fifth contour line. This makes it easier for you, the map reader, to not only visualise the shape of the land but to also count contours to determine height gained.

Auxiliary or Form contour lines are lines that sit in between the standard contour interval. These lines are generally broken lines and show important features that sit in between the standard contour interval. These help to determine the shape of the land that would over wise be missed on the map.

With practice and experience, you can start to visualise the shape of the ground around you. This is a very useful skill to learn for when you are in complex terrain in poor visibility and there are little to no features to help gather clues about your location.

When trying to visualise the ground, knowing some of the key shapes and details that create major landforms and slope types is very beneficial.

Contour basics

The shape of the ground in the hills and mountains of the UK is very complex. If you were to show every lump and bump in the topography of the land, our maps will be littered with lines and symbols and unreadable. With this in mind, maps in the UK show contours in either 5m, 10m or 15m intervals:

OS Maps (1:25,000 & 1:50,000 scale):

5m intervals in lowland terrain (1:25,000), 10m intervals in hilly and mountainous terrain

Harvey Maps:

15m contour intervals on most of their maps (1:25,000 & 1:40,000)

Interestingly, the Dartmoor British Mountain Map (1:40,000) has 10m contour intervals.

The spacing of contours tells us how steep the ground is. The closer the contour lines are to each other, the steeper the ground, the further apart they are, the flatter the ground. This is important information to know when determining the shape of the ground and for future planning as it is possible to determine slope angle using contours alone (this is important for winter navigation but we will not cover this in this article).

The image above shows an example of slope steepness:

This line shows a shallow slope. Notice how the contours are spaced far apart
This line shows a very steep slope as the contour lines are so close together it is difficult to distinguish individual lines.

In very steep areas, on OS 1:50,000 scale maps, some contour lines are completely omitted to show the ground is extremely steep. The thicker Index contour lines will still be shown but instead, they may be very close together. Take a look at the examples below.

Notice how in both examples the thinner contour lines in places just stop. Usually there will be four contour lines in between two thicker index contours. In places in the above examples, there is only a single contour line between two index contours. And there are even places where two index contours meet, indicating extremely steep ground! Can you find them?

Types of slope

Knowing how to determine how steep or shallow a slope is by how close together the contour lines are, we can start to understand the different types of slopes. These slope types are:

Uniform
Concave
Convex
Composite

A Uniform slope is a slope that rises or descends at an equal angle, neither getting steeper or shallower. Imagine it as a perfect straight line. Contours will be spaced equally on this type of slope

A Concave slope starts off shallow at the base and gradually gets steeper towards the top. The contour lines will be spaced far apart at the base and gradually get closer and closer together as you climb higher. From the top of a concave slope, you will be able to view the entire slope all the way to the base.

A Convex slope starts off steep at the base and gradually gets shallower as you approach the top. The contour lines at the base of the slope will be close together and gradually get spaced further and further apart as you approach the top. From the top of a convex slope, you will not be able to see down to the base.

A Composite slope is made up of all of the above: Uniform, Concave and Convex. From the base, the contour lines may be spaced far apart before gradually getting steeper (closer together) and as you approach the top they may start to spread further apart again.

Major Landforms

Beyond the types of slopes, contours also create shapes that we can recognise as specific landforms. The main major landforms to learn are:

Ridges
Valleys
Saddles
Knolls
Spurs
Re-entrants

You will need to learn how these landforms look on the ground, as well as how they look on a map to understand how to use them to your advantage when navigating.

relating the map to the ground contour lines

A Ridge is characterised by sharp pointed contours with steep slopes either side. The 'points' of the contours will point downhill. Spurs are similar to ridges, however the contour lines will not be as 'sharp'. A Spur can be a broad slope with contour lines forming a 'U' shape, or a tiny feature with perhaps only one or two contour lines displaying this shape.

A Valley and a re-entrant are very similar to a ridge and a spur in terms of the contour shape. The way to tell the difference is by looking at the contour heights and that the 'points' of the contours (V of the contour line) point uphill. The top of the number on the contour line points uphill also.

Other ways to tell the difference between a ridge/spur and a valley/re-entrant is the presence of a water course, for example a stream or river.

Another way to visualise spurs/ridges and re-entrants/valleys is by looking at your hand. Lay your hand flat on a surface and spread your fingers. Your fingers represent spurs/ridges and the the bits in between your fingers are valleys/re-entrants. Now imagine you poured water over the top of your hand. Where will the water run?

A saddle, or sometimes referred to as a Col, is a dip between two tops. Clench your fists and take a look at your knuckles from the back of your hand. Imagine the tip of two of your knuckles are the summits of a hill or mountain. The bit in between these two knuckles is what is referred to as a saddle or a col. The below two images show a saddle on the ground and how it appears on a map:

Great Staple Tor on the left, Roos Tor on the right. The Saddle/Col is the dip in between the Tors.

A knoll is a hump or bump, shown on a map as a circular contour known as a ring contour. They are usually found on or near the summits of hills and mountains or along ridges and spurs.

The best way to learn these major landforms is to get out into the hills and mountains and practice relating the map to the ground. Take advantage of this article and the question paper attached to gain a basic understanding of contour shapes.

Other features to be aware of are the following:

Bowl/Cwm/Corrie
Shoulder/Plateau

These may take a similar form in terms of contour shape to other major landforms, however they will differ slightly in the sense that they are much larger features. A bowl (Cwm in Wales, Corrie in Scotland) is a large amphitheater type feature.

A shoulder is another term used for a large, broad spur. Sometimes, a flattening on a spur or a ridge is referred to as a shoulder too.

Take a look at the image below for examples:

A: A prominent ridge. Notice how it also forms a saddle/col between the two peaks.

B: A large spur or shoulder

C: A re-entrant. Notice the presence of a water course and the contours pointing uphill.

D: A bowl/corrie.

E: A flattening on a spur, also referred to as a shoulder.

F: A shoulder on a spur but notice the form line indicating a knoll too.

By knowing these contour features and how they appear on the ground, we can use these major landforms to help us orientate the map. See the below section for examples.

How to interpret contour

In one of the first parts to my online navigation course, we looked at orientating the map using linear and area features. Gaining an understanding of what contours are and how they look on the ground, we can use major landforms to orientate our maps to the ground. Ridges can be viewed as linear features, valleys and shoulders are area features. This is why knowing what these features look like in their contour form on a map is important.

By using major landforms to orientate a map, we can gain a better understanding of how the ground looks around us compared to the map. We can visually see the shape of the ground, the different types of slopes and other contour features, either big or small.

Take a look at the image above. The red lines converge to my location and show the area shown in the image. The black lines are linking their map representations to the real life feature on the ground. Spend some time looking at the image and try to get an idea of how contours represent the ground. Another image is below for you to get an idea of contour interpretation:

To help gain a better understanding of contours, we can use our compass to interpret the ground on our bearing. When taking a bearing from your location to your next checkpoint, take a look at the contours along the line of you compass base plate. Take a look at the contour spacing along this line and the shape of the ground. You can use the changes in slope angle and direction as tick off features whilst walking this leg. Also take note of what the contours do either side of this line too and what features they form.

Take a look at the example below for a visual representation. The 'X' represents change in slope angle. You can use information like this as a tick off feature.

Use the magnifying glass on your compass to look at smaller contour details that may be hidden behind other symbols on the map. The example below shows how using the magnifying glass can show a greater level detail that otherwise may be missed at first glance. Notice how the path (circled) goes up a re-entrant between two spurs. Knowing this information can help you locate the start of Y Gribin ridge in poor visibility as you know you will be looking for this small contour feature.

We can also gain an understanding about contour features by measuring them on a map and seeing how they compare on the ground. In a previous article, we looked at judging and measuring distances. By measuring contour features on a map, we can then use this information whilst on our walk to help us navigate. For example, if we are walking up the spur in the image below, we can measure the shoulder to determine how big that feature is. In this case, it measures 10mm. This is a 1:25,000 scale map and 10mm equals 250m on the ground. With this information, we know that we are going to come across a large flattening on the spur. This information is vital in poor visibility. If for what ever reason we drift off the crest of the spur and come across a flattening on the ground, by knowing that the shoulder is 250m long and roughly the same in width at it's widest, we can use this information to look out for this feature and compare the distance to any other flattening we may come across.

The trick with contour interpretation is to be able to read 'between the lines'. Contour lines only show ground features at that height. Therefore, any features between two contour intervals that don't rise above 10m may not be included on a 10m contour interval. For example, you may be on a flat plateau, according to the map, in the middle of Dartmoor, yet you may come across an obvious hump or mound. To you it looks like a small incline, making you go up and over it. But it is not marked on the map. This may be due to this 'feature' not being tall enough to break into the next contour interval. See example below:

In the image above, both the examples represent what the ground could look like. Both are correct interpretations. Usually, a map will show some sort of indication as to what the ground may look like between contour lines.

Both Harvey Maps and OS maps have subtle clues that we can use to help us picture the shape of the ground in otherwise featureless terrain. On Harvey Maps, form lines or auxiliary contour lines are used to represent contour features that do not fall within the standard contour interval. OS maps sometimes use Spot Heights to help you work out the shape of the ground when there is little contour information.

Notice how the Harvey Map (left) shows form lines to represent the ground sloping which would otherwise be missed. Where as OS Maps (right) show a spot height to indicate the low point in the featureless area.

(Left: Harvey Map | Right: OS Map)

Even with the difference in contour intervals between these two maps (Harvey Map =15m / OS Map = 10m) you can see how the contour information is shown to include the extra height of the summit of Drosgl. Harvey introduce a form line and a spot height and OS use just a spot height. Without these bits of information, just looking at the contour lines one would assume the summit is flat, however it does dome gradually.

Using Contours

Being able to interpret and use contours whilst navigating is what makes a good navigator a great navigator! Reading the lines and feeling what's underfoot can make all the difference in relocating yourself in featureless terrain. But we can also use contour features as tick off and catching features to help us keep track of where we are on our route. See the example below:

The above is a leg of a route. We start at the Triangle and follow the track to the summit of Kitty Tor (Circled). In poor visibility, there are very few tick off features for us to be able to confirm our position along the track. In this case, we can use the shape of the ground to help us.

The track gradually ascends diagonally up the slope, crossing a re-entrant that we can use as a tick off feature.
This section of the track levels out and contours around the spur for 200m. We can feel this underfoot as the track flattens and use this as another tick off feature.
Up until now, we have had relatively steep slopes to our left. After the flat section of path in No.2, the slope to our left now reduces in steepness.
The track now levels out as we approach a saddle. We can expect to walk past a knoll to our left before the ground then gradually starts to descend into a large re-entrant.
The final slope to the summit of Kitty Tor is a uniform slope. The footpath here also heads up the slope at right angles to the contour lines. It is possible to take an aspect of slope here to confirm your location.
If in poor visibility we miss the summit of Kitty Tor, we can use the contour lines on the North East facing slope of Kitty Tor as a catchment feature. This is a convex slope and we can feel underfoot that we are descending.

If you can interpret contours in the above way on your walks in the hills and mountains, you will be able to utilise this information to help you relocate if you get lost or to ensure you can keep track of your location on a walk.

This brings us to a close of my CONTOUR series.

Below is the question sheet to this article.

Question Sheet: