Contours and Relief
Map Reading and Navigation
Course 1 · Chapter 3 · Contours and Relief: Seeing the Shape of the Land
A paper map is flat, but the land is not. Contour lines are how a flat map describes hills, valleys and slopes, and learning to read them is what turns a map of lines into a mental picture of the ground. This chapter explains what a contour line is, how the interval and the spacing tell you height and steepness, and how to recognise summits, valleys, ridges and saddles.
What a Contour Line Is
A contour line joins all the points that are at the same height above sea level. Imagine the sea slowly rising around a hill: the shoreline at each stage would be a contour line. Walking along a contour line, you would neither climb nor descend.
Contour interval
The height difference between one line and the next. It is stated in the map key. Many walking maps use 10 m, and some detailed maps use 5 m (check the key of your own sheet).
Index contour
A thicker line, labelled with its height, that appears at regular steps (for example every fifth line) so you do not have to count from the sea.
Spot height
A single printed number (often with a dot) giving the exact height of a point such as a summit or road junction.
Relief
The overall shape and height variation of the land: its hills, valleys and slopes.
Reading Height
To find the height of a point, start from a labelled line and count. If the interval is 10 m and a thick line is labelled 200, the next thin line up is 210, then 220, and so on. A point lying between two lines is between those two heights; you can only estimate it unless a spot height is printed.
Reading Steepness
The contours are always the same height apart (the interval), so how far apart they are on the map tells you how steep the ground is.
| What you see | What it means |
|---|---|
| Lines close together | Steep ground: a lot of height gained over a short distance |
| Lines far apart | Gentle ground, or nearly flat |
| Evenly spaced lines | A steady, uniform slope |
| Lines close at the top, wide at the bottom | A slope that is steep high up and eases off (a concave slope) |
| Lines wide at the top, close at the bottom | A slope that is gentle high up and steepens (a convex slope), which hides the ground below from the top |
| Lines merge into one | A cliff or crag, where the ground is almost vertical |
Landforms
Certain patterns of lines appear again and again. Once you know them you can read a whole landscape at a glance.
| Feature | Pattern on the map | What it is on the ground |
|---|---|---|
| Summit | Closed rings, each higher than the one outside it, often with a spot height in the middle | The top of a hill |
| Valley | V or U shapes with the point of the V aiming uphill; a stream often runs along the bottom | Low ground between two higher areas |
| Ridge | V or U shapes with the point aiming downhill | A long line of high ground, with the land falling away on both sides |
| Saddle (col) | Two sets of higher rings facing each other with a gap between them | The low point on a ridge between two summits, and often the easiest crossing |
| Spur | A small ridge-like tongue of lines pointing downhill from a bigger hill | A rib of land running down the side of a hill |
| Depression | Closed rings with the heights falling towards the middle (tick marks point down) | A hollow or pit with no outlet |
Using Relief
Reading relief changes how you plan a walk. A route that looks short on the map may include a long, steep climb; a longer route round the contours may be faster and easier. Because water runs downhill, following a stream down a valley leads to lower ground and often to roads and villages, which is a useful safety idea when you are lost (check local advice before relying on it, since valleys can also hold cliffs and bogs).
Common Mistakes
| Mistake | Fix |
|---|---|
| Assuming the interval is the same on every map | Read it in the key every time |
| Mistaking a ridge for a valley | Check which way the numbers rise and where the streams are |
| Judging steepness by the number of lines rather than their spacing | Close lines mean steep; many lines far apart can still be gentle |
| Forgetting small hills between the lines | A bump lower than the interval may not appear at all |
| Treating all hills as the same height because the shapes match | Always read the numbers |
Challenges
A map has a 10 m contour interval. Thick contours are labelled 150 and 200, with four thin lines between them. What is the height of the second thin line above 150? What is the height of the line directly above 200 and of the third line above 200? A path climbs from the 170 m line to the 230 m line: how many contour lines does it cross, counting both ends?
Name the landform in each case. (a) Closed rings labelled 300, 310, 320, 330 from the outside inwards. (b) V-shaped lines labelled 100, 110, 120 with the points aiming towards the 120 side and a blue stream along the middle. (c) V-shaped lines labelled 100, 110, 120 with the points aiming towards the 100 side and no stream. (d) Two sets of rings reaching 400 m facing each other, with a gap at 350 m between them.
Write a short program or spreadsheet formula that takes the contour interval (m), the spacing between two lines on the map (mm) and the map scale denominator (25000 or 50000), and returns the ground distance, the gradient as a percentage and the angle in degrees. Test it with 10 m, 4 mm, 1:25,000 and then 10 m, 2 mm, 1:50,000.
Quick Reference
- Contour: a line of equal height
- Interval: height between lines; read it from the key
- Close lines: steep; wide lines: gentle
- Summit: closed rings rising inwards
- Valley: points aim uphill, stream in the bottom
- Ridge: points aim downhill
- Saddle: low gap between two higher areas