Map Reading and Interpretation
Senior 5 · Geography
Map Reading and Interpretation
Lesson 3 of 4
25 min
Rwanda is called the land of a thousand hills, yet every map of it is flat. Mapmakers have invented several ways to show height on paper. Figure 5 shows one invented hill drawn in four of them. In every version the hill's eastern (right-hand) side is steeper than its western side. Before reading on, decide which method shows that most clearly, and which would let you read the height of any point.
Definition
Relief: the shape and height of the land surface: its hills, valleys, slopes and plains. Example: the relief of Map A is a valley between two hills.
Contour: a line on a map joining places at the same height above sea level. Example: on Map A the 1,500 m contour is drawn thicker than the others.
| Method | How it works | Advantages | Disadvantages |
|---|---|---|---|
| Contours | Lines of equal height at a fixed vertical interval; close lines mean steep ground | Exact heights; slopes can be measured; the basis of cross sections and gradients | Need training to read; give no picture at a glance; small features between two contours are missed |
| Hachures | Short lines drawn down the slope; thicker and closer on steep slopes | Give a striking picture of steep slopes and escarpments | No heights; flat hilltops and valley floors look the same; crowd out other detail |
| Hill shading | Slopes facing away from an imagined light (from the north-west) are shaded | Gives a three-dimensional impression; easy for anyone to read | No heights; dark areas hide other symbols; a shaded slope may be misread as low ground |
| Layer tints | Colours fill the spaces between chosen contours, usually green (low) to brown or red (high) | Show at once which land is high or low; good on atlas maps of large areas | Heights only within a band; slopes inside a band are invisible; sudden colour changes suggest cliffs |
| Spot heights | A dot with the height written beside it | An exact height at a point | Say nothing about the land between the dots |
| Trigonometrical stations | A triangle with a dot and its height, at a survey point, often a hilltop | A precise, surveyed height; good for orientation in the field | Very few on a sheet |
Modern topographic maps combine methods: contours for accuracy, spot heights and trig stations for exact values, and sometimes light hill shading for the picture. On Map A, contours, spot heights and a trig station are used.
Definition
Vertical interval (VI), also called the contour interval: the difference in height between two neighbouring contours. It is the same over the whole map. Example: on Map A the VI is 50 m.
Amplitude of relief (relative relief): the difference between the highest and the lowest points in an area. Example: on Map A, m.
Only some contours carry numbers. To find the value of the others, start from a labelled one and count up or down in steps of the VI, using the river (always the lowest ground in its valley) and the summits to tell which way is up. Between two contours you can give only a range: the school (259422) lies between the 1,400 m and 1,450 m contours, so its height is between 1,400 m and 1,450 m. A summit lies above its highest closed contour: Hill T's highest ring is 1,750 m and its trig station reads 1,779 m.
Worked example 1: finding the vertical interval
Problem. A map has unlabelled contours between a 1,500 m and a 1,700 m contour. You count three contours between them. What is the VI?
Solution. Three contours between two labelled ones make four equal steps:
Worked example 2: amplitude of relief
Problem. Find the amplitude of relief on Map A, and along the footpath from the bridge (256441) to the trig station.
Solution. The highest point on the map is the trig station on Hill T, 1,779 m. The lowest marked point is the spot height on the River Nyaga near the western edge, 1,255 m. Whole map: m. Along the footpath: the bridge is at 1,344 m, so m. Check it with the contours: the footpath crosses nine of them, from 1,350 m to 1,750 m, which span 400 m. Add the part step below the first (1,344 m to 1,350 m, 6 m) and above the last (1,750 m to 1,779 m, 29 m): m.
Contour spacing describes the slope. Closely spaced contours mean a steep slope; widely spaced ones, a gentle slope; evenly spaced ones, a uniform slope. Along the footpath on Map A the contours are wide apart near the bridge and crowd together near the summit of Hill T: the slope is gentle at the bottom and steep at the top, a concave slope. Hill V, where the contours from 1,450 m to 1,650 m form tight rings around the summit, is much steeper than the almost flat land of the tea estate to its north-east.
Warning
Common mistakes
Tip
Exam technique. When asked "how is relief shown on the map extract?", list every method actually used (contours, with their VI; spot heights; trig stations; any shading) and give one example of each from the extract, with a grid reference.
Ordnance Survey: "Understanding contour lines with Steve Backshall". Watch how he links the spacing of contours on the map to the steepness of the slope in front of him; the same rule works on Map A.
Practice
Practice (use Map A and Figure 5)
Summary
Sources and credits
Written by Ganzaa. Map A and Figure 5 are invented for teaching and drawn by Ganzaa; heights and counts were taken from the map's own data.
Video: "Understanding contour lines with Steve Backshall and Ordnance Survey", Ordnance Survey, embedded from YouTube.
