Map Reading and Interpretation

Lessons

Senior 3 · Geography

Map Reading and Interpretation

5 lessons

Lesson 2 of 5

Drawing and Interpreting a Cross-Section

25 min

Let's begin

A planner wants to put a mobile-phone mast where it can be seen from Gasave; a road engineer wants to know how steep a new track would be; a farmer wants to know why the tea is on one hillside and not the other. A contour map holds the answers, but it shows the land from above. To see the land from the side, as you would see it standing in the valley, you draw a cross-section. In this lesson you draw one across Map A and use it to explain what you see.

Definition

Cross-section (profile): a side view of the land along a straight line drawn on a map, showing how the height rises and falls along that line.

Section line: the straight line on the map along which the cross-section is drawn, usually between two grid references, e.g. X (GR 200430) to Y (GR 260430).

The section line on Map A

The line X–Y runs west to east along northing 43, from easting 20 to easting 26. It is 6 km long. It crosses the forested south shoulder of Kibonde Hill, the valley of R. Mpenge and the tea estate on the slope of Nyarutovu Hill.

Figure 2. Map A with the section line X–Y. Drawn by Ganzaa.

Step 1: collect the information with a paper strip

Lay the straight edge of a strip of paper along X–Y. Mark X and Y on the strip.
Make a small tick wherever a contour touches the edge, and write its height. Where contours are close together, write the heights at a slant.
With a different colour, mark where the edge crosses rivers, roads, paths and the edges of forest or plantations.

Figure 3. The paper strip for X–Y on Map A. Drawn by Ganzaa.

Distance from X (km)	0.33	1.13	2.00	2.29	2.61	4.13	5.20	5.72
Height (m)	1800	1800	1750	1700	1650	1650	1650	1600

The footpath crosses at 2.69 km, the murram road at 3.04 km and R. Mpenge at 3.25 km from X.

Definition

Vertical scale: the scale used for heights on the cross-section, e.g. 1 cm to 50 m.

Vertical exaggeration (VE): how many times the vertical scale is larger than the horizontal scale. It makes the relief easier to see:

\text{VE} = \frac{\text{horizontal scale denominator}}{\text{vertical scale denominator}}

Example: 1 cm to 50 m is 1 cm to 5000 cm, i.e. 1:5000. With a map scale of 1:50 000, $\text{VE} = \frac{50\,000}{5000} = 10$ .

Step 2: draw the frame

Draw a horizontal base line exactly as long as X–Y on the map (the horizontal scale must stay the map's scale). Label its ends X and Y.
Choose a vertical scale that fits your page and makes the relief clear, usually 1 cm to 20 m, 50 m or 100 m.
Draw a vertical axis at each end. Start it a little below the lowest height (here 1550 m, not 0 m) and end it a little above the highest height (here 1850 m).

Step 3: plot, join and label

Lay the paper strip along the base line and put a dot above each tick at its height.
Join the dots with a smooth freehand curve. Between two ticks of the same height the land must rise to a rounded top (a hill) or dip to a rounded bottom (a valley). Decide which by looking at the map: between the two 1800 m ticks the curve rises (towards the 1850 m contour, without reaching it); where R. Mpenge crosses, it dips below 1650 m.
Label the features with arrows, and write a title, the horizontal scale, the vertical scale and the VE.

Figure 4. The finished cross-section X–Y, with a line of sight from P, the highest point on the section, towards R. Mpenge. Drawn by Ganzaa.

Warning

Where marks are lost

Changing the horizontal length: the base line must be the same length as X–Y on the map.
Starting the vertical axis at 0 m: the section becomes a flat line at the top of the page.
Joining the dots with a ruler, or drawing a flat top between two ticks of the same height.
Forgetting the title, the two scales, the VE or the labels.

Everything Geography by Thapelo: a teacher works through a cross-section and an intervisibility question on a topographic map. Watch how the paper strip is moved from the map to the graph.

Reading the cross-section

A drawn section is evidence. Use it to describe the relief along the line and to explain how people use the land.

Relief: the land rises from about 1760 m at X to a rounded top of just over 1800 m, then falls east. The fall is gentle from P to about 2.0 km but steepens between 2.0 km and 2.6 km, where the contours are close together, before reaching the valley floor of R. Mpenge at about 1620 m. East of the river the land rises gently to about 1665 m, then falls to about 1585 m at Y.
Slope shape: the east face of Kibonde is convex: gentle at the top, steep at the bottom.
Land use: the forest reserve covers the highest, coolest land; the footpath and the murram road keep to the lower slope near the river, where movement is easier; the tea estate lies on the gentle slopes east of the river.

Definition

Intervisibility: whether two points can be seen from each other. Draw a straight line between them on the cross-section; if the land rises above that line anywhere, the view is blocked.

Dead ground: land that cannot be seen from a viewpoint because higher ground lies in between.

Worked example

Worked example 1: intervisibility. Can a person standing at P, the highest point on the section (just over 1800 m, about 0.7 km from X), see the point where R. Mpenge crosses the section line?

On Figure 4, rule a straight line from P to the river (about 1620 m, 3.25 km from X).
At 2.0 km from X the line is at about 1720 m, but the land there is 1750 m (a contour tick).
The land rises about 30 m above the line of sight.

Answer: No. The convex shoulder of the slope near 2.0 km blocks the view, so the river bank is dead ground from P.

Worked example

Worked example 2: vertical exaggeration. A pupil draws a section from a 1:50 000 map with a vertical scale of 1 cm to 20 m. Find the VE.

Vertical scale: 1 cm to 20 m = 1 cm to 2000 cm, i.e. 1:2000.
$\text{VE} = \frac{50\,000}{2000} = 25$ .

Answer: 25 times. The hills on this section look 25 times steeper than they really are, so never read true slope angles off an exaggerated section.

Worked example

Worked example 3: gradient from the section. Find the average gradient of the steep part of the slope, between the 1750 m tick (2.00 km from X) and the 1650 m tick (2.61 km from X).

VI: $1750 - 1650 = 100\,\text{m}$ .
HE: $2.61 - 2.00 = 0.61\,\text{km} = 610\,\text{m}$ .
Gradient: $\frac{100}{610} \approx \frac{1}{6.1}$ .

Answer: about 1 in 6.

Tip

Save time in the exam. Write the heights on the paper strip as you go, not from memory afterwards, and check the number of ticks against the number of contours you can count along the line. One missed tick shifts the whole curve.

Practice

Use Map A and Figures 3 and 4.

How long is the section line X–Y in kilometres?
State the height of the land at 5.72 km from X.
A pupil draws the section X–Y with a vertical scale of 1 cm to 100 m. Calculate the vertical exaggeration.
Which part of the section is steeper: from 1.13 km to 2.00 km, or from 2.00 km to 2.61 km? Give a reason from the strip.
Calculate the average gradient between the 1800 m tick at 1.13 km and the 1750 m tick at 2.00 km.
Explain why a pupil must not draw a flat line between the two 1650 m ticks at 4.13 km and 5.20 km.
Using the section, explain why the tea estate is on the land east of R. Mpenge and not on the slope between 2.0 km and 2.6 km.
A telecommunications company wants a mast that can be seen from the whole of R. Mpenge's valley along X–Y. Using Figure 4, would P be a good site? Explain.

Show answersHide answers

6 km (from easting 20 to easting 26, six 1 km grid squares).
1600 m (the 1600 m contour tick).
1 cm to 100 m = 1:10 000; $\text{VE} = \frac{50\,000}{10\,000} = 5$ .
From 2.00 km to 2.61 km. The land falls 100 m (1750 m to 1650 m) in 0.61 km there, but only 50 m (1800 m to 1750 m) in 0.87 km on the other stretch.
VI $= 50\,\text{m}$ ; HE $= 2.00 - 1.13 = 0.87\,\text{km} = 870\,\text{m}$ ; gradient $= \frac{50}{870} \approx \frac{1}{17}$ , about 1 in 17.
Two ticks of the same height mean the land rises above 1650 m or falls below it between them. The map shows a rise there (the slope of Nyarutovu Hill; no 1700 m contour is crossed), so the curve must rise to a rounded top below 1700 m.
East of the river the slope is gentle (about 45 m of rise in 1.4 km), so the soil is less easily washed away and tea can be planted and picked in rows. Between 2.0 km and 2.6 km the slope is steep (about 1 in 6), so it suffers soil erosion and is hard to work.
No. The convex slope below P hides the valley floor (dead ground): the line of sight from P to the river passes about 30 m below the ground near 2.0 km. A site lower down the slope, or on the hill east of the river, would see more of the valley.

Sources and credits

Topographic profile and Vertical exaggeration: Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.

Video: Geography mapwork: Cross section/mapping and intervisibility, Everything Geography by Thapelo, embedded from YouTube.

Map A, Figures 2 to 4, the worked examples and the practice exercises are written by Ganzaa. Map A shows an invented area.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1The section line on Map A

2Step 1: collect the information with a paper strip

3Step 2: draw the frame

4Step 3: plot, join and label

5Reading the cross-section

The section line on Map A

Step 1: collect the information with a paper strip

Step 2: draw the frame

Step 3: plot, join and label

Reading the cross-section