Relief and Drainage on a Topographic Map – Map Reading and Interpretation (Senior 3 Geography) | Ganzaa

Map Reading and Interpretation

Lessons

Senior 3 · Geography

Map Reading and Interpretation

5 lessons

Lesson 1 of 5

Relief and Drainage on a Topographic Map

25 min

Let's begin

Lay a topographic map on the desk and the ground seems flat: it is only paper. Yet a trained reader can say where the hills rise, which slopes are too steep to farm, where rain water will run and in which direction a river flows. All of that is coded in thin brown and blue lines. In S2 you used a map to find direction, grid references, distance and area. This unit reads what the map shows: the land, the people, and how the two are linked. We start with the land itself, its relief and its drainage.

Map A: the extract for this unit

Map A is a practice extract drawn for this unit. The area and its names are invented, but it follows the style of a real 1:50 000 survey sheet: each grid square is 1 km by 1 km, and the contour interval is 50 m. Study the key first. You will use this map in every lesson of the unit.

Map A. Gasave: an illustrative topographic extract (invented area). Drawn by Ganzaa.

Definition

Topographic map: a large-scale map (usually 1:50 000 or larger) that shows both the natural features of an area (relief, rivers, vegetation) and the human features (roads, settlements, buildings), using standard symbols explained in a key.

Relief: the shape and height of the land surface: its hills, valleys, plains and slopes.

Drainage: the water features of an area and the way water flows across it: rivers and streams, lakes, swamps and dams.

How a map shows height

Paper has no height, so map makers use several methods. Most survey maps combine two or three of them.

Method	What you see	Strength	Weakness
Contours	Brown lines, each joining points of the same height	Show both height and the shape of slopes	Take practice to read
Spot height	A dot with a number, e.g. • 1782	Exact height of one point	Say nothing about the land around it
Trigonometrical station (trig point)	A small triangle with a height, often on a hilltop	A carefully surveyed, exact height	Only a few on each sheet
Layer tinting	Bands of colour for height ranges, e.g. green for low land, brown for high land	Gives a quick picture of high and low ground	Exact height inside a band is unknown; green may be misread as vegetation
Hill shading	Shadows drawn as if light came from the north-west	Makes the land look three-dimensional	Gives no heights
Hachures	Short strokes drawn down the slope, thicker where it is steep	Show steepness well	Give no heights and crowd the map

Definition

Contour: a line on a map joining places of the same height above mean sea level. Example: every point on the 1650 m contour is 1650 m above sea level.

Contour interval: the constant difference in height between one contour and the next. If contours are labelled 1600, 1650, 1700, the interval is 50 m.

Spot height: a point whose exact height is printed beside a dot. A trigonometrical station is a surveyed point marked with a triangle.

Five rules for reading contours

Spacing gives steepness. Contours close together mean a steep slope; contours far apart mean a gentle slope; evenly spaced contours mean an even (uniform) slope.
A convex slope bulges outwards: its contours are far apart at the top and crowded at the bottom. A concave slope curves inwards: crowded at the top and spread out at the bottom. So from the top of a convex slope you cannot see its foot.
Contours never cross or split. A point cannot have two heights. Where they seem to merge, the land is a vertical cliff or an escarpment face.
A closed ring of contours is a hill if the values rise towards the centre, and a hollow (depression) if they fall.
Read the numbers, not only the shapes. The same V-shape is a valley or a spur depending on which way the values rise.

Landforms and their contour patterns

Figure 1 shows the patterns you must recognise. Heights are in metres and the contour interval in every panel is 50 m.

Figure 1. Contour patterns of common landforms. Drawn by Ganzaa.

Landform	Contour pattern	How to be sure
Conical hill	Roughly circular closed contours, evenly spaced	Values rise to the centre; often a spot height or trig point on top
Ridge	Long, narrow closed contours	High ground much longer than it is wide; often a watershed between two rivers
Spur	U- or V-shapes pointing towards lower ground	The tip of each V lies on lower ground than its arms reach
Valley	V-shapes pointing towards higher ground	A river often runs through the tips of the Vs
Saddle (col)	A low dip between two sets of closed contours	Lower than both hills, higher than the land on either side of the dip
Plateau	Contours crowded round the edge, a wide space with no contours on top	High and flat: the top lies above the highest contour
Escarpment	Many contours crowded along one side, widely spaced on the other side	One steep face and one gentle back slope

Warning

Spur or valley? Both look like a row of Vs. Many candidates guess from the shape and lose the mark. Read two contour values: if the Vs point up the values (to higher ground), it is a valley; if they point down the values (to lower ground), it is a spur. A river line through the Vs confirms a valley.

Ordnance Survey, Britain's national mapping agency: Steve Backshall shows how contour lines on a real survey map reveal hills, valleys and steep slopes. Watch how the spacing of the lines changes where the ground steepens.

Reading the relief of Map A

Apply the rules to Map A. Always give a grid reference for any feature you name.

Feature on Map A	Where	Evidence on the map
Highest point: trig point, 1935 m	GR 214435	Triangle symbol inside the closed 1900 m contour
Kibonde Hill, an elongated hill (ridge)	Grid squares 2043 to 2244	Long oval closed contours from 1800 m to 1900 m, stretching from south-west to north-east
Nyarutovu Hill, a conical hill	Spot height 1826 m at GR 247439	Almost circular closed contours from 1650 m to 1800 m
Saddle (col)	Spot height 1636 m at GR 234437	The low point between the two hills: lower than both summits
Spur	Grid squares 2241 and 2242	The 1600 m and 1650 m contours bend south-east, towards lower ground, between the seasonal stream and R. Mpenge
Valley of R. Mpenge	Grid squares 2341 to 2343	Contours make Vs that point north, upstream, where they cross the river
Flat valley floor of R. Kamiro	Southern part of the map, e.g. spot height 1492 m at GR 223408	Wide space with no contours along the river; swamp

Notice also the thicker contours at 1500 m and 1750 m. These are index contours: every fifth line is drawn thicker so that you can count heights quickly.

Drainage on the map

Perennial rivers flow all year and are drawn as solid blue lines. Seasonal (intermittent) streams flow only in the rainy seasons and are drawn as broken blue lines.
Swamps (marshes) appear as short blue tufts on flat, low land, usually on a valley floor. Lakes, ponds and dams are shown in blue fill.
The source of a river is where it begins, on high ground; its mouth is where it ends, in a lake, sea or larger river. A confluence is where two rivers join, and the smaller one is a tributary.

Which way does the river flow? Water flows downhill, so use any of these clues:

the contour values the river crosses fall in the direction of flow;
the Vs of the contours crossing a river point upstream;
tributaries usually join the main river at an acute angle that opens upstream, so the "arrow" they form points downstream;
a river flows away from its source and into a lake or a larger river.

Definition

Watershed (drainage divide): a ridge or line of high ground that separates the areas drained by two different river systems. Rain falling on one side runs to one river, and on the other side to another.

Drainage pattern: the layout that a river and its tributaries make on the map. You met dendritic (tree-like), trellis, radial and parallel patterns in S1.

Reading the drainage of Map A

R. Kamiro is the main, perennial river. It flows from west to east: the 1500 m contour crosses it in grid square 2141 with its V pointing west (upstream), the floor is 1492 m at GR 223408, and it leaves the map below the 1450 m contour in the east.
R. Mpenge is a perennial tributary. It rises in grid square 2343, just south of the saddle, flows south past Gasave and joins R. Kamiro in grid square 2340.
Two seasonal streams: one rises on the south slope of Kibonde (grid square 2142) and flows south-south-east to R. Kamiro; the other rises north of the saddle and flows north, off the map.
A swamp lines R. Kamiro in grid squares 2440 and 2540, where the valley floor is widest and flattest.
The saddle at GR 234437 is part of a watershed: rain falling just south of it drains to R. Mpenge and R. Kamiro, rain falling just north of it drains north.

Working out gradient

An examiner may ask how steep a slope is. The gradient compares the rise in height with the distance on the ground:

\text{gradient} = \frac{\text{vertical interval (VI)}}{\text{horizontal equivalent (HE)}}

Both must be in the same unit (usually metres), and the answer is written as a ratio "1 in n".

Definition

Vertical interval (VI): the difference in height between two points, e.g. $1750 - 1500 = 250\,\text{m}$ .

Horizontal equivalent (HE): the real ground distance between the same two points, measured on the map and converted with the scale.

Gradient: VI divided by HE, written "1 in n". A gradient of 1 in 10 means the land rises 1 m for every 10 m you travel.

Worked example

Worked example 1: gradient. On a 1:50 000 map, point P lies on the 1750 m contour and point Q on the 1500 m contour. P and Q are 5 cm apart on the map. Find the average gradient from P to Q.

VI: $1750 - 1500 = 250\,\text{m}$ .
HE: $5\,\text{cm} \times 50\,000 = 250\,000\,\text{cm} = 2500\,\text{m}$ .
Gradient: $\frac{250}{2500} = \frac{1}{10}$ .

Answer: 1 in 10.

Worked example

Worked example 2: direction of flow. In Figure 1, panel D, the river crosses the 1650, 1600, 1550 and 1500 m contours from top to bottom, and each contour's V points towards the top of the panel. In which direction does the river flow?

The values fall from 1650 m to 1500 m towards the bottom, and the Vs point upstream, towards the top. Water flows from high to low ground, so the river flows from the top of the panel to the bottom: southwards, if north is at the top.

Worked example

Worked example 3: gradient on Map A. Find the average gradient from the trig point (GR 214435) to the spot height at GR 223408.

VI: $1935 - 1492 = 443\,\text{m}$ .
HE: the points are 0.9 km apart east–west and 2.7 km apart north–south, so the straight distance is $\sqrt{0.9^2 + 2.7^2} = \sqrt{8.1} \approx 2.85\,\text{km} = 2850\,\text{m}$ . (On paper you would measure it with a ruler and the scale bar.)
Gradient: $\frac{443}{2850} \approx \frac{1}{6.4}$ .

Answer: about 1 in 6. That is steep: a loaded bicycle cannot be ridden up it.

Tip

Exam habit. Before you name any landform, find two labelled contours next to each other and write down which way the land rises. Thirty seconds on that saves the spur-or-valley mistake and tells you the direction of every river on the sheet.

Practice

Use Map A for questions 1 to 6.

State the contour interval of Map A, and name the two index contours shown on it.
Give the six-figure grid reference and the height of the highest point on Map A.
Name the landform at GR 234437 and explain how the map shows it.
Name the relief feature in grid squares 2241 and 2242, between the seasonal stream and R. Mpenge, and give the map evidence.
In which direction does R. Mpenge flow? Give two pieces of evidence from the map.
Explain why a swamp has formed along R. Kamiro in grid squares 2440 and 2540.
A slope has contours far apart near the top and crowded near the bottom. Name this type of slope and say what a person standing at the top cannot see.
Point X is at 1680 m and point Y at 1580 m. On a 1:50 000 map they are 3 cm apart. Calculate the average gradient from X to Y.

Show answersHide answers

50 m. The index contours are 1500 m and 1750 m (drawn thicker).
GR 214435: the trig point, 1935 m.
A saddle (col), 1636 m high. It is the lowest point on the high ground between two sets of closed contours: Kibonde Hill (1935 m) to the west and Nyarutovu Hill (1826 m) to the east.
A spur. The 1600 m and 1650 m contours bend south-east, pointing towards lower ground, with a valley on each side (the seasonal stream to the west and R. Mpenge to the east).
South. Any two of: the contours form Vs that point north (upstream) where they cross it; it starts near the saddle above 1600 m and ends at R. Kamiro at about 1475 m; it joins R. Kamiro, which is the larger river.
The valley floor there is wide and nearly flat (no contours cross it for about 2 km), so the river flows slowly, overflows its banks in the rainy seasons, and the water cannot drain away quickly.
A convex slope. From the top you cannot see the foot of the slope, because the steeper lower part bulges out of view.
VI $= 1680 - 1580 = 100\,\text{m}$ ; HE $= 3 \times 50\,000 = 150\,000\,\text{cm} = 1500\,\text{m}$ ; gradient $= \frac{100}{1500} = \frac{1}{15}$ , i.e. 1 in 15.

Sources and credits

Contour line and Topographic map: Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.

Video: Understanding contour lines with Steve Backshall and Ordnance Survey, Ordnance Survey, embedded from YouTube.

Map A, Figure 1, 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.

1Map A: the extract for this unit

2How a map shows height

3Five rules for reading contours

4Landforms and their contour patterns

5Reading the relief of Map A

6Drainage on the map

7Reading the drainage of Map A

8Working out gradient

Map A: the extract for this unit

How a map shows height

Five rules for reading contours

Landforms and their contour patterns

Reading the relief of Map A

Drainage on the map

Reading the drainage of Map A

Working out gradient