Geography · Lesson 8

Map reading & interpretation: scale, bearings, contours & gradient

JAMBWAECNECO

What the syllabus asks

JAMB: Practical Geography A–C: maps; scale and measurement of distances, areas, reduction and enlargement, directions, bearings and gradients; map reading and interpretation, cross profiles, intervisibility, physical and human features and their relationships on topographical maps

WAEC: Map work: maps (meaning, types, uses); map reading and interpretation of contoured survey maps: scale, measurement of distances, direction and bearing, map reduction and enlargement, identification of physical features (spurs, valleys etc.) and cultural features (settlements, routes etc.), measurement of gradients, drawing of cross profiles, intervisibility, drainage characteristics and pattern,…

NECO: Map work: maps (meaning, types, uses); map reading and interpretation of contoured survey maps: scale, measurement of distances, direction and bearing, map reduction and enlargement, identification of physical features (spurs, valleys etc.) and cultural features (settlements, routes etc.), measurement of gradients, drawing of cross profiles, intervisibility, drainage characteristics and pattern,…

What you will learn

Define a map and state its types and uses, and read marginal information.
Convert scales between statement, R.F. and linear forms, and between metric and imperial units.
Measure straight and curved distances, and areas, on topographical maps.
Reduce and enlarge maps and explain the effect on scale and area.
Give directions, bearings, back bearings and grid references.
Interpret relief from contours and identify landforms.
Calculate gradients, draw cross profiles with vertical exaggeration and test intervisibility.
Describe drainage, settlement, communication and land-use patterns and relate them to relief.

Key points

A map is a scaled drawing of the earth’s surface on a flat surface; large-scale maps show small areas in detail.
Scale can be a statement, an R.F. (no units) or a linear scale; 1 km = 100,000 cm; 1 mile = 63,360 in.
At 1:50,000, km = cm ÷ 2; measure curved routes with string or the paper-edge method.
Area = length × breadth (in km) or whole squares + half the part squares; area scales with the square of length.
Reducing a map makes its scale smaller (denominator larger); enlarging makes it larger.
Bearings: clockwise from north, three figures; back bearing ± 180°.
Grid references: eastings first; four-figure for a square, six-figure for a point.
Contours: close = steep; valley V points uphill; spur V points downhill.
Gradient = V.I. ÷ H.E. in the same units = 1 in n; smaller n is steeper.
V.E. = horizontal scale denominator ÷ vertical scale denominator.
Intervisible if no ground rises above the line of sight.
Relate relief to drainage, settlement, routes and land use; spend no more than 35 minutes on Question 1.

Start here: Maps: meaning, types and uses

When a surveyor plans a new road from Ibadan to a village in Oyo State, when a farmer wants to know whether a slope is too steep to plough, or when soldiers plan a route, they all use a map. In WAEC Paper 3, Question 1 is a map-reading question on a contoured survey map of a part of West Africa. It is compulsory and carries 25 of the 70 marks, so the skills in this topic are worth more than any other single question.

A map is a representation, drawn to scale, of the whole or part of the earth’s surface on a flat surface such as paper or a screen.

Types of maps

By scale. Large-scale maps (e.g. 1:2,500, 1:10,000, 1:25,000) cover a small area in great detail, such as a town plan. Medium-scale maps (e.g. 1:50,000, 1:62,500, 1:100,000) are the usual topographical sheets used in exams. Small-scale maps (e.g. 1:1,000,000 and smaller) show large areas such as a country or continent with little detail; atlas maps are small-scale.
Topographical maps show relief (contours, spot heights), drainage, vegetation and human features such as settlements, roads and railways.
Thematic (special-purpose) maps show one theme: rainfall, population density, geology, soils, vegetation.
Cadastral maps show land ownership and plot boundaries.
Political maps show countries, states and boundaries; atlas maps combine several themes.
Sketch maps are simple, not exactly to scale, drawn to show the main features of an area.

⚠️ Common mistake: thinking a large-scale map covers a large area. It is the opposite: the fraction 1/2,500 is larger than 1/1,000,000, so features appear large and the area covered is small.

Uses of maps

Finding direction, location and distance; route planning for travel.
Planning roads, railways, dams, towns and farms.
Military planning and national defence.
Census and administration (demarcating enumeration areas, wards, constituencies).
Teaching and research; storing information about the earth.
Tourism, and showing the distribution of resources such as minerals.

Marginal information

Before reading the map itself, read the margin: title and sheet name, scale (R.F. and linear), contour interval, the key (legend) of conventional signs, the north arrow, grid numbers and sometimes the date of survey. Conventional signs are standard symbols, e.g. a dashed blue line for a seasonal stream, a cross for a church or mission, black squares for buildings.

Worked example 8.1

Arrange these scales from the largest scale to the smallest: 1:250,000; 1:25,000; 1:1,000,000; 1:50,000.

Write each as a fraction: 1/25,000 is the biggest fraction because its denominator is smallest.
Order of denominators, smallest first: 25,000; 50,000; 250,000; 1,000,000.

1:25,000 > 1:50,000 > 1:250,000 > 1:1,000,000

Try it yourself. Which would show the individual houses in a village better: a 1:10,000 map or a 1:500,000 map?

Show solution

The 1:10,000 map. It is the larger scale, so it shows more detail over a smaller area.

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Practice questions

1. A scale of 1 : 50,000 means 1 cm on the map represents

0.5 km on the ground
5 km on the ground
50 km on the ground
50 m on the ground

Show answer

A: 0.5 km on the ground
50,000 cm = 500 m = 0.5 km.

2. The representative fraction of the scale ‘2 cm to 1 km’ is

1 : 20,000
1 : 200,000
1 : 500,000
1 : 50,000

Show answer

D: 1 : 50,000
1 km = 100,000 cm; 100,000 ÷ 2 = 50,000.

3. On a 1 : 50,000 map two towns are 6.4 cm apart. The ground distance is

6.4 km
12.8 km
3.2 km
32 km

Show answer

C: 3.2 km
6.4 × 0.5 = 3.2 km.

4. A square of 2 cm × 2 cm on a 1 : 50,000 map represents a ground area of

0.25 km²
1 km²
4 km²
2 km²

Show answer

B: 1 km²
Each side is 1 km, so 1 × 1 = 1 km².

5. If a map is reduced to half its linear size, its area becomes

one quarter
one half
twice as large
the same

Show answer

A: one quarter
Area changes by the square of the linear factor.

Key words

Map: A drawing, to scale, of part or all of the earth’s surface on a flat surface.
Topographical map: A detailed map at a large or medium scale showing both relief and human features.
Scale: The ratio between a distance on a map and the corresponding distance on the ground.
Representative fraction (R.F.): A scale written as a ratio or fraction with no units, e.g. 1:50,000.
Linear scale: A line divided into units of ground distance, drawn on the map.
Bearing: The angle of a direction measured clockwise from north, in three figures.
Back bearing: The bearing from the second point back to the first; it differs by 180°.
Grid reference: A set of figures (eastings then northings) that locates a point on a gridded map.
Contour: A line on a map joining places of equal height above sea level.
Vertical interval (V.I.): The difference in height between two points or between successive contours.
Horizontal equivalent (H.E.): The horizontal ground distance between two points.
Gradient: The steepness of a slope: V.I. ÷ H.E., usually given as 1 in n.

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