Elements of a Map
Senior 1 · Geography
Elements of a Map
Lesson 2 of 4
22 min
Measured in a straight line on a map, Musanze is about 68 km from Kigali. The road between them is a little over 90 km long. Both numbers come from maps, and both are correct. By the end of this lesson you will be able to find both kinds of distance yourself.
Definition
Scale: the relationship between a distance on a map and the real distance on the ground. Example: on a map with a scale of 1:50,000, 1 cm on the map stands for 50,000 cm (0.5 km) on the ground.
Figure 1. The same scale shown in three ways.
Definition
Representative fraction (RF): a scale written as a ratio of map distance to ground distance in the same units, with 1 on the left. Example: 1:25,000 means 1 cm on the map is 25,000 cm on the ground.
Important
The conversions you need
Here "scale" means the number after the colon (the denominator of the RF). Work in centimetres, then divide by 100,000 to get kilometres.
Worked example 1: turn an RF into a statement
Change 1:50,000 into a statement in kilometres.
Step 1 1 cm on the map = 50,000 cm on the ground.
Step 2 , so 50,000 cm = 0.5 km.
Answer. "1 cm represents 0.5 km" (or 2 cm represent 1 km).
Worked example 2: a straight-line distance
On a map of Rwanda with a scale of 1:1,000,000, the straight line from Kigali to Musanze measures 6.85 cm. Find the real distance.
Answer. About 68.5 km in a straight line ("as the crow flies").
Roads and rivers bend, so a ruler cannot measure them in one go. Use the edge of a strip of paper (or a piece of thread laid along the road and then straightened).
Figure 2. Measuring a curved distance with the edge of a paper strip (the diagram itself is not at 1:50,000).
Worked example 3: a winding road
On a 1:50,000 map, the road from a school (A) to a market (B) is measured with a paper strip (Figure 2). The marks add up to 7.4 cm. How long is the road?
Answer. 3.7 km. Check with the scale bar: lay the strip on the bar with A at 0; B falls between 3 and 4 km, closer to 4. The answer is sensible.
Worked example 4: find the scale, then a map distance
(a) A straight road 2 km long is 4 cm long on a map. What is the scale? (b) On the same map, how long would a 12 km road be?
(a) 2 km = 200,000 cm. The ratio is 4 : 200,000. Divide both sides by 4 to put 1 on the left:
(b) 12 km = 1,200,000 cm, so
| Large scale | Small scale | |
|---|---|---|
| Example | 1:2,500 or 1:10,000 | 1:1,000,000 or smaller |
| Area shown | Small (a village, a school, a town centre) | Large (a country, a continent) |
| Detail | Much: single buildings, paths | Little: only main towns, big rivers |
| Used for | Plans, town maps, land plots | Atlas maps, maps of Rwanda or Africa |
Warning
The bigger the number, the smaller the scale. 1:10,000 is a larger scale than 1:1,000,000, because is a bigger fraction than . A large-scale map shows a small area in great detail.
Important
Case study: why Rwanda's roads are longer than the straight line
Straight from Kigali to Musanze is about 68 km; the road is a little over 90 km, about one third longer. Rwanda is a land of steep hills and deep valleys. A road cannot climb straight up a steep slope, so it winds along valley sides and zig-zags up hills (you will see this on the way to Musanze and on the road down to Rubavu). Each bend adds distance. A traveller needs the road distance to plan time and fuel; a radio engineer placing a mast, or a pilot, needs the straight-line distance.
How to use a Map Scale to Measure Linear Distance, by Mr Gedge's Geography Channel (4 min). Watch how the paper edge is laid on the scale bar, including the part left of zero.
A moto driver says: "Straight-line distances are useless in Rwanda." Using the Kigali to Musanze example, explain one situation in which he is right and one in which he is wrong.
Tip
Exam technique. Always write the units at every step, convert centimetres to kilometres by dividing by 100,000 (move the decimal point five places left), and check your answer against the scale bar. A school-to-market road of 370 km is clearly wrong.
Practice
Remember
Sources and credits
Written by Ganzaa: the lesson text, diagrams, worked examples and practice. Distances between towns were calculated from their latitude and longitude.
