Map Reading and Measurements of Distances on Maps – SS1 Geography Lesson Note (Third Term)

Map reading is the ability to recognise conventional signs on a map and use them to interpret it.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain what map reading and a map scale are;
2. name the types of map and the types of scale;
3. use statement scale and representative fraction to find ground distances;
4. describe how to use a linear scale;
5. distinguish between small scale and large scale maps;
6. measure straight and curved distances on a map.

What is map reading?

Map reading is the skill of recognising conventional signs on a map and using them to interpret the map.

• Other name: the map reading aspect of geography is often called Practical Geography.
• Map work: it involves recognising the signs on a map and using them to understand the area shown.
• Conventional signs: agreed symbols and colours that stand for features such as roads, railways, rivers, bridges and buildings.

What is a map?

A map is a representation of the earth’s surface as it is seen from above, drawn on paper.

• Coverage: a large area such as a town, a village or a whole country can be shown on one page.
• View: features are drawn as if seen from directly above.
• Reduction: the area is drawn smaller than it is on the ground, using a scale.

What are the types of map?

There are four types of map: topographical maps, atlas maps, plan maps and sketch maps.

1. Topographical maps: show natural and man-made features of an area in detail, such as relief, rivers, roads and settlements.
2. Atlas maps: maps collected in a book called an atlas, showing countries, continents or themes such as vegetation and population.
3. Plan maps: very detailed maps of a small area, such as a school compound or a building.
4. Sketch maps: rough hand-drawn maps that show the general position of places and are not drawn accurately to scale.

What is a map scale?

A map scale is the ratio between a distance on the map and the actual distance on the ground.

• Purpose: it shows how much the real area has been reduced on the map.
• Use: it lets us measure ground distances from the map.
• Example: if the scale is 2 cm to 1 km, then 2 cm on the map represent 1 km on the ground.

What are the types of scale?

There are three types of scale: the statement scale, the linear scale and the representative fraction.

What is a statement scale?

A statement scale is a scale given as a statement with figures, such as 1 cm to 2 km.

• 1 cm to 2 km: 1 cm on the map represents 2 km on the ground.
• 2 cm to 1 km: 2 cm on the map is equal to 1 km on the ground.

Worked example: finding ground distance from a statement scale

Question: The scale of a map is 2 cm to 1 km. What is the ground distance if two towns are 10 cm apart on the map?

1. Write what is given: map distance = 10 cm, scale = 2 cm to 1 km.
2. Write the ground distance as x km, so 2 cm = 1 km and 10 cm = x km.
3. Cross multiply: 2 × x = 10 × 1.
4. 2x = 10, so x = 10 ÷ 2 = 5 km.

Answer: the ground distance is 5 km.

What is a linear scale?

A linear scale is a line drawn and divided to show the relationship between map distance and ground distance.

• Primary divisions: the major divisions on the right side of zero.
• Secondary divisions: the minor divisions on the left side of zero.
• Origin: the mark zero (0), where measuring starts.

How do you use a linear scale?

1. Measure the distance between the two places on the map, with a ruler or a thread.
2. Place the measured length on the scale, starting from the origin (0). Stretch it to the right along the primary divisions.
3. Read where it ends. That is the ground distance in km or miles.
4. If the length is longer than the scale, record where the scale ends. Measure the remaining part again from zero to the right, then add the two readings.
5. If a small fraction is left, smaller than one major division, measure it from zero to the left on the secondary divisions. Add this to the first reading.

What is a representative fraction (R.F.)?

A representative fraction is a scale written as a ratio or fraction, such as 1:50,000.

• Numerator: the map distance, always 1.
• Denominator: the ground distance for that 1 unit.
• Same unit: both numbers are in the same unit, so 1:50,000 means 1 cm on the map is 50,000 cm on the ground.
• Examples: 1:100,000, 1:50,000 and 1:200,000.

Common conversions

• 1:100,000: the same as 1 cm to 1 km.
• 1:50,000: the same as 2 cm to 1 km, or 1 cm to ½ km.
• Why: 1 km = 1,000 m = 100,000 cm.

How do you use a representative fraction?

1. Note the two places involved.
2. Measure the distance between them on the map.
3. Relate the measured distance to the scale to get the ground distance.

Worked example: finding ground distance from an R.F.

Question: The scale of a map is 1:50,000. What is the ground distance if two towns are 10 cm apart on the map?

1. Write what is given: map distance = 10 cm, scale = 1:50,000.
2. Convert the scale: 1 cm = 50,000 cm = 500 m = ½ km.
3. Multiply: 10 cm = 10 × ½ km = 5 km.

Answer: the ground distance is 5 km.

Worked example: finding map distance from ground distance

Question: The scale is 1:100,000. What map distance represents 8 km on the ground?

1. 1:100,000 means 1 cm represents 1 km.
2. So 8 km is represented by 8 × 1 cm = 8 cm.

Answer: 8 cm.

Worked example: changing a statement scale to an R.F.

Question: Express 1 cm to 2 km as a representative fraction.

1. Change 2 km to cm: 2 × 100,000 = 200,000 cm.
2. Write 1 cm to 200,000 cm as a ratio.

Answer: 1:200,000.

What is the size of a scale?

The size of a scale is either large or small, depending on the denominator of the representative fraction.

What is a small scale map?

A small scale map shows a large area with little detail, so only very important features appear.

• Rule: the larger the denominator, the smaller the scale.
• Examples: 1:1,000,000, 1:2,000,000 and 1:250,000.

What is a large scale map?

A large scale map shows a small area with more detail of the features in it.

• Rule: the smaller the denominator, the larger the scale.
• Examples: 1:5,000, 1:250 and 1:20.

What is the difference between small scale and large scale maps?

How are distances measured on a map?

Distance on a map is the interval between two points, and it can be straight or curved.

How do you measure a straight distance?

1. Locate the two places on the map.
2. Use a long ruler to measure the distance between the two points.
3. Relate the measured distance to the scale given to get the ground distance.

How do you measure a curved distance?

There are three methods of measuring a curved distance: dividers, thread and the straight edge of paper.

• Pair of dividers: walk the dividers in small equal steps along the curve, count the steps and multiply by the step length.
• Piece of thread: lay thread along the curve, then straighten it against a ruler or scale.
• Straight edge of paper: mark the curve on the paper edge in short sections, turning the paper at each bend.

Of the three, the easiest and best is the use of thread.

How do you measure a curve with a piece of thread?

1. Place one end of a piece of white thread at the start of the route.
2. Stretch the thread along the curve gradually, carefully and accurately.
3. Follow the curve and do not let the thread move out of it.
4. Mark the end of the distance on the thread with a biro or pencil.
5. Straighten the thread and transfer the length to the linear scale.
6. Or measure it with a ruler and calculate with the statement scale or R.F.

Evaluation

1. What is a map scale?
2. Name the three types of scale.
3. State two differences between a small scale map and a large scale map.
4. If the scale of a map is 2 cm to 1 km, what ground distance is represented by 6 cm on the map?
5. List the three methods of measuring a curved distance and say which is the best.

Assignment

A map has the scale 1:100,000. Explain what the scale means, write it as a statement scale, and find the ground distance for 7.5 cm on the map.