Map Reading And Measurements Of Distances On Maps SS1 Geography Lesson Note

INTRODUCTION TO MAP READING

The map reading aspect of geography is often referred to as Practical Geography. Map work or map reading involves the ability to recognize conventional signs on a map and use the signs to interpret the map. 

A map is a representation of the earth’s surface as it is seen from above on paper. A large area of land e.g. town, village or even a whole country can be represented on one page of paper.

Maps are of different types. They include: 

1. Topographical Maps 
2. Atlas Maps 
3. Plan Maps and 
4. Sketch Maps 

MAP SCALE

The Scale of a map indicates the relationship between the distance of two points on a map and the actual distance on the ground. That is, the map scale is the ratio, proportion or relationship between the measurement on the map and the actual measurement on the ground.  If the scale of a map is 2cm to 1km, it means that two centimetres on a map represent one kilometre on the ground.

TYPES OF SCALE 

We have three types of scale. They include the Statement scale, Linear scale and Representative scale. 

1. Statement Scale 

This is a scale that is given in the form of a statement and figures e.g. 1cm to 2km (means 1 cm on the map represents 2 km on the ground), 2cm to 1km (means 2cm on the map is equal to 1 km on ground).

Question: 

If the scale of a map is 2cm to 1km, what is the ground distance if the distance between two towns on the map is 10cm? 

Solution

Map distance = 10cm 

Ground distance =?

Map scale = 2cm to 1km 

Since 2 cm = 1 km 

10 x 1 =? 

10 x 1 = 2 x x (cross multiply) 

10 = 2x 

: .x = 10= 5 km

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2. Linear Scale: This is a line drawn to represent the relationship between the distances on the map and that of the actual distance on the ground. The linear scale is divided into two parts: primary divisions and secondary divisions. The primary division is the major division on the right side while the secondary division is the minor division on the left. 

HOW TO USE LINEAR SCALE 

(i) Measure the distance between the two places requested on the map either with a ruler or a thread. 

(ii) Take the distance measured in cm or inches to the scale and begin the measurement from the Origin marked zero (0) and stretch it to the right (Primary division). Where it ends represents the distance on the ground either in km or in miles. 

(iii) If the length is longer than the scale, first record the distance where the scale ends and then measure the remaining part again starting from zero (0) to the right and finally mark where the distance ends. Later add the first measurement with the last one. 

(iv) If any fraction is left which is far smaller than the major division, such fraction is taken to the Secondary division starting from zero (0) to the left. This length is finally added to the first. 

3. Representative Fraction: Here, the scale is expressed in the form of a fraction or ratio. Thus, the distance on the map is the numerator and is always one (1) while the denominator stands for the distance on the ground. Both the numerator and the denominator are in the same unit. 

E.g. 1: 100,000     1: 50,000     1: 200,000 etc 

Note: 1: 100,000 is the same as 1 cm to 1km while 1:50,000 is the same as 2cm to 1km or 1cm to 1/2 km

HOW TO USE REPRESENTATIVE FRACTION

(i) Note the two places involved

(ii) Measure the distance between the two places 

(iii) Relate the distance measured to the scale 

Question: 

If the scale of a map is 1:50,000, What is the ground distance if the distance between two towns on the map is 10cm? 

Solution

Map distance = 10cm 

Ground distance =? 

Map scale = 1:50,000 

= 1 cm = ½ km 

: 10cm = 5km 

Ans = 5km

SIZE OF A SCALE

The size of a scale may be large or small 

1. Small Scale Map: This shows a large area but cannot include a great amount of details. Only very important features are shown. 

The larger the denominator of a scale, the smaller the scale E.g. 1:1,000,000 1: 2,000,000, 1:250,000 etc 

2. Large Scale Map: This shows a small area but with more details of the area and also important features are shown. The smaller the denominator of a scale, the larger the scale e.g. 1:5,000, 1:250 and 1: 20 etc 

MAP DISTANCES

Distance on a map is the interval between two points on a map. It can either be straight or curved. 

MEASUREMENT OF STRAIGHT DISTANCES

(i) Locate the places involved on the map. 

(ii) Use a long ruler to measure the distance between the two points or places.  

(iii) Relate the distance measured on the map to the scale given and get the ground distance.

MEASUREMENT OF CURVE DISTANCES

This can be done through three methods:

(i) The use of a pair of dividers.

(ii) The use of a piece of thread.

(iii) The use of the straight edge of a paper.

Among the three methods, the easiest and the best is the use of thread.

1. Use of a piece of thread: Stretch a piece of white thread along the route or curve that is to be measured gradually, carefully and accurately. Carefully follow the curve and do not allow the thread to move out of the curve. Mark the end of the distance on the thread with a biro or a pencil and transfer it to the linear scale or calculate the distance in a statement or R.F Scale.