Reviewing map reading
Marjorie Henry, Contributor

In last week's lesson, I started to discuss the map reading question given in the June 2000 examination. This week we continue the discussion of that question. Here is part (a) (ii):

Using the line scale, determine the length of the road (along the coast of East Harbour) from the junction of the Class B and Class C roads used in (i) to the junction of the road to Folly Point. Give your answer in kilometres and metres to the nearest 100 metres.

Using a scale to measure distance is the skill tested in this part of the question. The question requires that you use the line scale to do the measurement. Do you all know what that is and where it is found? Just in case there are some of you who do not know what this is about, let me explain. Probably I should go back a bit to define a scale. Our text tells us that a scale is the ratio of the size of the map to the actual land, for example, 1 centimetre on the scale represents 1 kilometre on the actual land. All maps must have a scale. This can be expressed in different ways:

1. In the form of a direct statement (i.e. expressed in words); for example, 'one centimetre to one kilometre'.

2. In the form of a fraction, where the numerator - 1 - indicates the length on the map and the denominator indicates the length on the actual ground. The fraction 1 sometimes written 1:100,000, indicates the scale of 100,000.

One centimetre on the map represents 100,000cm on the actual ground. In other words, one unit on the map represents 100,000 units on the ground. This fraction is called the representative fraction.

3. Sometimes a line is drawn and subdivided so that distances on the map can be directly measured and easily read from it. This is a linear or plain scale.

Scale of the map

If you have a copy of the map extract for the question, that of Port Antonio, look at the bottom of the map for the scale. You will notice that it has the scale expressed as a representative fraction and also a linear scale. Those of you without the map referred to should look at any map extract you may have.

To measure the length of the road, as indicated in the question, you can use a bit of thread or small cord. This you guide along the road with the help of a sharp-pointed pencil or a divider. Put a knot at one end of the thread or cord before you begin. Place that at the junction of the Class B and Class C roads, the starting point, then guide the cord along the road (to the east) to the junction of the road to Folly Point. Use your pen or pencil to mark the cord where you end. Extend the cord along the linear scale to obtain your distance. The point you marked on the cord to show the end of the distance being measured should be placed on the 1 km point of the scale. The remainder will fall in the section that has been subdivided into metres.

Make a note of the reading there. Please be aware that a bit of paper can also be used in the same way as the cord. Ensure that you have a straight edge before you begin. Simply fold the paper and run your fingers along the fold. Whatever you choose to use to do the measuring, accuracy is absolutely important.

The question has instructed you to give your answer in kilometres and metres to the nearest 100 metres. Pay special attention to this. Your answer for this question should be 1km 600m.

Practise measuring distances, whether along a road or between two points in a straight line (often stated as the crow flies), using whatever map extract you may have.

Students from across the island march along the Central Village main road in Spanish Town, St Catherine, while participating in the Jamaica Red Cross National Youth Rally, earlier this year. The rally is the organisation's major national youths development activity for the year, with approximately 1,000 Red Cross youths from 60 schools across Jamaica coming together to network. -Ian Allen/Staff Photographer