Practical skills (part 5)
Marjorie Henry, Contributor

The practical skill that I revised last week was that of using the scale to measure distances. To reinforce the skill, here are three exercises given in reference to the map below.

(Source: Skills in Geography in Secondary School)

Give the approximate lengths of the road.

(a) From the intersection at the bridge southwards to the edge of the map.

(b) From Town A to Town B (use the edge of the circles).

(c) From the junction of the road south of the farm shed in a north westerly direction to the edge of the map.

Remember now that you must use a piece of thread or cord, along with a sharp-pointed pencil or a divider to guide the thread along the bends of the road. Aim at getting accurate measurements. Check your answers after you have done the exercises.

As I continue to discuss some of the practical skills you must master, I will now turn my attention to gradients. The specific objective 1.7 states that you must be able to 'calculate gradients using ratios'. The gradient is the inclination of a slope expressed as a ratio between the highest and lowest points and the horizontal distance between them. The gradient of a slope between two points is obtained by dividing the rise between those points by the horizontal distance between them. In reference to the diagram below, the gradient of the road AB is the rise BC over the distance AC. The rise is the difference in height and is also referred to as the vertical interval. The distance, that is, the horizontal distance, is also called the run.

There is a formula then for calculating the gradient of the slope. It can be expressed as

Another way of stating the formula is Difference in Height/Horizontal Distance

Note that the gradient is expressed as a fraction whose numerator is one. The horizontal distance AC is the distance as measured on the map. A common unit of measurement must be used when computing the answer, hence the need to multiply the kilometre by 1,000 to convert it to metres. There are 1,000 metres in one kilometre. The gradient of the road in the diagram above is, therefore, written as 1:25. Please note that your answer is in whole numbers. If after you do your division you have a remainder of more than half the numerator, then go to the nearest whole number. If the remainder is less than half, it is not included in the answer.

Whenever you are asked to calculate gradient, you MUST outline the procedure you followed. Here is a hypothetical situation.

'The spot height is 350m and the bridge is located at 150m. The distance between them is 6km. Calculate the gradient of the slope between the bridge and the spot height.'

Marjorie Henry is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com