The following method should be used to calculate the area of an irregular area (Fig. 12.9).
1. Using a red pen, tick all of the squares that are at least half-filled by the feature being measured.
2. In cases where one is unsure whether the square is at least half-filled with the feature being measured, divide the square in half with a diagonal, vertical or horizontal line using a ruler and pencil.
3. Number the squares that have a red tick – this number represents the approximate area of the feature in square kilometres.
Example (Fig. 12.9): 15 grid squares are more than 50% filled with water. Therefore, the area of the map covered by the sea is 15 km2
. 04 24 23 1 22 2 3 21 04 05 06 6 07
4 5
7 8
9 08
10 11
12 09
13 14
15 21 10
Fig.12.9 Calculating the area of an irregular shape. Notice how the grid square boxes numbered 1 and 3 have been divided using a diagonal line to clarify that over 50% of the grid square is filled with water.
CHECK YOUR LEARNING
Examine the 1:50,000 Ordnance Survey map of Kenmare on page 194 and answer the following questions.
1. What direction would you be travelling if you went from: a. The post office in Kenmare to the car park at Moll’s Gap? b. The post office in Kenmare to the post office in Assroe?
2. What is the straight-line distance from: a. The peak of Knocknaguish (V 919 768) to the peak of Derrygarriff (V 872 774)? b. The post office in Kenmare to the post office in Assroe?
3. What is the curved-line distance along the N71 from where it enters the map at V 940 684 to the point it crosses the Kenmare River at V 912 698?
4. What is the total area of the map? 5. What area of the map is covered by the Kenmare River?
05 06 07 08 09 10 24 EXAM TIP 23
Number each grid square being counted so as to avoid a mistake.
A-Z
Irregular-shaped area: This is typically used to measure the area of: • a town • an island • a body of water (sea or lake)
• an area of forestry • an area of upland.
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SKILLS ACTIVITY
What area of the map in Fig. 12.9 is covered in land?
