Ordnance Survey maps with a scale of 1cm:50 000cm (which means 1 cm on the map is 50 000 cm on the ground or 1 cm is 500 m) have boxes called grid squares. The side of a full grid square measures one kilometre (km).
Calculate a regular-shaped area
This method works for any regular-shaped (rectangular) area you wish to measure.
1. Count the number of grid squares across the base of the area you wish to measure.
2. Count the number of squares going up the side of the area. 3. Multiply both numbers. This gives you your answer in square kilometres (sq km/ km²).
Calculate an irregular-shaped area
The total area of each grid square on a 1:50 000 map is 1 km². To calculate the approximate area of any irregular-shaped feature, e.g. a lake, a bay, a forest, use this method.
z„
Tick off and count all the squares that are at least half-filled by the feature. This number will represent the approximate area of the feature in km².
7
85
92 93 94 95 96
85
84
Square (almost!) completely filled with the irregular- shaped feature (Westport Bay)
83 83 82
2. Westport Bay, Co. Mayo
82 81
Square more than half-filled with the irregular- shaped feature (Westport Bay)
92 93 94 95 9681
1. Calculate the area of the map in figure 2. 2. Calculate the area of the bay in figure 2.
