Straight-line distance The following method should be used to measure the straight-line distance on a map (Fig. 12.6).
1. Mark the two places being recorded X and Y and draw a straight line linking both places.
2. Place the straight edge of a piece of paper on the line linking X and Y on the map. 3. Mark the edge of the paper where it touches both X and Y at either end of the straight line.
4. Place the straight edge of paper against the OS map’s linear scale, with the second mark (Y) placed at the last full kilometre that can be measured.
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On 1:50,000 scale maps, a ruler can also be used to measure the straight line distance between two points. Measure the distance with the ruler, e.g. 12.4 cm. To calculate the distance in kilometres, divide by two, e.g. 12.4 cm ÷ 2 = 6.2 km.
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SKILLS ACTIVITY
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What is the distance from Castlebridge Post Office (X) to Curracloe Post Office (Y)?
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Fig.12.6(a) Measuring straight-line distance.
X Y
Fig.12.6(b) Placing the Y at a whole number 4 allows a more accurate reading; you can use the broken line scale that comes before 0 to measure a part of a kilometre.
Curved-line distance
The following method should be used to measure the distance along a curved line (Fig. 12.7).
1. Place a straight-edged strip of paper at the starting point. Mark and number both the map and strip of paper (1).
2. Hold the edge of the strip of paper along the centre of the line being measured until the first turn is reached. Mark and number (2) the map and strip of paper at the point where they are no longer in contact.
3. Using the last mark (2) as the new starting point, hold the strip of paper along the centre of the line until the next turn. Again, mark and number (3) the map and strip of paper at the point where they are no longer in contact.
4. Repeat the process until the required distance is covered. 5. Place the straight-edged strip of paper against the map’s linear scale, with the last mark placed at the last full kilometre that can be measured. 6. Record the distance between the two points in kilometres from the scale.
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Curved-line distance: Typically a measurement along a road or railway line.
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Straight-line distance: Also known as the distance ‘as the crow flies’.
