An important property of a map is the scale. It can be indicated by a scale bar and/or a ratio 1:n. This enables the map user to measure a distance on the map and determine the distance on the ground. These days maps are usually produced under the metric system which makes it easy to perform this task. Maps are usually produced at standard scale factors of (say) 1:10,000 or 1:50,000 or 1:100,000 and all you need to do is take a measurement on the map (in mm) and multiply that figure by the scale factor to determine the distance on the ground.

Calculation: Measure 466 mm on a map which is scale 1 : 50,000 ... 466 x 50,000 = 23,300,000 mm ie 23,300 metres ie 23.3 kilometres

However, maps from some countries may use imperial units, especially older maps. These maps displayed scale ratios but often they were of the form 60 chains to 1 inch or 2 miles to 1 inch, etc.

In this case measure mm on the map and multiply by the scale factor below (where 1 yard = 36 inches, 1 chain = 22 yards) ...

40 chains = 1 inch >> 40 x 22 x 36 >> 1 : 31,680 (scale factor) 60 chains = 1 inch >> 60 x 22 x 36 >> 1 : 47,520 ( " ) 1 mile = 1 inch >> 63,360 x 1 >> 1 : 63,360 ( " ) 2 miles = 1 inch >> 63,360 x 2 >> 1 : 126,720 ( " ) 4 miles = 1 inch >> 63,360 x 4 >> 1 : 253,440 ( " )

Calculation: Measure 132 mm on a map which is scale 60 chains to 1 inch ... 132 x 47,520 = 6272,640 mm ie 6,272.6 metres ie 6.27 kilometres

Cartography | Maps

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