Scale (map)

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A variable scale for measuring maps

The scale of a map is the ratio of a single unit of distance on the map to the equivalent distance on the ground. The scale can be expressed in four ways: as a ratio, a fraction, in words and as a graphical (bar) scale. A scale in words can be called a lexical scale.

The statement 'one millimetre represents 25 metres' is an expression of scale in words.

Scale expressed as a fraction, 1/25,000, means that any distance on the map is 1/25,000 th the distance on the ground. It expresses the amount of reduction of distances used to represent detail on the map. The 25,000 value is called the scale denominator.

Due to showing the curved surface of the earth on a flat map surface, the scale varies from place to place. Thus a representative fraction is stated for scale which is correct at the centre of the map and which varies elsewhere. While called representative fraction, it really is the representative ratio.

A graphical (bar) scale is a ruler with ground distances added, included in the margin of most maps. The graphical scale is used to measure distances on the map. The distance on the map is marked on the edge of a sheet of paper, which is then placed over the graphical bar scale and the distance read.

Maps are usually produced at standard scales of (say) 1:10,000, 1:25,000, 1:50,000, 1:100,000, 1:250,000, 1:500,000. The distance on the ground equals the distance measured on the map multiplied by the scale denominator.

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

Some older maps use imperial units expressed in words, such as 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)
60 chains = 1 inch >> 60 x 22 x 36 >> 1 : 47,520 (scale)
1 mile = 1 inch >> 63,360 x 1 >> 1 : 63,360 (scale)
2 miles = 1 inch >> 63,360 x 2 >> 1 : 126,720 (scale)
4 miles = 1 inch >> 63,360 x 4 >> 1 : 253,440 (scale)

Calculation: Measure 132 mm on a map with scale 60 chains to 1 inch

132 x 47,520 = 6272,640 millimetres
ie 6,272.6 metres
ie 6.27 kilometres

Maps are sometimes referred to by relative descriptions of large scale or small scale. A large scale map displays objects so they appear relatively large. For example, an island displayed on a 1:10,000 map will appear larger than if displayed on a 1:100,000 map. Thus, the former is large scale. Maps with a ratio of 1:50,000 or larger (for example, 1:25,000 would be larger) are considered large scale. Maps with a ratio of 1:50,000 to 1:250,000 are considered medium scale. Any maps with a smaller scale (for example 1:500,000) are considered small scale.^[1] A trick to remember this sometimes unintuitive description schema is to remember that small scales have tiny ratio values (i.e. 1:500000 = 1/500000 = 0.000002) and large scales have larger ratio values (1:500 = 1/500 = 0.002).^[2]