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On a New Zealand Aeronautical Chart
The ratio of the length of a line on a chart to the actual distance represented,
e.g. a scale of 1,000,000 (1 to 1,000,000) literally means that a line of 1 cm on the chart represents 1,000,000 cm on the surface of the earth.
There are a number of methods used to describe chart scale.
For example, the scale of a chart can be expressed as a fraction eg. 1/1,000,000 or 1:1,000,000 (one in one million).
One centimetre on the chart will therefore represent 1,000,000 cm on the earth’s surface.
One nautical mile on the earth’s surface will be represented on the same chart by a distance equal to one millionth of a nautical mile.
Graduated Scale Line
The graduated scale line is usually located at the bottom of the chart.
Using a graduated scale line the distance between any two points on the chart can be measured and compared with the scale line.
The correct scale line (nautical miles) must be used as there may be several scales for different units (such as nautical miles, statute miles and kilometres).
Even if there is no scale line provided on a chart, it is always possible to measure distance by comparison of the chart length, between any two points, with the latitude graduations marked on the chart (but not longitude), given that 1′ of latitude = 1 nm, and 1° of latitude = 60 nm.