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At 60° N the scale of a direct Mercator chart is 1: 3 000 000. What is the scale at the Equator?
  • A
    1: 1 500 000
  • B
    1: 3 500 000
  • C
    1: 6 000 000
  • D
    1: 3 000 000

Charts represent the “reduced earth”.

The chart scale represents the relation between the length represented on the chart compared with the real distance on planet earth.

Chart Scale = Chart length / Earth distance

On a Mercator chart, scale expands away from the Equator.
The scale expansion is proportional to the secant of the latitude.
Questions about scale in a Mercator chart, come in 1 of 4 basic types:

  • Given scale at the Equator, find Mercator scale at some other latitude.

  • Given Mercator scale at some latitude, find scale at the Equator.

  • Given Mercator scale at some latitude, find Mercator scale at some other latitude.

  • Given a fixed chart distance between meridians, find Mercator scale at a specific latitude.

The equation to remember to solve this questions is the following one:

Scale denominator A x cos (Latitude B) = Scale denominator B x cos (Latitude A)

3 000 000 x cos (0º) = Scale denominator Equator x cos (60º)
Scale denominator Equator = 6 000 000
Scale at the Equator = 1 / 6000 000

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