Consider the triangle made up of:
A: 61°30'N on first meridian
B: 58°30'N on second meridian
C: 58°30'N on first meridian
The track AB at A is 120°T so the angle in the triangle at A = 180° -
120° = 60°
The angle in the triangle at C = 90°
Side AC = 3° latitude = 180nm
AB = hypotenuse (h) and AC = adjacent (a) to angle A
CosA =
a/h therefore h = a/cosA = 180nm/cos60° = 360nm =
667.3km
Scale = CD/ED = CD/667.3km = 1/1,000,000
Therefore CD = 667.3km/1,000,000 = 66,730,000cm/1,000,000 = 66.73cm