On
a mercator:
Scale
= Chart distance = Scale at
equator x 1 .
Earth
distance
cos lat
Transpose:
Chart distance = Scale at equator x Earth distance
cos lat
Since
it is the same chart distance at 45ºN and 30ºN:
Scale
at equator x Earth dist. at 30ºN = Scale at equator x Earth dist at 45ºN
cos 30º
cos 45º
Or:
ED at 30º = ED at 45º
cos 30º
cos
45º
ED
at 30º = ED at 45º x cos 30º
= 70 x 0.866 = 85.75 nm
cos
45º
0.707