Latitude Scale as Distance Scale

Definition

One minute of latitude equals one nautical mile by definition, so the vertical border of a Mercator chart is the distance scale. Because the projection stretches latitude with the secant of latitude, dividers must be applied at the mid-latitude of the leg being measured, and a long leg is measured in short steps. The longitude scale along the top and bottom is never used for distance, since a minute of longitude shortens with cosine of latitude.