Mercator modification equalised the North-South and East-West scale expansion making it orthomorphic
Scale expansion of a Mercator chart in the the North-South and East-West is proportional to secant latitude
Scale at Any Latitude = Scale (Equator) x Secant Latitude
Mercator is a Non-Perspective Orthomorphic Cylindrical projection
Scale Expansion and Orthomorphism of Mercator Charts
Large Areas and High Latitudes distorts shape of natural features in a Mercator Chart
Chart convergence is zero everywhere making it correct only at equator but constant throughout the chart
Scale at any latitude can be applying secant of latitude to equatorial scale
Scale expansion is less than 1% within 8° North and South and less than ½ % within 6° North and South
ICAO accepted 1% scale error as a constant scale chart
Therefore, Mercator can be considered to be a Constant Scale Orthomorphic chart between 8 degrees North and South latitudes
Outside 8 degrees north and south, scale expansion is more than 1 % so the scale printed on the chart cannot be used directly but should be measured using local scale
Rhumb Lines and Great Circles
Rhumb lines are straight lines since it cuts all meridians at same angle
Equator and parallels of latitudes are straight lines
Meridians equally spaced straight lines perpendicular to equator
Great circles are curves concave to equator or convex to nearer pole
Equator and meridians are straight lines since they are also rhumb lines