Rhumbline vs Great Circle
13 June 2008
The great circle on a spherical surface is the path with the smallest curvature, and, hence, an arc is the shortest path between two points on the surface. The distance between any two points on a sphere is known as the great-circle distance. The great-circle route is the shortest path between two points on a sphere; however, if one were to travel along such a route, it would be difficult to manually steer as the heading would constantly be changing (except in the case of due north, south, or along the equator). Thus, Great Circle routes are often broken into a series of shorter Rhumb lines which allow the use of constant headings between waypoints along the Great Circle.
Most nautical charts are based on Mercator Projections and hence my manual charting and plotting will be straight lines aka rhumb lines.
The difference in distance between the rhumb line and great circle routes from the Golden Gate to Kaneohe Bay, Oahu is about 7 miles.