When someone specifies a "line" between two points on the surface of the earth, they will usually be referring to one of two types:

* loxodromic - constant bearing, and shown as a straight line in Mercator

* geodesic - conceptually closer to the "straight line"

Drawing a way representing a loxodrome is easy - plot two points, draw a way between them, project it in Mercator, and add intermediate points on the resulting straight line if necessary (i.e. if ways are not already assumed to be loxodromes).

Here comes the problem - I need to draw a line which is defined as a series of geodesics between points, rather than loxodromes. How on earth do I do it? (No pun intended)