This calculates distance between points on the earth, measured
along the surface. The method involves the solution of the geodetic
inverse problem, using T. Vincenty's modification of Rainsford's
method with Helmert's elliptical terms. If the first argument is an object of class section, the result
is vector containing the distances of the individual stations from the
first one in the section.
If the first argument is a list, then the "1" and "2" lists are of
equal length, then the result is the pairwise distances. However, if
the length of "2" is shorter than the length of "1", then only
the first value in the "2" list is used, repeated over and over to
match the length of "1".
A common use is for "1" to contain a vector of positions along a
cruise track, and for "2" to contain a reference point;
e.g. geod.dist(lats,lons,lats[1],lons[1]) gives distance along
the track starting from zero.