The "along track distance" is the distance from the start point (p1) to the closest point on the path to a third point (p3),
following a great circle path defined by points p1 and p2. See dist2gc for the "cross track distance"
Usage
alongTrackDistance(p1, p2, p3, r=6378137)
Value
A distance in units of r (default is meters)
Arguments
p1
longitude/latitude of point(s). Can be a vector of two numbers, a matrix of 2 columns (first one is longitude, second is latitude) or a SpatialPoints* object