A rhumb line (loxodrome) is a path of constant bearing (direction), which crosses all meridians at the same angle.
distRhumb(p1, p2, r=6378137)
distance in units of r (default=meters)
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
as above; or missing, in which case the sequential distance between the points in p1 is computed
radius of the earth; default = 6378137 m
Robert Hijmans and Chris Veness
Rhumb (from the Spanish word for course, 'rumbo') lines are straight lines on a Mercator projection map. They were used in navigation because it is easier to follow a constant compass bearing than to continually adjust the bearing as is needed to follow a great circle, even though rhumb lines are normally longer than great-circle (orthodrome) routes. Most rhumb lines will gradually spiral towards one of the poles.