Mal Grey wrote:A rough and ready trace of the route on the map planner suggests 11 miles is right, so it might be your son's GPS thing that hasn't tracked properly. Did it also include mileage done by pottering about before or after the walk?
A map trace only gives horizontal distance ( 2 dimensions), going up and down adds to the length actually traversed.
GPS calculates distance between each reference point recorded, i.e. in three dimensions.
Think triangles, the hypotenuse is always longer than the base.