Consider a route on a map between two waypoints. Google Maps will calculate the distance between the two, and display it for you. This is very handy. Thank you Google Maps!
Consider a route where the route is straight, and the distance 'as the crow flies' between those points is 1 mile. Change that scenario to include a hill at a 30 degree slope between each point.
If the route contains hills with significant inclines or declines, does the calculated distance represent the true distance?