Hi Chris - sorry I missed this one when you first posted the question. The simple answer is no, the measurement is 2D, but the extra distance you travel horizontally by running up hills is negligible.
Take a look at the following example of a runner running 5 miles up and down a hill from A to B (not to scale!). The total climb is 60m (a typical average climb of 12m per mile).
If we split the hill into two equal triangles, we can calculate the length of the slope (the hypotenuse) of each triangle using Pythagorus’ theorem: a2 + b2 = c2.
The length of the each slope (c) would therefore be √(4023.362 + 602) . So, we get the following results:
Route length measured in 2D = 8046.72m.
Route length measured in 3D = 8047.62m.
A difference of less than 1 metre!
Even with very hilly routes such as those experienced by fell runners, the extra distance travelled from running up and down hills is negligible and well within the normal margins of error of any measurement system.
John
Does the distance calculation take into account "3D" distance? In other words, If I run up a steep hill, will it measure the distance taking into account the slope or as if it was a flat distance (shorter)?