@Mushroom
But it is not a triangle. It is trying to closely estimate with visual representations a three dimensional traverse from a two dimensional depiction.
It is, in fact, a triangle. The map distance gives you a horizontal distance. The contour lines give you a vertical distance. That gives you two vectors โ both distance and direction โ at right angles to each other. Those vectors form the two legs of a right triangle. The actual three-dimensional distance between the points is the length of the hypotenuse of that triangle. And, as I said, the length of the hypotenuse is not the sum of the legs. It is, per Pythagoras, the square root of the sum of the squares of the legs.
For example, if you walk 4 km map distance up a constant slope that rises 200 meters over that distance, the actual distance you have traveled is not 4000 + 200 meters. It's โ(4000ยฒ+200ยฒ) == 4005 meters.
The sum of the legs is the upper bound of the length of the hypotenuse (the lower bound is the length of the longer leg), and only accurate as one leg or the other approaches zero โ if you're traveling across a perfectly flat plain or climbing a vertical cliff.
Of course, simply finding the three-dimensional distance between the start and end points is only exact for a constant, smooth slope. For more complex curves, you can get better approximations by breaking the calculation up into the sum of a series of smaller ones โ the smaller, the better. (This is the basic premise behind calculus.)
If you're measuring on a topo map, in practice the accuracy of the approximation will be limited by the resolution of the contour lines. The best approximation you'll be able to get is measuring the distance between each successive pair of contour lines along the path and calculating the hypotenuse of the triangle with the base the distance between lines and the altitude the contour resolution, then sum all of those small approximations.
In theory, you can imagine making those measurements closer and closer together, until they're infinitesimally small and the series of tiny hypotenuses exactly conform to the terrain.
Trust me, I have been doing this for decades. It can never be exact, as not every aspect even on a map is "perfect". Especially as the maps normally give increments in 10-20 meters, and does not even count in any additional changes that might happen in the distance between contour lines. But it is far closer than just the distance taken alone without adding the additional vertical distances figured in.
The Marines taught you a method of making a rough approximation that you can do in the field without needing to have a calculator or figure square roots in your head. It may even be more useful for figuring the difficulty of traversing the terrain than the actual geometric distance. But that doesn't mean that it's an accurate measurement of the actual distance, which is what we're talking about here.