Measuring difficulty is a very subjective thing to do. For example, hiking on the PCT some may consider easier than the AT and they might be right, but getting water is significantly more challenging on the PCT particularly in the desert sections.
Some people may find distance very difficult, while others can walk on flat ground for 100s of miles but at the sight of a hill crumble and quit.
With all of this in mind we have tried to come up with a basic system for grading trails difficulty, that we override where we think there are exceptional factors that affect a trails difficulty.
The core of what we do to decide on the difficulty of a trail is elevation gain per kilometer of trail. On this scale, one of the most difficult trails on Hiiker for example is infamously punishing New Hampshire section of the AT, where you will gain, on average 69 meters of elevation for 1000 meters of distance.
Whats important to remember is that this method is far from fool proof. For example you can have a section that is only moderately inclined but can have massive boulders on it, making it much more difficult than could be perceived from the data.
We may have missed some, so we ask you to let us know or pop me an email (firstname.lastname@example.org) if you see a trail that you feel should have a higher/lower difficulty ranking and why. We have started adding test to our difficulty where we override it to explain our thinking.