An eye for the forest

Here two boys at the camp measured the slope a patch of ground using a kilometer. One boy held it up to his eye, while pointing it at the other boy, who was about the same height. The instrument told him the difference in slope between the two of them was 10 percent.

Then the boys drew the landscape around them in their camp journals.

I really like this photo, as both of them look so intense on what they're doing