In a few episodes of "Man vs. Wild", survival expert Bear Grylls was seen jumping off a cliff into a pool of water below. He made some rough estimation of the depth of the pool before making the decision to jump. I am puzzled as to how he managed to do it with just quick visual observation. Any wrong judgement could lead to a broken leg or even death.
If the terrain becomes too difficult, a jump from the cliff to the pool of water below might be the only way to make any progress. How would a person go about estimating the depth of the pool and the height of the jump by quick visual observation? Is there a proper guideline for jump height versus pool depth?