But from back in my parachute days I know that winds ~50 feet are often five to perhaps ten knots higher than on the ground. They can also be in a different direction. This is a result of surface friction and other obstructions (trees). As a result when I'm using a hand held anemometer I add five as a rule of thumb. Your idea of comparing with another is good, but remember theirs may be off also. I only use the anemometer as a guide and perhaps for "bragging rights". I base my actions on sea state and how the boat is actually handling the conditions, not the "measured" wind speed.