As the clinometer is rotated through angles (a) and/or (b) it reports height
in relation to the distance (C)

To measure tree height, walk the distance (C) for which the clinometer
is calibrated. Look at the top of the tree and note the height on the scale,
and then look at the base of the tree and note the height on the scale.

Note: The clinometer will read 0 when the observer is looking at a point
equivalent to eye level. In most cases the base of the tree will be below
eye level and will show up on the negative scale. If the base of the tree is
above eye level, then the lower value should be subtracted from the upper
value rather than added.