ABC's of Measuring Tree Heights
With a clinometer, a measuring tape, and a little mathematics (calculators are OK), it is relatively easy to determine tree heights. First, and the clinometer. It has two scales, most often a percent and a degree combo. The degree scale (left side) is necessary to convert slope distance from the tree to horizontal distance. If you are on flat ground, or level with the tree, no conversion is needed. The percent scale (right side) is used to take the height of the tree. Now put the clino to your right eye, keep BOTH eyes open and view the horizontal line. This is what you "aim" with. Practice superimposing that line with your left eye while tilting you head back and forth. You will need to "shoot" the top of the tree and base of the tree, requiring a decent view of each. The readings you take are based on being 100ft from the tree (this is an absolute requirement to remember, based on the science of math). Don't worry if you can't measure 100ft from the tree. For example: if you measure 75ft, you would multiply your result by 0.75; or if 48ft, you would multiply by 0.48 to determine the actual height.
Now you are ready.
Let's start on flat ground and measure 65ft from the tree. Look eye level at the tree (percent scale at Zero). You will have a minus percent to the base (usually about 8%) and a plus percent to the top. These will be added together (above and below eye level) to get total height. Now, let's say the top measurement is +132 and the base measurement is -8; your total is 140. Multiply that by your distance from the tree (in this example 0.65) to get a total height of 91ft for the tree.
Now you are on steep ground, and best view of the tree is sidehill AND uphill, so your distance measurement is slope and not horizontal, and must be converted. Take a reading using the left scale (degree) at approximately your eye height relative to the tree.