Wednesday, January 12, 2011

Know Your 800 Pound Gorilla, Part Two: Determining Tree Height

Foresters are usually more concerned about merchantable height in trees than the overall height, but if you are cutting trees down, you need to be able to determine how tall a tree is on a fairly regular basis.  You need this information in order to tell if a tree will fall short of things you do not want to hit, such as power lines and houses.  You also need to know the height to estimate whether you can handle the back lean and side lean of trees before you stick your saw in them.

A quick and fairly accurate method for height measurement is nothing more complicated than a stick in your hand for creating similar triangles.  Look at the diagram above, (Click on photo to enlarge.) and note that segment ab is the same length as segment bc.  Point a (A), is your eye, which must not move due to head bobbing during the measuring process.  If you set up your stick properly, the distance AB will be equal to distance BC.

Back away from the tree you wish to measure, tip your head back so your eyes can easily roll to the top of the tree, extend the arm which holds your stick, and place the top of the stick at the corner of one eye. 

 Bring the stick up to vertical, and sight over the top of your hand to the bottom of the tree. Holding your head still, roll your eye up to the top of the stick and compare it to the top of the tree. If the tree stands taller than the stick, it can reach you, so back up and re-measure. If the stick stands taller than the tree, move in closer. When the segment bc matches BC, you are the same distance from the tree as the top of the tree is from the ground.

 This is about how it looks as you move forward and back to adjust your measurement.

Next, you pace in to the tree to determine the height.  Pacing is the quick, easy way to measure distances and height while working in timber.  Briefly, a pace is two normal steps.  Foresters alway start with the left foot, and count each pace on the right foot.  Set up a measured course for determining your pace, and check yourself occasionally.  Biltmore sticks, which are made for measuring merchantable height in trees, are set up to be used at 66 feet from a tree, or one chain, so every forester knows how many paces he has per chain.  A course 100 feet long is also handy, and in much of the country, you can pace across twenty acres to go ten chains.

Hardwood trees usually have broad crowns, so the top center of the tree is often hard to spot.  You have to be careful not to measure to the edge of a broad crown, or the height will be exaggerated greatly.  Practice this a bit while the leaves are down to get a feel for it. 

Next time: Diameter measurement, and segments.
Link Back to Part 1

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