Re: Height Recording   Colby Rucker
  May 18, 2004 14:39 PDT 

You state that you're measuring items of known height, but the numbers don't come out. Perhaps the "known" heights aren't accurate.

There are two things you can do with your rangefinder and clinometer. The first is to measure trees - lots of trees of all sizes, contribute some data and have lots of fun doing it. The second is to get all balled up in worrying about calibration, light effects, cosines, tangents and so forth, which will frustrate you, and won't lead to anything useful.

So, simplify, simplify! The fewer variables you crank into your work, the more consistent you will be. Always back up to clickover, and always establish a target a measurable distance above the central basal contour. If you can eliminate the lower triangle, do it. Measure angles to 1/10 degree, and taped distances to the nearest 1/2 inch.

Blast away with the laser - your first laser shot can be a yard too long. If the crown structure allows you to take a near-vertical reading, do it. That will almost eliminate clinometer error.

If there's only a small window, and you can't back to clickover, kneel, squat, sit, lean, etc. to get clickover, and sight level to your target on the trunk.

You will be able to devise all sorts of reliable approaches to measure problem trees as you go along, but all of them will be dependent on a clear vision of how each triangle is part of the whole, and you'll want to use the simplest approach you can to avoid overlapping or separating triangles. The pole method is slower than just shooting two triangles, but it's much more accurate. In all cases, pay as much attention to locating the true base of the tree as you do in sighting the top.

So, Simplify, Simplify! Good solid techniques will result in consistency, so that you'll get compatible readings in the future. Forget all the complicated stuff, and have some fun!

RE: Height Recording   John Eichholz
  May 15, 2004 08:37 PDT 


As a recent convert to tree measuring, I can say you are not alone. It
does take some getting used to and practice to get consistent readings
you can be confident of. Having someone else try the equipment, or
comparing your results with Bob's is a good start. Barring any obvious
misunderstandings, you should be within a few feet (3 to 5) regardless
of the technique you use. With experience you will be able to see how
different techniques affect the results. I am still trying to adapt for
what I call "phantom bounces" from the rangefinder. By grazing the edge
of a limb I can often get a higher reading, probably due to an imbalance
of the harmonics of the lightwaves returning to the sensor. I have also
noticed an effect of the background sky, with clear blue skies giving a
longer reading than cloudy skies, probably due to the effect on the
sensor of background radiation from the sky.    The clinometer also
sticks in cold weather, which is important to accomodate with technique.
These quirks can (and have to) be dealt with to get consistent readings
within two feet or less. Practice in a forest setting, take your time,
go back to the same spot from time to time and compare your results.
Once you can get consistent results you are probably getting accurate
results as well.

RE: Height Recording... Error checking    Paul Jost
   May 15, 2004 09:46 PDT 

Phil, and others,

When measuring trees with a rangefinder and clinometer, you MUST verify your
measurements and calculations with law of cosine or difference of cosine
calculations to see if the points make sense.

On conifers, the height should usually be very close to the cosine law
calculation. On trees whose peak is over the base, the height will be
approximately equal to the straight line distance. I have had trees where
the difference was less than 0.01%. It will usually be within 1% on
conifers. If they aren't, then you measured other branches in the line of

Similar errors on spreading hardwoods or multileader conifers can be
detected in the field with the difference of the cosines calculation. We
calculate heights by summing the vertical components of the triangles whose
hypotenuses we had measured with the rangefinder and whose hypotenuse's
angle of inclination or declination that we had measured with the
clinometer. [The vertical components of the triangles are the product of
the range and the sine of the clinometer angle. (If both the upper and
lower triangle are above or below our position, then the base triangle must
be subtracted from the peak triangle.)

Furthermore, if, instead, you calculate the difference of the horizontal
components, then you can visually compare the offset of the peak and base on
the ground to see if it looks like it could be the branch or leader that you
intended to measure. Again, large differences or calculations far from what
we expected will imply a measurement in error. [Remember, the horizontal
components of the triangles are the product of the range and the cosine of
the clinometer angles.]

If you don't have a feeling for the ranges that you should be getting, then
you really should do additional calculations to verify your original
readings. I use a programmable calculator to store the four measurements
(two ranges and two angles) and then automatically calculate all these
formulas in the field to speed things up and make their use practical. Even
if you don't verify these measurements in the field, you should postprocess
the raw measurement data when you return from the field. You can usually
return to correct bad data.

For reference, the calculations are as follows:

The law of cosines is a modification of the Pythagorean theorem to allow it
to be used on triangles without a right angle. Using it on our range/angle
data to endpoints of a tree will produce the straight line distance between
those points. The calculation is as follows:

A=distance to peak
a=angle to peak
B=distance to base
b=angle to base
C=straight line distance between peak and base
D=plumb line or vertical component distance between peak and base
E=horizontal offset of peak from base in plane of measurement

Keep in mind that clinometers give positive measurements to angles above the
horizon (inclination) and negative measurements to angles below horizontal

Vertical or plumb height:

"As the crow flies" or straight-line distance between base and peak:
C=(A^2 + B^2 - 2*A*B*cos(a-b) )^(1/2)

where ^2 means squared and ^(1/2) means square root.

Horizontal offset of peak from base:

Hopefully, I typed that all correctly...


Paul Jost