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

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

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
sight.
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
(declination).
Vertical or plumb height:
D=A*sin(a)B*sin(b)
"As the crow flies" or straightline distance between
base and peak:
C=(A^2 + B^2  2*A*B*cos(ab) )^(1/2)
where ^2 means squared and ^(1/2) means square root.
Horizontal offset of peak from base:
E=A*cos(a)B*cos(b)
Hopefully, I typed that all correctly...
Regards,
Paul Jost

