| Truly
the TruPulse |
Robert
Leverett |
| Oct
10, 2006 13:13 PDT |
ENTS,
Over the long weekend, I took advantage of the
clear weather to make
good use of my TruPulse 200. It came in handy, but its
limitations were
also further confirmed. The TruPulse doesn't shoot through the
narrow
openings and through intervening clutter that the Nikon Prostaff
440
does. So, I found that I could not dispense with my Nikon in
dense
forests. In the open, the TruPulse was a pleasure to use.
In another experiment, I shot the height
of a straight-appearing red
pine on the property of the motel in which Monica and I were
staying.
Visibility was perfect. I used the built-in height routine and
compared
the results to the addition of the vertical components of the
top point
and base of the pine. The height determination via the built-in
routine
was too high by 6 feet. The reason was totally predictable. The
top of
the pine was horizontally offset from the trunk in my direction.
That
determination was easily made by taking the HD return when
shooting the
crown point and comparing it to the trunk HD distance. It was a
classic
case of shooting the crown angle of a point that looked to be
directly
over the base, but was not.
Now, hundreds, if eventually not
thousands, of adoring advocates of
the TruPulse 200 will routinely use the built-in height routine
and
swear that they are measuring height the right way. After all,
the gurus
of laser design gave them accuracy to under a foot, in most
cases under
half a foot. How could they be wrong? Simple. LTI followed
time-honored
methods of tree measuring that when adopted served the intended
purposes. Well, lots of folks once thought the Earth was flat.
However, there is more to this story
than a rant about using the
slope method for measuring tree height. There is a very
practical reason
that nons-serious tree measurers will discover for using the
built-in
height routine - despite its obvious flaws (obvious to Ents).
Measuring
angles with the TruPulse utilizes a tilt sensor that is not
dependent on
the distance of the object being measured or its visibility to
the
laser. What did I just say? Oh yes, there were those pesky
problems I
experienced shooting to the top of a tree through clutter. I
didn't get
through. But if I could see the trunk at some point, see the
base, and
see the crown, I could use the built-in routine to get a
calculated
result. The angles could be measured even if distances to the
targets
couldn't.
The time-consuming challenge of finding a
vantage point where one can
get a laser bounce from the highest crown points and a laser
bounce from
the base will be sure to frustrate most users of the TruPulse
200.
Perhaps the designers understood this. Then again, perhaps they
didn't
and just followed the time-honored method as in the Earth is
flat
thinking.
Now the big question for the list
is this: Will Bob cheat and use
the built-in routine when he can't get the laser to see the
crown or
base points? No, he will use his trusty Nikon Prostaff 440 or
his even
trustier Bushnell 800, unless he's just getting an idea of
height? But
he will not report a height that is based on the approximating
tangent
method. To do so would bring down the wrath of the Dale
Luthringer tree
spirit, the Will Blozan tree spirit, the Jess Riddle tree
spirit, the Ed
Frank spirit, etc., etc., etc. on his head. Fellers, I promise
not to
cheat!
Bob
Robert T. Leverett
Cofounder, Eastern Native Tree Society
|
| TruPulse |
Robert
Leverett |
| Oct
12, 2006 07:17 PDT |
ENTS,
Yesterday evening I took the opportunity to
continue collecting data
on the TruPulse 200. My intention was to first shoot a tree with
the
TruPulse using the built in height routine and then separately
obtain
the vertical components of the tree, using the VD returns of the
instrument. I would then compare the results. The target tree
was a
handsome sugar maple street tree next to a neighbor’s house.
Results of
the test follow.
HT = Horizontal distance to trunk = 150.5 (HT is the return from
the
TruPulse, using the built-in height routine)
CA = Crown angle = 26.9 from TruPulse, using the built-in height
routine
BA = Base angle = -1.3 from TruPulse, using the
built-in height
routine
HR = Height from built-in routine = 79.8 ft from TruPulse, using
the
built-in height routine
HC = Height from eye level to crown = 76.4, breaking up the 79.8
feet
into two components with HC being height above eye level
HB = Height from eye level to base = 3.4
, breaking up the 79.8
feet into two components with HC being height below eye level
The above sequence flows from the built-in
height routine, which
computes the maple’s height as 79.8 feet.
Shooting the tree again, using the VD
returns, I got the following.
VD1 = 66.5 from TruPulse using the VD return from shooting the
crown
point
VD2 = 3.5 from TruPulse using the VD return from shooting the
base.
The difference between the above 3.4 and the 3.5 results from
one being
a calculated result and the other being a distance return from
the
laser. The TruPulse returns distances to the nearest half foot
for the
SD, VD, and HD modes.
HVD = 70.0 additive result of VD1 and VD2. So, this alternative
approach leads to a height of 70.0 feet for the maple.
Is the height of the maple 79.8 feet or 70 feet? The difference
is a
significant 9.8 feet, worth resolving.
The answer is simple. The horizontal distance to the crown point
using
the HD return from the TruPulse is 131.0 feet while the
horizontal
distance to the trunk is 150.5 feet. The crown point is offset
by 19.5
feet. Had the 131-foot distance been used in the height routine,
the
vertical component HC would have been as shown below.
HC = tan(26.9)*131.0 = 66.5. This matches the
VD1 return of 66.5
feet. The height routine calls for obtaining the horizontal
distance to
the trunk, which is 150.5 feet. But a separate shot to the crown
point
using the HD return shows it to be at a horizontal distance of
131.0
feet – not 150.5. Therefore, shooting the trunk, using the
built in
height routine can lead to an obviously unreliable result. No
news here.
We’ve known it for years, but how simple it would have been
for LTI to
have based their height routine on first the crown shot and then
the
base shot – the equivalent of consecutive VD returns, but
added together
for the user.
I can think of only one legitimate
reason that LTI didn’t do it. It
is because, unless you have a clear shot at the crown target,
you can’t
get a laser distance return, whereas you can get the crown
angle. You
can usually find a spot on the trunk to get a laser distance
return.
That’s all fine and well, but the built-in tangent-based
height routine
will frequently return erroneous results. Being of 3 or 4 feet
on
conifers in a plantation may carry no severe penalty, but
applying the
routine to broad-crowned hardwoods is a disaster.
Since the TruPulse has the individual
returns of SD, VD, HD, and INC
all from one target shot, as explained in a previous e-mail, the
user
can compute the height of the tree accurately by adding the VD
return
from a shot to the top or bottom, followed by a shot to the
other end,
and adding the two VD returns. No big deal. Still, it would be
nice to
have a built-in feature to take two shots and get a calculated
height.
As a final comment, my distance
from the tree was sufficient to see
the highest crown point visible from the side I was on. At 150.5
feet
away. My angle to the top was a modest 26.9 degrees. So the test
wasn’t
a case of being too close to the target to see what appears to
be the
top point. Had I moved closer, the difference between the VD-VD
calculation and the built-in height routine calculation would
have
become greater, because I would have been hitting the ends of
outstretched branches.
As I pointed out in a previous e-mail, the
TruPulse 200’s VD returns
are equivalent to our sine top-sine bottom method. So anyone
with money
burning a hole in his/her pocket may want to buy the TruPulse. I
absolutely love the instrument – but I cannot function
effectively in
the forest without my Nikon 440 or Bushnell 800. One can’t
have too many
gizmos.
Bob
Robert T. Leverett
Cofounder, Eastern Native Tree Society
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