Truly the TruPulse   Robert Leverett
  Oct 10, 2006 13:13 PDT 


   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

   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

     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


Robert T. Leverett
Cofounder, Eastern Native Tree Society
TruPulse   Robert Leverett
  Oct 12, 2006 07:17 PDT 


   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

BA = Base angle = -1.3   from TruPulse, using the built-in height

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

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


Robert T. Leverett
Cofounder, Eastern Native Tree Society