| RD
1000 - the latest modelings |
Robert
Leverett |
|
Sep
26, 2005 08:01 PDT |
ENTS:
Over the weekend, and with the help of PhD
candidate Nancy Rich from
Antioc Graduate School, 4 more white pines in MTSF were modeled,
the
Jani Tree, the Clutter Pine, the Joe Norton tree, and the Ed
Perle tree.
Volumes range from 411 cubes to 546, with the Joe Norton tree,
somewhat
surprisingly, being the lowest in volume. The 4 trees bring to
17 the
number modeled using the RD 1000 and laser rangefinder. What
lessons
have I learned?
Well, modeling with the RD 1000 is
superficially simple and
relatively quick, especially if you have someone to write down
numbers.
However, getting accuracy requires lots of checks and balances
and lots
of follow-on remeasurements. It is much more labor intensive
than first
modelings might indicate. As number accumulate, troubling
patterns
emerge that beg for resolution. As a consequence, there remain
lots of
questions to be answered about the method of calibration that
Laser Tech
used for the RD 1000 and why. One must learn how to deal with
the often
inconsistent results one gets from separate measurements of the
same
spot on a tree's trunk. Let me explain. Suppose I find a spot
that has
good trunk visibility and shoot the level distance to the trunk
and it
is 60 feet. I set this distance into the RD 1000 for its
diameter mode
usage. I then set the angle to the base of the tree using the RD
1000's
tilt sensor. After that, I'm ready to scan upwards from the base
and
read off diameters in inches at various heights above the base
on feet.
Now suppose I read a diameter at a particular point up the
trunk, using
the RD 1000, that I can easily identify from other locations. I
also
take a diameter reading with a D-tape and it is 32.3 inches. The
hope
is that I can come very close to the taped 32.3 inches with RD
1000.
However, there may be no value from the scale that is exactly
32.3
inches. For instance for the distance of 50 feet to level and
for the
angle of the spot on the trunk, the potential values that I can
choose
from are: ..... 31.2, 31.8, 32.4, 33.0, 33.6, 34.2, 34.8
.......As can
be seen, one value, namely 32.4 comes within 0.1 inches of the
taped
value, which might or might not represent the cross-sectional
distance
of the trunk at that point. However, the good news is that we
have a
reading from the instrument that is virtually the same as the
D-tape
result. The rub is that, this reading may not have appeared to
match the
trunk diameter at the particular distance. That was, in fact,
the case
in a test I ran. My eyes may be bad, but not that bad. The best
fit
appeared to be 33.0. Now had I been 100 feet away from the
target, the
sequence I would have to choose from was:
....... 28.8, 29.9, 31.1, 32.2, 33.4, 34.5, 35.7..... Here
again, we
have a number, 32.2 that is very close to the taped result and
that was
the best fit measurement also. So it appears that from closer or
farther
away there is some value on the RD 1000 scale that is very close
to what
could be gotten from a D-tape. But what if it isn't the most
obvious
reading. If you can't get to the tree to measure that spot, you
have no
immediate way of knowing in which direction the closer reading
might be.
At 120 feet, the RD 1000 sequence is: ....... 27.7, 29.0, 30.4,
31.8,
33.2, 34.5, and 35.9......... My best match reading was 31.5,
which
understates the diameter by 0.8 inches. My next best choice is
32.9,
which is 0.6 inches off. This pattern and the one developed by
John
Eichholz on the Saheda Pine led us to believe that at distance
of over
100 feet, the RD 1000's best match to the diameter is actually 1
to 2
readings higher than what appears to be the best fit as seen
through the
view finder. Oh Boy! What about at very close distances? It
appears to
go the other direction, i.e. the actual best fit is 1 to 2
readings
below what appears to be the best. One quickly comes to
recognize that
the built in tables of values represent compromises and it isn't
apparent in which direction the errors go in any given situation
- thus
the need for cross-checking to look for patterns.
Now, let's look at the impediments to
pointing and shooting and
relying on a single reading at a point up the trunk. The
diameter you
read can be off because:
1. You are too close or too far away and
in the zone where what you
see off, but by how much?
2. The target is not clearly visible
3. The sequence of values does not have
a single value that comes
close to the actual value, which you can't know unless you
measure the
tree.
4. The form of the tree is not round, so
that at a particular point
on the trunk, radically different readings at least partially
reflect
trunk form - but without you knowing it.
5. The scale cannot be expanded or
contracted enough to match the
diameter of the tree. Being unable to contract the scale
sufficiently
occurs regularly when high on the trunk for the very tall trees.
Conversely, being to close to a large trunk results in the
situation
where the scale can't be expanded enough. This expansion
situation can
always be resolved by backing up sufficiently. The opposite is
not true.
When the spot being measured is 130 feet up the trunk and the
trunk is
very narrow, the scale cannot be shrunk enough. Climbing the
tree is the
only option.
6. The tilt sensor is off calibration.
The amount and direction of
error here is hard to predict, but that has to be an effect. The
the
tilt sensor is just off level, but correct for interval
measurement,
then the problem is manageable. At present, this appears to be
the case.
I have to do more tests to determine how the instrument performs
on the
differentials as opposed to the absolutes. The one thing I don't
want to
do is to mix measurements from laser rangefinder-clinometer to
compute
height intervals with the intervals calculated by the RD 1000. I
made
that mistake once.
With Nancy Rich working with me, I have
the opportunity to do a lot
of work in the white pine stands of MTSF in the next few months.
This is
exciting because Nancy has lots of time to devote to the
questions were
trying to answer. I'll present those questions in another
e-mail.
Bob
Robert T. Leverett
Cofounder, Eastern Native Tree Society
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