This monocular thing   Will Blozan
  Jan 29, 2006 13:46 PST 


Several posts in the last few weeks have mentioned the use of a monocular
reticle for determining remote widths, as in tree trunks. Jess introduced
the device in his recent report on the giant tuliptree in Cosby, TN. We
wanted to take some time to fully discuss this instrument and the
implications for its use.

jess_using_reticle.jpg (121563 bytes) Jess using Reticle

Jess's father, long-time ENTS member Doug Riddle, is an electrical engineer
(who is particularly fond of gadgets) and has used his monocular reticled
macroscope (hereafter reticle) to check specifications of installed
components that could not be reached. His is a model made by PENTAX that is
an 8X30 monocular with a field of view of 6.2 degrees. It has an internal
scale (reticle) that can be optically transposed across an object. The scale
runs from 0-4 (mm) with ten graduations between each number. This gives an
effective resolution of 40 increments. The "zero" (left hand of scale) is
demarcated by a long vertical line that is useful for "averaging" the bark
or trunk profile. We found that the extreme sharpness and clarity of the
scale easily allows optical divisions of each gradation into 10ths (as we do
with clinometers). In our tests Jess and I often estimated to the same
1/10th gradation. On sharply defined objects 1/100th estimations are
entirely possible making the scale extremely accurate. The gradations are
then multiplied by the distance and a factor to get the actual width of the


Our intended use for the instrument was to see if it could be used in
determining diameter of tree trunks aloft to help measure displacement
volume. Bob Leverett has been extensively testing the RD1000 for such
applications and has found a myriad of inherent limitations. The reticle is
extremely simple and fits right in with our existing instruments (laser and
clinometer) for a low-budget three dimensional measuring outfit. The catch
is- is it accurate enough?

To assess its applications into tree volume modeling Jess and I devised a
series of tests to ascertain the range of use and accuracy. We ran
duplicates (Jess and I independently) of the following tests:

Width of trees and flat objects
Same tree trunk at various distances,
Window sill at various distances,
Tree trunk; four locations up trunk, each at the four cardinal directions.

Fixed round objects
Steel billboard post; diameter at various distances,
Steel billboard post; volume,
Steel billboard post; volume- laser versus taped baseline.

And finally, the ultimate test;
Tree climb; diameters and volume versus climb data.

Rather than go into detail of each test and lose everyone on the list I will
summarize the findings of each test. But I'll tell you know we have stumbled
upon a revolution in volume modeling. The device tested well above our
expectations. Accuracy does not diminish with distance and is not restricted
to distance or scale size like the RD1000 (it also costs much less). It does
not use batteries and is very easy to carry.

Here are the results of the above tests.

Width of trees and flat objects
Same tree trunk at various distances
Target: white oak measured diameter (w/ tape) at 1.007 feet. Maximum
error was 5.8 % while working at 30 feet. After getting into the working
range of the device (45'+) error dropped to less than 2.5% (.3 inch).
Average diameter over a range of 30 feet to 165' was 1.007 for Will and .999
feet for Jess. Widths were slightly overstated at long distances and
slightly under at close distances. Jess and I differed by slightly over 1%
in our reticle readings. This was the first time either of us had used it.

But, trees are not round and thus the diameter of 1.007' was a calculation,
not an actual width. So the next test was of a flat object with no curves.

Window sill at various distances
Target: a white window sill taped at 73.5 inches wide. We measured it from
125-200 feet away at 15 foot intervals. Will averaged 73.49 inches and Jess
73.52 inches. Maximum error was .15 inches over for Will and .375 inches
under for Jess.

jess_and_flagged_trunk.jpg (73287 bytes) Jess and flagged trunk

Tree trunk; four locations up trunk, each at four cardinal directions.
Target: a relatively straight tuliptree measured from four
locations, 90 degrees different from each other. Four locations from the
base to over 20'were marked with yellow flagging tape perpendicular to the
trunk. The girth of each point was measured with a tape. Each flagged point
was then measured with the reticle from four directions paired from opposite
sides (same baseline distance). Jess and I independently measured each
point. The maximum difference in reticle scale reading between our
measurements was a miniscule 0.015 of a gradation so I will combine our
results. Since volume determination is based on girths (obtained to convert
to diameter) I will summarize the results in girth. I will point out that
parts of the trunk were elliptical with a maximum linear difference (width)
between 90 degree measurements being over 5.6 inches (23.34 inches one way
and 17.4 in the other). The measurements obtained at opposing positions were
nearly identical with a maximum difference of just 0.049 feet (.59 inch)
which was incidentally obtained on a "shot" through brush. Here are the

Trunk point Reticle girth Taped girth Elliptical offset
20.08' 58.81" 61" 3.2"
14.58' 63.16" 64.625" 5.6"
6.58' 66.21" 66.375" 1.92"
Base 77.04" 77.5" 1.8"

I suspect the volume calculated from the two opposing reticle values (an
ellipse) would more accurately represent the actual volume since the tree is
not round. Taped girths would tend to overestimate the volume. However,
later field tests may show otherwise (see below).

Fixed round objects
Target: steel billboard post; diameter at various distances
The post was taped to 3.01 feet in diameter and assumed to be round.
Reticle indicated a diameter of 3.02 feet with a maximum error of 3.06 feet
(+1.3%). We measured at distances of over 280 feet.

Steel billboard post; volume
Using the rangefinder and clinometer we measured the height of the
post and various points along the length with the reticle to determine
volume. Girths used in the calculations were on the average .05 feet off (.6
inches circumference) which yielded a volume of 456.65 ft3 over a length of
63.2 feet. Actual volume based on the taped girth and baseline was 459.88
ft3, indicating an understatement of .7% or 3.23 ft3.

Steel billboard post; volume-laser versus taped baseline
Interestingly, the taped baseline with reticle girths overstated the
volume by 4.1%. We are baffled by this and suspect human error.

And finally, the ultimate test.
Tree climb; diameters and volume versus climb data.

will_setting_the_targets.jpg (147942 bytes)

Will setting the flagging

tapes_set.jpg (133258 bytes)

Tapes set

Jess and I selected a huge hemlock tree with easy access and visibility to
test the reticle against a tree climb. I climbed the tree and set up five
marked intervals like in the tuliptree above. I wrapped yellow flagging tape
around the trunk and measured the girths at that point. We then set up the
reticle in two locations roughly 90 degrees from each other so we could get
the widths at each point and determine the elliptical numbers. We then
calculated the volume of the marked section using the climb data and the
reticle data. We were impressed!

trunk_from_first_position.jpg (118933 bytes)

Trunk fro first Position

trunk_with_flagging.jpg (62550 bytes)

Trunk with flagging

I'll cut to the chase. For the 47.5 foot long modeled section the climb data
indicated a displacement volume of 535.74 ft3. The reticle data collected by
Will indicated a volume of 529.7 ft3 and for Jess, 526.32 ft3. The
difference was 9.42 ft3 at the most, an impressive difference of only 1.8%!
These numbers were from a taped baseline to the middle of the trunk with no
lean taken into account (very minor). The laser distances were somewhat
longer indicating a long reading that must be calibrated. As such, the laser
(uncorrected) estimated the volume as 562.99 ft3 for Will and 559.34 ft3 for
Jess. Still, the results were within 5.1% of the climb data. One of the
numbers for the lasered hypotenuse distance was erratic and added nearly 1.8
feet to the length of a large section which added about 22 ft3. Correcting
this section alone would reduce the "error" to a 4.3 ft3 overestimate or
less than 1% off.

Obviously, the more we use it and refine our techniques and learn the
nuances of the equipment we should be able to obtain remote volume
estimations within 95-99% of a climb. Jess and I were hoping for 95%
accuracy and with a bit more practice should be able to obtain that goal

I am not sure that further testing of the reticle is needed but a visit to
trees that have already been climbed and measured could prove useful.
Comparing a random series of shots at various locations to calculate volume
and compare it to the climbed results could give insight into how many shots
are needed and where. The problem is we never really know the absolute
volume. To us, the reticle seems to be the best option other than climbing.

Jess and I have already begun to model hemlocks and Jess has measured two
big tuliptrees for main trunk volume. We intend to use the reticle
extensively in Congaree next month; loblolly pine and cherrybark oak are on
the menu.

Where can you get one?
Jess and I searched with no luck for the Pentax model. It is no longer
manufactured. We did find some through a company named Cole-Parmer
Instruments in Illinois and I ordered one. I also ordered one from Ben
Meadows Company (1-800-241-6401) which has miraculously just appeared in
their new 2006 catalog. The biggest issue with all the models is close
focus, as the reticle scale will go out of focus at close distances. The
units, when used as telescopes, have the reticle in focus when set at
infinity. The unit from Cole-Parmer (made by Specwell) has its focus set at
over 300 feet- not useful for ENTS. Although a camera repair shop believes
the focus can be reset I returned the Specwell units in favor of the Ben
Meadows model which arrived this week. The Ben Meadows model (catalog #
6JB-221152, page 573) has the advantage of a wider field of view than the
Pentax (hence more gradations; 0-50) and a scale that starts at 0 on the
left (the Specwell model had "0" in the center- very awkward). It is also
$70 cheaper! The focus is good as set so it can be used instantly. I did
retain the tripod adapter (a fancy clamp) from Cole-Parmer to attach it
securely to the tripod (Item # WD-03099-62 on website). However, simple
attachments can be made with wood and straps.

To check its accuracy Jess and I have put the "new" model through some
tests. The first test (diameter of a steel light pole) sucked. We made a
mistake of not visiting the pole first- it was not round! We also determined
that the factor supplied with the instrument was significantly incorrect. It
may have been a relic manual from previous model that was not updated for
this model. In any case, Jess' math prowess solved the problem and the new
factor revealed excellent results in the next test of a 5' wide board.
Widths were no more than .26 inch off over distances from 100-200 feet away.
The average error was .0765 inch. Looks like it will work just fine!

Field observations and techniques

The narrow field of view in Doug's PENTAX model (6.2 degrees) necessitates
being rather far back from large trees. So in our search for a suitable
model for our regular use we looked for one with a wider field of view. The
first model we ordered (Specwell) had a field of view of 8.2 degrees- vastly
more useful for large trees at close distances. The scale could span a six
foot wide object at 95'. However, as mentioned above, we soon found the
unit's focus with regard to the reticle set way to far away. This makes
sense for its use as a telescope but the reticle scale was sharp only at
over 300 feet away. Focusing in on closer objects would eliminate the scale
from view. Not good. The Ben Meadows model has a wider field of view than
Doug's but not as wide as the Specwell model. I am not sure of the
specifications but it is substantially wider. Being able to work within a
reasonable range is helpful since the farther you have to move back the more
obstacles position themselves between you and the tree.

Another crucial component of the ultimate accuracy of this technique is
getting an accurate distance to the tree. We have come to realize that
estimating to 1/10th yards is entirely reasonable by finding click-over and
checking against the eyepiece of the reticle, which is stationary. This
means that a properly calibrated laser will be highly accurate for distances
even at +/- 1 yard resolution. When modeling, we measure to the midpoint of
the trunk. All points are referenced to a common base target which is
usually a tape or flagging wrapped around the tree. All measurements from
any location can then be easily referenced against each other. Heights of
the aerial targets are measured by placing the clinometer on or next to the
reticle and using the distance to the nearest 1/10th yard. We also measure
the taped reference point to compare the actual girth to the reticle
estimation. We have found that they are often just an inch or two different.

Some limitations which we find to be rather minor are that you MUST use a
tripod (and not in the wind!). The reticle is so precise that a slight shift
will throw off the measurements. Even standing on the adjacent duff layer
can shift the tripod! We are using a tripod made by Samsonite ($50 at
Circuit City) that has a functional height of 66"- good for the neck! You
also need a vantage point with a view of as much of the tree as possible.
This can take some time to find, but once the reticle is set up many
measurements can be made from one side. These points can then be compared to
another set-up and if a common reference be found, elliptical sections can
be used for volume calculations.

Jess and I have successfully used the reticle in hemlock forests in low
light conditions, but the reticle can be hard to see if it is dark in the
background. Also, laser distances can be hard to obtain through the
branches. As such you are limited to portions of the trunk that can both be
seen with the reticle and hit by the laser. We have not found this to be a
huge issue, mainly because there is not much volume in the upper canopy
portion (of conifers) and more measurements will not gain but a miniscule
increase in accuracy.

So, there it is formally introduced. Nowadays the device is not left at home
when we go into the woods. It has become a standard part of our gear
arsenal. More significantly though is how it has changed the way we look at
trees. With more and more experience in modeling we can now ascertain the
relationships of tree growth (volume) within and between species. We intend
to begin determining annual volume accrual by accurately measuring the
displacement volume and coring the tree for age. Questions start to present
themselves; at what age and on what sites do tuliptrees grow the fastest?
How much wood can they grow in a year? Where do they put down the most wood?
Do they slow down or increase wood production over time? If so, by how much?
Could they grow faster when they are big? What about white pine? .Oh no, Bob
Leverett will have an aneurysm.

Now it appears that we have the tools to answer these questions.

Will Blozan and Jess Riddle