Tree Measuring
Guidelines of the
Eastern
Native Tree Society
Prepared
by Will Blozan, ENTS President, October 2004
Revis=
ed March
2008
Introduction
The
Eastern Native Tree Society is a diverse, non-profit group of ecologists,
educators, naturalists, and world-renowned canopy researchers dedicated to a
better understanding our Eastern forests. Accuracy is the premise of our
mission, and ENTS has developed research techniques unmatched by any other
institution. There are =
three
key measurements made to characterize the size of a tree: 1) Height; 2) Girth; and 3) Crown Spread. The next steps beyond
taking these basic size measurements is calculation of te wood volume of the
tree, and mapping of the structure of hte tree itself in three dimensions.<=
span
style=3D'mso-spacerun:yes'> The final considereation included =
in
this basic guidelines are the concepts of Tree Dimension Index, which relat=
es
the size of a particular tree to the largest known for the species, and the
Rucker Height Index which is the numerical avarage of the tallest individua=
l of
each of the ten tallest species on a site.=
This RHI provides a useful characterization of the overall composite
maximum heights of the tree species found on a site.
The most significant difference bet=
ween
ENTS measured trees and those measured via conventional forestry methods is=
a
much higher standard of accuracy. In fact, the “ENTS method” of
laser-based tree height measurements is so accurate that it is being employ=
ed
in height growth monitoring projects by several universities and premier fo=
rest
ecologists. Perhaps the most important value of the ENTS method is that it =
is repeatable,
a basic premise of any scientific project. Users of the method, with a few
minutes of training, can produce measured results within 1% or less of a
seasoned ENTS researcher. Seasoned ENTS researchers can measure a tree from=
the
ground to within inches of a direct tape drop. This process takes but a few
minutes for most trees.
_______________________________________________________________=
_
NOTE: This is a dr=
aft
document and is the sole property of Will Blozan and the Eastern Native Tree
Society. Unauthorized copies and distributions are forbidden.
For permission to =
use
this document please contact me at:
Will Blozan, Presi=
dent
Eastern Native Tree
Society
102 Fourth Street<= o:p>
Black Mountain, NC=
28711
(828) 669-7435
(828) 273-5302
Tree Height Measurement
Getting an accurate tree height is the nemesis of many
potential tree hunters, and the leading source of point errors on champion =
tree
lists. Although the techniques are very simple, employing them accurately is
another story, which will be dealt with later in this section. Tree heights=
are
typically remotely obtained using a clinometer or transit for angles and a
measuring tape or infrared laser rangefinder for distance. By using simple
trigonometry and laws of similar triangles and right triangles, the true he=
ight
of a tree can be easily obtained. In all cases, the height obtained is the =
vertical
distance between the top and base, not trunk length. Leaning tre=
es
and hardwoods have longer trunk and branch systems than indicated by vertic=
al
height, but is beyond the scope of my efforts or champion tree lists to
measure.
In
standard forestry methods only a clinometer is used. At a set distance from the tree, t=
he
angle (A) to the top is measured.
Using a simple formula
tan(A) x distance to base =3D height of the tree. This method requires the use of a =
series
of untenable assumptions. One=
of
the flaws of this tangent method is the assumption that the top sprig of a =
tree
is directly over the base. Pr=
eliminary
analysis of a set of 1500+ measurements indicates that the top is on average
offset more than 13 feet from the base of the tree. This value is even higher for broad
canopy deciduous trees. Shoot=
ing at
a reasonably steep angle this will result in height measurements exaggerati=
ng
the tree height by upwards of tens of feet. There are a number of electronic
instruments on the market that measure tree heights using a built-in laser
rangefinder, clinometer, and auto-calculating routines. Unfortunately, the vast majority of
these instruments employ the same flawed tangent calculation methodology wh=
ich still
assumes the tree top is directly over the base. So even though the instrumentation=
is
technologically advanced, the height results will still on average provide a
height with a built in error of feet to tens of feet. Sources of error are discussed in =
more
detail later in this document.
The
ENTS Method Laser Technique
The ENTS method requires the use of a laser
rangefinder, a clinometer, and a basic calculator. This low-cost m=
ethod
has the advantage of being the quickest, simplest, and most accurate. A
clinometer and a laser rangefinder is a relatively minor expense (<$300)=
and
easily justified by the speed, accuracy, and foremost, the REPEATABILITY of
your results! It is certainly
cheaper than the all-in-one measurement instruments that use the erroneous
tangent calculations to determine heights.
The
laser rangefinder is a device that sends out a pulse of infrared laser ligh=
t.
This light reflects off a target and bounces back to the laser unit. A clock
inside times the bounce and calculates the distance based on elapsed time.
Since the laser requires a return bounce, this method has the distinct
advantage of automatically measuring a physical part of the tree, as oppose=
d to
an extrapolation of a part of the tree via cross-triangulation or conventio=
nal
methods.
Only four numbers are needed to complete the tree
height calculation, and no tape is necessary, nor is direct contact with the
tree. This last bonus can be useful for trees across a river, road, a mean =
dog
lair or other obstacle. When searching for champions, a quick height reading
will tell you if further exploration and contact with the tree, the dog or =
its
owner are necessary. Since the hypotenuse of the triangle is the baseline a=
nd
it is measured from a physical part of the tree any lean or slope correction is irrelevant. You are simply crea=
ting
two right triangles to an imaginary (but fully real) level plane (eye-level=
or
tripod, etc.) that is the base of the top triangle and the top of the lower
triangle.
Use the laser to explore the crown looking for t=
he
highest point. “Skate&#=
8221;
the laser over the surface of the crown and in “nested” pockets=
and
places you may not expect a high part to be. The highest point may be well
below what appears to be the tallest part. Look for the farthest distance
first, then the highest angles with far readings. Once you become familiar =
with
a species and its architecture, you will know how to narrow your search
down. A general rule of thumb=
is
that at a similar vertical angle, the tallest point will be the one farthest
away. A laser rangefinder giv=
es you
instant feedback on the distances to different points in the crown and enab=
les
you to identify the true top from many similar looking options.
Figure 1: The
“ENTS Method”
Total tree heig=
ht=3D
H1+H2, where H1=3D [(SIN) A1*D1] and H2=3D [(SIN) A2*D2]
Measuring
From Two Different Points
Tree heights can be measured additively from two
different points if the top of the tree and the base can’t both be se=
en
from a single position. Shoot=
them
from the best locations and reference the triangles to a common point easily
seen from both sites, i.e. lowest branch, a burl, or bend in the trunk. First shoot the=
top
and calculate the height above your position. Then shoot to the distinctive poin=
t on
the tree and note its height. The difference between the two is the
height of the tree above the distinctive point. Then from a second
location, from where you can see the base and the distinctive point, calcul=
ate
the height above the base to the distinctive point as if it were the top of=
the
tree. Then add the height from the first measurement to this second
height to get the total height of the tree. Creating two triangles from the two (or mor=
e)
locations allows you to measure tree heights that could not be measured using other methods.
Laser
Calibration
The laser measurement accuracies listed in their
respective specifications essentially is a statement that the actual distan=
ce
will be within so much of the distance displayed. The precision of the instrument is
actually much higher. Before =
you
use a new laser, it must be calibrated. To do this, stretch out a long
measuring tape flat on the ground. Have an assistant stand at various locat=
ions
on the tape with a reflective target. Place yourself in a position so the
eyepiece of the laser is over the “0” mark on the tape. Alterna=
tively,
you can do this by yourself by affixing the “0” end to a reflec=
tive
target and walking down the tape, shooting back at the target and noting yo=
ur
position at click-over. Shoot a known distance; say to 40 yards (or meters).
Have the assistant move the target closer or away from you until you get to=
the
“click-over”, or inflection point of the laser for 40 yards (or
meters). Note where the target is in relation to the tape. Do this calibrat=
ion
over a wide range of distances to see the variation and correction factor to
use (if needed). For example, if the laser reads 40 yards at a distance of =
40.6
yards based on the measuring tape, then you would use that figure when your
laser gives the click-over reading for 40 yards. By calibrating your laser,=
you
can actually be mere inches off in the distance measuring part of the tree
height.
Figure
2. Using a reference point to create triangles from two positions
Shooting Straig=
ht Up
The
laser is calibrated in .5-1 yard increments, and shooting straight up seems=
to
be a logical way to at least rough-out a trees height. This is true, and
shooting straight up is in fact a fully legitimate and appropriate method to
measure a tree that has a crown conducive to it. Dense conifers and fully
leaved hardwoods are impossible to measure this way, but hardwoods in winter
are typically fine. Some trees, such as oaks and sycamore are easy to measu=
re
by this method during early leaf-out. Careful exploration of the canopy is
necessary to find the highest point. Figures obtained from straight-up shots
are usually recorded as “NLT”- “not-less-than”. I u=
se
this technique to help determine if more careful searching is needed or to =
find
the highest leader for more detailed measurements.
Since
a straight line leaning 11 degrees off vertical is still over 98% of vertic=
al
length, this technique gives you a full 40' circle of exploration on a 100'
tree from one spot. Figures obtained by shooting straight up are seldom less than one foot different than the two
triangle ENTS technique (often listed as SIN+SIN)
described above. All you need to do is find the inflection or click-over po=
int
and sight the level point on the trunk and add it to the laser reading. See
Figure 9 below:
Figure 3: Explo=
ring
the crown by shooting straight up
Measurement
Accuracy
The ENTS website has a table of trees that appea=
red
on various state and national champion lists that upon re-measurement using
ENTS laser/Sin techniques were found to have dramatic height errors.
A similar comparison of tree heights measured fr=
om
the ground using ENTS laser/sin techniques and the actual heights of the tr=
ee
determined by later tree climbs and tape drops demonstrates the remarkable
accuracy of the ENTS methodology.
The list includes all trees that were measured by this dual criteria
from 2000 until the list was compiled in 2005. http://www.nativetreesociety.org/measure/measure=
d_trees.htm
Girth
Girth is a dimension taken at a point 4.5 feet (BH) a=
bove
average soil level (A). This measurement is called circumference at brea=
st
height (CBH). If a burl or other atypical growth formation is encounter=
ed
at this point the least distorted girth below this point is used (B); other=
wise
above BH. When a tree is growing on a slope the girth is taken at a
point that is the average of the highest point and the lowest point the tree
trunk appears to contact the soil (Mid-slope-C). This mid-slope rule is use=
d to
follow the American Forests
guidelines for measuring champion trees. In all cases the girth is t=
aken
perpendicular to the axis of the trunk at BH, not parallel to the
soil. Measured girth is the best approximation of size, since it is a r=
eal
number, not a calculation based on fictional premises. Even girth has its
limitations, as a sinewy or contorted trunk will have lots of hollows and
ridges that are not accounted for in the measurement. Diameters calculated =
from
such trees, or measured with a diameter tape, will be overstated (diameter=
=3D
CBH/ 3.142). For volume measurements, “footprint” maps must be
obtained to calculate the “functional” diameter and girth. The
functional diameter is always smaller than the calculated diameter. For this reason the actual measure=
ment
of girth should be recorded. =
For
some types of calculations gi=
rth
must be converted to an approximate radius or diameter with the assumption =
of
circularity, but there is no reason to introduce these assumptions into the=
raw
dataset.
Figure 4. Girth
measurement locations
<=
span
style=3D'font-size:12.0pt;font-family:"Arial","sans-serif"'>When most peopl=
e ask
how big a tree is they want to know the diameter, not the girth. Diameter is
useful to calculate since it is an attribute readily understood by most peo=
ple.
It also seems to be one of the most overstated dimensions other than height,
with many “5-6 foot diameter” trees being closer to 3-4 feet in
actual diameter. Lack of an accurate reference is often to blame, as is in =
the
case of overstated heights.
Why
is Girth measured at 4.5 feet? This
4.5 foot value is a measurement grandfathered from decades of forestry
measurements. It was developed because of the simplicity and ease of
measurement. There is no one
“ideal” height at which to measure girth. Trees flair outward at their base.=
In some trees this flair extends o=
nly a
short distance up the trunk, while in others it may extend thirty feet up t=
he
tree. Ideally the girth would=
be
measured every few feet along the length of the trunk to characterize the s=
hape
of the entire trunk. This is =
what
is done in trunk volume modeling.
Since the height at which girth is measured is essentially arbitrary,
then the best point to measure is at one which is the simplest and easiest =
to
measure, therefore the old forestry standard is still used.
Multitrunk Tree=
s
I use a “pith test” to define what i=
s a
multitrunk tree. If the tree has more than one pith at ground level it is a
multiple-stemmed tree. Note I did not say 4.5 feet above the ground. This is because the 4.5 foot height is a fore=
stry
standard and is an arbitrary and convenient place for most people to measur=
e a
tree. Some trees, like flowering dogwood or rhododendrons, may branch well
below 4.5 feet but have a single pith at ground level. In the case of such
trees, I would measure the narrowest point below the lowest fork. More detailed discussions of how to
measure multitrunk trees and trees with other odd forms is presented on the
ENTS website.
For champion tree listings Measure the attribute=
s of
the target stem only. Do not include the crowns or heights of the other
sprouts. To me, the entire po=
int of
a champion tree list and the ENTS research is to assess the capabilities of=
the
eastern species. The best way to assess this is to study individual stems or
trees of the species. By focusing on individuals we can accurately assess t=
he
potential and find benchmarks for restoration efforts or whatever the goal =
may
be. To me, a champion tree is one that represents the best development of an
individual, and therefore I do not include multi-stemmed trees in my resear=
ch
or nominations. Many will argue that a clump of sprouts fused into a huge t=
runk
that originates from one root system is a single tree. I would agree, but i=
t is
not a single stem and thus does not represent the potential of an individua=
l.
Average
crown spread
Average crown spread is obtained by measuring the longest and shortest extent of the crown and averaging the figures. Crown spread is taken independent of trunk position. I measure to the tips of the limbs, not to “notches” = in the crown shape. Try for a ninety-degree difference in measuring location.<= o:p>
Figure 5. Measu=
ring
crown spread
<=
!--[if gte vml 1]> Longest spread Shortest spread