Diameter
The diameter of a tree provides a measure of
tree performance and is a useful starting point for estimating
tree volume. By convention, the diameter of forest trees is
measured in centimetres at 1.3 metres above the ground, and
is termed the Diameter at Breast Height or DBH. Because
trees are measured with the bark on, this may also be called
the Diameter at Breast Height Over Bark (DBHOB). When
measuring live trees most information is presented as ‘over
bark’ dimensions.
It is possible to estimate the depth of the
bark by cutting through the bark to the wood and measuring
bark depth or by observing the bark of recently felled trees
and then converting DBHOB to the Diameter at Breast Height
Under Bark (DBHUB).
DBHUB = DBHOB  (Bark
Thickness x 2)
To measure DBH, it is first necessary to determine
where "breast height" or where 1.3 metres is on
one’s own body. Standing on the uphill side, a diameter
tape should be wrapped around the tree at 1.3 metres high.
Care should be taken not to twist the tape. The diameter is
read from where the diameter scale starts. (Obviously the
tape can be used to measure the diameter at any point on a
tree or log.)
Tree diameter is measured at 1.3 m above
the ground. The tape simply shows the diameter that corresponds
with the circumference.
Using
the MTG tape to measure Tree Diameter at Breast Height Over
Bark. The diameter of this tree is 41.1mm
How it works
The circumference of a circle
is equal to p x d where p (or Pi)Ý = 3.142 and d is the diameter. If the
diameter of a tree is 30cm the circumference will be 30 x
3.142 = 94.3cm. The “30” mark on the diameter tape
matches 94.3cm on the conventional metric measure. The markings
on the diameter tape side are simply 3.142 centimetres apart
so that diameter doesn’t need to be calculated. It is
simply read off the tape.
Precautions when measuring
diameter
 Be
sure to read the scale on the correct side of the tape.
 The
tape must be held tightly around the tree at right angles
to the main stem, and any loose, peeling bark should be
removed.
 On
sloping ground, breast height should be measured from the
uphill side. This is because the slope will determine the
ultimate height of the stump.
 Obvious
swellings, distortions or branches at 1.3 metres need to
be avoided. If there is a distortion at 1.3 metres, measure
the diameter 10 cm up (at 1.4m) and again at 10 cm down
(at 1.2m) and take the average of the two measurements.
 Measure
diameter to the closest 10th of a centimetre as shown by
the graduations on the tape (eg the diameter in the above
photo is 41.1cm).
Other ways to measure tree diameter
Sometimes calipers are used to measure diameter
because they are quicker, especially with small trees. For
larger trees however, if callipers are used it is important
to measure both the largest and smallest diameter and calculate
the average diameter from these two measurements.
If calipers are used to measure diameter
it necessary to average two measurements if the trees are
asymmetrical in shape
Many American farm foresters use a "Biltmore
stick" to measure diameter. The straight wooden stick
is graduated for direct readings of tree diameters by an operator
looking from a set distance, say 75 cm.Ý Although the user
cannot set the full diameter of the tree, the stick is carefully
calibrated (the marks get closer together as tree diameter
increases) making it possible to measure a tree larger than
the length of the stick. It is understood that no metric version
of the Biltmore stick is currently available.
Measuring diameter with a carefully calibrated Biltmore stick.
Back to top
