IEEE Geoscience and Remote Sensing Magazine - September 2022 - 37

general, manual field mensuration can collect a wide range
of forest attributes. However, the problem is that significant
resources are required in most cases, which practically limits
the amount, scale, and frequency of the practice.
SENSORS
Calipers and diameter tapes are the most commonly used
instruments for DBH measurement, while Biltmore sticks
and Bitterlich sector forks can be used with less precision
[18]. In operational forest inventory, when many sample
plots and trees have to be measured, calipers are preferred
since they are easy and efficient to use [20], [37]. Tree height
is defined as the vertical distance between the top of the
crown and the base of a tree stem. Estimating individual
tree heights is not a straightforward task, due to limited visibility
of treetops and stem bases because of occlusion effects.
The accuracy of tree height measurements has always
been the focus of forest investigation and practice [38] since
it can significantly influence the estimation and prediction
precision of other important tree and forest attributes (such
as volume and biomass) that are not directly measured in
the field [24], [35].
Tree heights can be measured directly or indirectly in
the field. Many techniques have been developed during the
past decades to facilitate measuring procedures and provide
more reliable tree height estimates. Tape measurements of
felled trees offer the most accurate results [39], [40], but this
is destructive and rarely applicable. The height of standing
trees can be directly measured using telescopic height poles
for small trees, usually up to 10 [41], 15 [17], and 21 m [18].
Similarly, direct tree height measurement can be conducted
by climbing trees [35], [41], [42].
Indirect methods employ ultrasonic and laser-based
hypsometers and trigonometric principles [38], [41]. In the
tangent method, tree height is calculated based on the tangent
function by using a measured vertical angle between
the tree base and treetop and a slope distance to the tree
stem. In the sine method, tree height is calculated based
on the sine function via the measured slope distance to the
treetop and base and a vertical angle between the tree base
and top. Applying tangent and sine indirect approaches requires
clear visibility to the tree base and treetop, which
can be limited in complex and dense forest structures. The
tangent method, in particular, can produce errors in tree
height measurement when trees are leaning, while the
sine technique is much more resistant to errors. In general,
there is a tendency to overestimate tree heights through
the tangent approach and underestimate them via the sine
method [18], [41].
Other length and height measurements typically use the
same sensors as tree height estimation, e.g., the first branch
height and the crown size. Absolute positions in forests are
commonly collected by a GNSS device or the combination
of a GNSS device and total station. Relative locations with
respect to known positions, e.g., within a plot, are measured
by, e.g., a range finder and a bearing compass.
SEPTEMBER 2022 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
DATA ACQUISITION
The DBH is usually measured in sample plots for all
trees above a specified minimum threshold (e.g., 5, 7.5,
or 10 cm, depending on the regulations) and for sample
trees selected through the angle gauge method. Since trees
do not have a circular form, the DBH is usually measured
with calipers from two perpendicular directions (i.e.,
cross measurement). The average (arithmetic) value is calculated.
The work in [20] demonstrated that cross measurements
considerably improved precision compared to
single measurements. Diameter tapes measure circumference
and give the DBH by recalculation, assuming a circular
shape for the cross section. They are efficient when
large trees (e.g., DBH >60 cm) are measured in dense
forest structures [18] and when repeated and consistent
measurements in permanent plots are needed [33]. DBH
measurement using calipers and diameter tapes may seem
straightforward but includes many potential sources of errors,
such as the nonperpendicular orientation of devices
with respect to the vertical axis of the stem, branches and
other deformities of stems at breast height, steep slopes,
leaning trees, limited operator experience and expertise,
and so on [20]. Approaches for solving these issues can
be found in forest inventory manuals and books, e.g., [17]
and [18]. Several studies compared DBH measurements
using different devices, and all reported no significant differences
in accuracy between calipers and diameter tapes
for operational use [33], [43]-[45].
Individual tree height estimates typically have lower
accuracy and precision than DBH measurements [20].
With commonly used hypsometers, errors in individual
tree height estimates can be 1-5 m and more [19], [24],
[46], [47], mainly due to the limited visibility of treetops,
especially in more complex forest environments with
dense canopy layers. Tree heights are the costliest data to
collect in a practical forest inventory [48]. To reduce the
expense, field tree height measurements are often conducted
only on a selected number of trees within sample
plots (a representative subsample), a considerably lower
number than what is involved in DBH estimation. For
trees without actual measurements,
the height is usually
estimated using local species-specific DBH-height
models [17], [18]. Further statistical modeling is also required
to obtain unmeasurable parameters (e.g., volume,
biomass, and carbon stock) at plot, stand, and national
levels. However, error propagation is expected but often
not considered in practice [16].
All three direct tree height measurements are relatively
slow and thus are not suitable for measuring a large number
of trees in a practical forest inventory [41]. Compared
to direct methods,
indirect approaches using hypsometers
(ultrasonic and laser based) are more effective for
larger samples, and therefore they are commonly used
in practical forest
inventories [19], [20], [49]. Regardless
of the instrument and indirect method, tree height
measurements are influenced by various factors, such as
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IEEE Geoscience and Remote Sensing Magazine - September 2022

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