Speedier Tree Height Measuring without Taking
WSFR, Univeristy of Georgia
Accurate height estimation is basic to tree and plot volume estimation, as well as to
estimation of top height and other height-based stand statistics. Frequently, routine
height measurements use telescopic height poles to obtain direct height measurements for
trees up to 15 m tall. For taller trees, clinometers together with measuring tapes are
used to obtain indirect estimates. With the latter, often the most time consuming task is
the measuring of distances from the observer to points where both the tip of the crown and
the butt of the stem are visible, while also keeping the measurement angle within
desirable values. On sloping terrain, this distance measurement may be particularly error
prone and tiresome to acquire. Little time can be saved even if a person holding the
clinometer can see a few trees well from the same position, because the tape has to be
rerun between the trees rather than just moved on a radius.
On the other hand, as concluded by Curtis and Bruce (1968) and Bell and Gourley (1980),
eliminating distance measurements from the height measurement procedure can appreciably
improve the speed of this work. Much time can be also saved by using small-size field
computers, or data loggers making it easy to compute any trigonometric calculations needed
for estimating tree height from measured angles at requisite points along the tree stem
and to easily compute additional corrections on the measurements.
The basic approach of taking heights without measuring distances is described in Curtis
and Bruce (1968) and it relies on measuring angles from a horizontal level to: 1. the top
of the tree; 2. the top of the pole of known height; and 3. the base of the pole. The
measurements we were taking with this method were collected and computed using hand-held
field computers running DOS-based programs and containing storage capacity sufficient for
holding a whole days worth of fieldwork data as well as the previous measurements on
the same plots. Using the data logger made any trigonometric functions, evaluations and
computations transparent to the user, so that we needed to concern ourselves only with the
measuring aspect of the fieldwork, the work efficiency, and the reliability or accuracy of
the final estimates.
We compared the new height measurement technique to our current standard in remeasuring
10, 45-year-old lodgepole pine permanent sample plots on hilly terrain. Tree heights
ranged between 15 and 20 m. Using the Spiegel relascopes angle scale and a 10 m
height rod for the distance-less technique, and % scale and a 30 m tape for the standard
technique with measuring horizontal distances, the two approaches gave similar accuracy as
long as all the steps were done carefully and good measurement practices observed.
The advantage of not having to measure distances for our measurements really showed in
measuring speed. On the average, we found over a 20% reduction in the time required to
complete the necessary height measurements on each plot. The technique was particularly
advantageous on steep slopes, where the distance measurements required for the standard
technique were particularly onerous. Indeed, above a certain slope gradient, getting
accurate horizontal distances becomes impractical. On such slopes, the fact that the
pole and angle technique requires no distance measurements alone would justify
its use. Further, any extra height measurement, e.g., live crown base, whorl location,
etc., requires only one extra reading, and from these the program can automatically
computer the desired statistics.
It is possible that in some situations it may not be possible to locate the pole on the
same level as the tree and/or parallel to the tree. Thus, further improvements to the
functionality of this system were achieved by incorporating into the data logger program
corrections for different cases of such difficulties in taking the measurements. The
corrections accounted for situations of the pole not being parallel to the tree and being
located on a side of a tree or between the tree and the clinometer while still in the same
plane with the clinometer. Such corrections depend on whether the top or the bottom of the
pole is further from the stem.
The general conclusion from our experience was that the technique described in Curtis
and Bruce (1968), in combination with a data logger and appropriate data collection
program, provides an accurate and fast method for obtaining total stem and other height
values for individual trees. Accuracy is similar to the other standard techniques, while
measurement productivity is likely to increase (in our case over 20%). This technique is
particularly useful on hilly terrain, and where multiple height estimates are required on
the same stem, or where many trees need to be measured on each plot. A program available
from the author can be used to provide accurate and instantaneous height values at any
point on the stump, such as internodes, crown base, defects, squirrels, and bird nests.
Curtis, Robert O., and David Bruce. 1968. Tree heights without a tape. J. For.
Bell, John, F., and Robert Gourley. 1980. Assessing the Accuracy of a Sectional Pole,
Haga Altimeter, and Alti-Level for Determining Total Height of Young Coniferous Stands.
South. J. Appl. Forestry. 4:136-138.