Speedier Tree Height Measuring without Taking Horizontal Distances

by C.J. Cieszewski
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 day’s 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 relascope’s 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.

Literature Cited

Curtis, Robert O., and David Bruce. 1968. Tree heights without a tape. J. For. 66:60-61.

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.

Originally published July 2000

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