Tree volume is one of many parameters that are measured to document the size of individual trees. Measuring it serves a variety of purposes, some economic, some scientific, and some for sporting competitions. Measurements may include just the volume of the trunk, or the volume of the trunk and the branches depending on the detail needed and the sophistication of the measurement methodology.
Other commonly used parameters, outlined in Tree measurement: Tree height measurement, Tree girth measurement, and Tree crown measurement. Volume measurements can be achieved via tree climbers making direct measurements or through remote methods.Blozan, Will. 2004, 2008. The Tree Measuring Guidelines of the Eastern Native Tree Society. http://www.nativetreesociety.org/measure/Tree_Measuring_Guidelines-revised1.pdf Accessed March 4, 2013.Blozan, Will F. and Riddle, Jess D. 2006. Tsuga Search progress report October 2006. http://www.nativetreesociety.org/tsuga/oct2006/tsuga_search_oct2006.htm In each method, the tree is subdivided into smaller sections, the dimensions of each section are measured and the corresponding volume calculated. The section volumes are then totaled to determine the overall volume of the tree or part of the tree being modeled. In general most sections are treated as frustums of a cone, paraboloid, or neiloid, where the diameter at each end and the length of each section is determined to calculate volume. Direct measurements are obtained by a tree climber who uses a tape to measure the girth at each end of a segment along with its length. Ground-based methods use optical and electronic surveying equipment to remotely measure the end diameters and the length of each section.
The largest trees in the world by volume are all Giant Sequoias in Kings Canyon National Park. They have been previously reported by trunk volume as: General Sherman at 52,508 cubic feet (1,486.9 m3); General Grant at ; and President at . The largest non-giant Sequoia tree currently standing, Lost Monarch, is, at , larger than all but the top five largest living giant sequoias. The Lost Monarch is a Coast Redwood (Sequoia sempervirens) tree in Northern California that is in diameter at breast height (with multiple stems included), and in height. In 2012 a team of researchers led by Stephen Sillett did a detailed mapping of the branches of the President tree and calculated the volume of the branches to be . This would raise the total volume for the President from 45,000 cubic feet to surpassing the volume of the General Grant Tree.Cone, Trace. December 1, 2012. Upon further review, giant sequoia tops a neighbor. , Boston.com Accessed March 14, 2013.Quammen, David. 2012. Scaling a Forest Giant. National Geographic Magazine, Vol. 222, No. 6, December 2012, pp. 28–41. The branch volume of the General Grant and General Sherman Trees have yet to be measured in this detail.
Direct volume measurements – trunk Tree climbers can physically measure the height and circumference of a tree using a tape. The distance from the highest climb point and the top of the tree is measured using a pole that extends from the tree top to the anchor point of the tape. This height is noted and the diameter of the tree is measured at that point. The climber then rappels down the tree measuring the trunk circumference by tape wrap at different heights with the height of each measurement referenced to the fixed tape running down the trunk.
Direct trunk measurements are obtained by a tree climber. The climber will ascend into the tree until he reaches the highest safe climbing point. Once this point is reached, the climber drops a weighted throw line straight to the ground. A measuring (reference) tape is then attached via a small carabineer to the dropped throw line and pulled up to the top, following the vertical path of the weight's descent. The tape is affixed to the trunk at this point via several thumbtacks at this point and allowed to hang freely down the trunk. The exact position of the tack relative to the top of the tree is noted. If the top of the tree was not safely reachable a pole or stick is used to assist in measuring the remaining distance to the high point of the tree.
The climber pulls up an extendable pole and uses it to reach to the top of the tree from the point at the upper end of the tape. If not vertical, the slope of the leaning pole is measured and the length of the pole is measured. The vertical distance added by the pole to the tape length is (sin Θ x pole length). The lower end of the tape is terminated at the base of the tree. If on sloping ground this is the mid-slope point between the lowest and highest sides of the tree. The total height of the tree is equal to the measured distance from the base at mid-slope to the upper end of the tape where affixed to the tree plus the vertical height measured to the actual top of the tree. Girth measurements are made by wrapping the tape around the tree perpendicular to the bole at successive intervals as the climber rappels down the tree. All points of measurement are referenced for height above ground as measured on the fixed reference tape. Measurement intervals are subjectively chosen based on changes in trunk taper. An area where a change in profile is observed (in or out) is measured with a tape. Clear sections of trunk are selected so as to not include branch collars, burls, etc. For greatest accuracy, measurements are taken on single-trunked trees at no more than intervals. Additional measurements are generally required where the trunk branches or bifurcates or where there are trunk reiterations.
Reiterations are identified by an upturned branch that had gained apical dominance and formed an additional branch supporting trunk. Reiteration lengths are terminated at the point of trunk contact. Trunk reiterations are measured and added to the final trunk volume. A bifurcation is defined as a split or fork in the trunk that forms two or more often similarly sized ascending trunks. Bifurcations often form an irregularly shaped fused section that cannot be accurately measured with a tape for the purpose of computing cross-sectional area. All bifurcation lengths are terminated at estimated pith origination from the main stem.
Frame mapping As part of the Tsuga Search Project, a frame mapping technique was developed to allow characterization of significantly large fusion areas at forks in the trees. With two climbers, each on opposite sides of the tree, an area of fusion is selected to be measured. Two poles, longer than the diameter of the fused section, are lifted in place and connected by a thin rope threaded through opposite ends so they are adjustable. The poles are temporarily tensioned and the distance between the ends measured. Adjustments are made until they are parallel and perpendicular to the axis of the trunk. The slight tension between the poles holds them steady against the trunk. Tents stakes wedged in the bark can also be used to level and steady the frame. One end is designated the y axis, and the adjacent side the x axis. Measurements are made with a carpenters tape from the frame to the edge of the trunk and the profile of the trunk shape is plotted. The data is then entered into a trapezoidal area function in a spreadsheet and converted into cross sectional area so as to calculate the equivalent circumference to use in the volume formula.
Footprint mapping Many trees flare outward significantly at the base and this basal wedge has a complex surface of bumps and hollows. This becomes an even more complex volume in trees growing on a slope. Approximations of the volume of this basal segment using best estimates of the effective diameters exhibited may be used in many cases. In other cases footprint mapping is an option. In footprint mapping a level, rectangular reference frame is placed around the base of the tree, to create a horizontal plane. The position of the multiple points on the trunk surface is measured with respect to the frame and plotted. This process repeated at different heights creating a series of virtual slices at different heights. The volume of each individual slice is then calculated and all are added together to determine the volume of the basal wedge.
Remote volume measurements – trunk Remote measurements of trunk volume are usually made from a position on the ground where the observer has a clear view of the entire length of the trunk. Measurements may be made using professional surveying equipment such as a total station or an instrument such as the Criterion RD1000, using a combination of a monocular w/reticle, laser rangefinder, and clinometer, using photographic methods combined with a laser rangefinder and clinometer, or by using cloud mapping techniques.
Electronic surveying instruments such as a total station allows the observer to measure the position of each diameter measurement and the length of the trunk section between each measurement. With most of the instruments, the diameter is determined by measuring the angle of azimuth between the opposite sides of the trunk. Laser-measured distances to the sides of the trunk representing the ends of the diameter and the included angle are used with the law of cosines to calculate the diameter. The Criterion RD 1000 has a special feature that allows the diameter to be measured through a visible display. These length and diameter values then can be used to determine the volume of the individual section.
Another technique is available for those who possess instruments that will measure horizontal angles. The following diagram shows how to measure diameter remotely using a laser rangefinder to shoot the distance to the middle of the trunk and a transit or compass or another device to measure the horizontal angle created by the diameter. Note that in this method, the measurer shoots to the middle of the trunk instead of either edge. Also, the full diameter does not have to be visible from the point of measurement. It is a common misconception that closer distances lead to errors because the measurer cannot see the full diameter. However, if the trunk is round, closeness is not a factor. In the diagram d = diameter, D = distance from measurer to middle of the tree, a = angle from the middle to the edge of the trunk. A variation of this method is to measure the complete angle taken up by the image of the trunk and divide it by 2 to get angle a.
A combination of a monocular w/reticle, laser rangefinder, and a clinometer can be used to do go from simple diameters to a full measure of trunk volume. A monocular w/reticle is a small telescope with an internal scale visible through the glass. The monocular is mounted on a tripod and the trunk of the tree is sighted through the monocular. The width of the trunk is measured as so many units of the reticle scale. The height above, or distance below, instrument and distance of the target point is measured using the laser rangefinder and clinometer. The distance is measured to the center (side) of the tree. With the distance known, the diameter of the tree measured expressed as units of the reticle scale, and an optical scaling factor for the monocular w/reticle, provided by the diameter of the tree at that point can be calculated:
diameter = (reticle scale) × (distance to target) ÷ (optical factor)
To assure accuracy, the calibration of the optical factor should be checked for each instrument rather than solely relying upon the manufacturer's specifications.
A series of tree diameters up the trunk of the tree are systematically measured using this procedure from the base of the tree to the top and their height is noted. Diameters can sometimes be measured with the monocular w/reticle in sections where it is difficult to obtain accurate laser distances because intervening thin brush or branches. Distances to the obscured section may be interpolated from measurements made above and below the obscured section.