Measuring Trunk Diameters with a Laser Rangefinder
Colby B. Rucker
Although we primarily use laser rangefinders to measure the height of trees, some may be used to determine the trunk diameter at any height, allowing us to calculate trunk volume in cubic feet or board feet. By a fortunate coincidence, the Bushnell YP500 rangefinder displays a horizontal row of ten rectangles to indicate "target quality." This line of rectangles may be used like a ruler. The Nikon rangefinders do not have this, and cannot be used for diameter determination.
First, you must determine how many feet the ten rectangles span at a distance of 100 yards. Actually, itís more convenient to determine the span at 50 yards, and just double your finding. If you sight to targets on a fence, building, or garage doorframe, you may find the rectangles span about 8 feet 2 inches at 50 yards, so the base span is 16.3 feet at 100 yards. Your rangefinder may vary, so itís important to check.
If you use the ten rectangles like a ruler, estimating width to one-tenth of a rectangle, you can determine the diameter of an object of an appropriate size, like a tree trunk. At first youíll be dismayed by how much your hands waver, but with a bit of practice, youíll be able to envision whether the trunk is 3.2, 4.7, 8.1, etc. rectangles in width.
If youíve shot the laser distance to the spot on the trunk where youíre measuring diameter, you have the third essential element for measuring diameter. Of course, youíll also need to measure the angle with a clinometer, so you can determine the elevation of your diameter measurement later on.
Letís say the trunk was 4.7 rectangles in width at a distance of 31 yards. Multiply 4.7 times 31, times 16.3, which gives 2374.91. Thatís not quite right, because the distance is actually 31 percent of 100 yards, and 4.7 rectangles is actually 47 percent of the span (16.3 feet) for ten rectangles. Therefore, you must divide by 1000 by moving the decimal point three spaces to the left. That gives an answer of 2.37491, which we can record as 2.37 feet, or 2 feet 4 inches.
You can change the base from 16.3 to .0163, (or whatever distance your rangefinder spans), thereby eliminating the need to divide by 1000, if you want. The main thing is to understand what the numbers mean, and why they work.
To actually model the entire trunk of the tree, you need to establish the central basal contour ("the elevation where the acorn sprouted") so that your measurements will be taken at known distances above a fixed point. Take a number of circumferences of the base with a tape, up as high as you can reach, recording the elevation of each one. Attach a target to the trunk so that you can shoot angles and laser distances to that known elevation, and thereby establish the elevation of all subsequent laser/clinometer readings.
If possible, find an elevated location, perhaps a nearby ridge, where you can see the target and the structure of the trunk. Make a sketch of the trunk, showing the major limbs, and any loose bark or other targets on the clear part of the trunk. Shooting distance and angle to the basal target will establish the elevation of your eye at the location where youíre working. Although you may want to back your distances to clickover, this can change the elevations of your measurements unless youíre standing on level ground. If youíre working on a slope, itís better to ignore clickover, and take all measurements from the same spot. Record the diameter immediately below and just above each major branch, and perhaps every ten or fifteen feet along the clear part of the trunk.
Once you have completed your model, you can divide the trunk into sections, determine the volume of each section, and ultimately, the volume of the entire trunk. If estimating board feet, make an allowance for bark thickness at the small end of each log. Model where the trunk might be divided into logs, usually making cuts below major limbs. Allow perhaps four inches of overage on each log. Although the logger might discard sections with large knots, you can include them for a higher total, and also divide the trunk into shorter logs with less taper.
So, see if your rangefinder has ten rectangles, and donít worry if your hands waver at first.