Measuring Tree Heights by the Pole Method
Colby B. Rucker
The ability to obtain accurate tree heights creates an opportunity for some interesting forest studies. One of these is the construction of a forest profile, in which all the tree species, large and small, are included. By including information on topography, exposure and soils for the tallest example of each tree species, those specimens may be arranged by height into habitat groups, which show the need for trees to find a niche in which they are height competitive, and can receive enough sunlight to survive.
While laser rangefinders are excellent for tall trees, most cannot measure a distance less than 54 feet. Itís neither practical nor accurate to back off in a thick woodland to measure some eighteen-foot mountain laurel, much less some smaller immature tree that represents its species.
For the smallest trees, a carpenterís six-foot folding ruler works well. Above the rulerís reach, a pole is needed. An aluminum painters pole telescopes to nearly twelve feet, and works quite nicely. It can be adjusted to the height of a small tree, and the pole measured with a steel tape hooked to one end. It can be raised to the top of a slightly taller tree, and the distance to the ground measured with the carpentersí rule.
For additional reach, two aluminum extensions can be made that fit inside one another, and both fit inside the pole. I used a sturdy aluminum ski pole for the top piece. That extends the pole to about twenty feet, which is convenient for most work. Occasionally, additional height is needed. I found a 30-foot champion poison sumac in a swampy thicket, and had to make an extension that fitted below the painters pole. I didnít climb the tree.
Of course, there are other uses for a pole beside direct measurements. I measured a champion spicebush with the pole, but the top was over a steep slope. I marked the ground spot under the total measurement, and then extended the pole from the base of the tree to over the spot, leveled it with the clinometer placed upon it like a level, and measured the distance from the underside of the pole to the ground spot. This amount was subtracted from the total vertical distance.
I found that measuring big trees with a laser wasnít as easy as I expected. A tuliptree five feet thick and 154 feet tall stood in a narrow ravine. It defied measurement from a variety of locations because I couldnít see the treeís base. This was even true for trees on a mowed lawn. In some cases I could thumbtack a piece of paper or other target a measured distance above the base, but in areas thick with spicebush, I needed a target at least fifteen feet up. The telescoping pole solved the problem.
With an elevated target, I soon found that the lower triangle wasnít needed. I could shoot the top to clickover, mark the location, adjust the pole to that eye elevation, re-shoot the top triangle, and just add the length of the pole. On level ground, the elevation was less than the pole length, so I tied a handkerchief neatly around the pole, and slid it down until the top edge of the fabric was level with the base of the top triangle.
The advantage of eliminating the lower triangle was considerable. On rough terrain, the top triangle was backed up to clickover, but the lower triangle couldnít be backed up without changing the elevation of the eye. The two triangles could overlap or be separated. Also, a direct measurement (the pole) is always more accurate than calculations based on sightings (the lower triangle).
On slopes, the pole is often set on the uphill side of the treeís base. Of course, thatís not "where the acorn sprouted," the central basal contour. Whenever possible, the pole should be set on that contour, but sometimes visibility requires that it be set higher, or even lower. In such cases, a basal height adjustment must be made by measuring the difference in elevation between the base of the pole and the central basal contour. This is done by extending the pole horizontally, leveling it with a clinometer placed upon it, and measuring the vertical difference with a folding ruler.
The pole can be essential where triangulation is more difficult. I recall a handsome old sweet cherry in a dense woodland with a thicket extending to the roadside. The site was threatened unless I could get accurate measurements, and prove the tree was a significant specimen. The road was the only place I could see the treeís top. I knew the treeís base was higher than the road, but I could only see a small section of trunk about fifteen feet up. I shot a triangle to the treeís top, and one to the pole tip visible against the exposed trunk. Subtracting the pole length from the lower triangle left the elevation of the base above my eye, and subtracting that from the top triangle gave the actual height of the tree.
The pole can be handy in other ways. I recall measuring the trunk of a huge tuliptree standing at the head of a steep narrow ravine. The central basal contour was about 2 Ĺ feet below the grade on the high side, but the roots on the low side plummeted down, almost vertically, for about fifteen feet. There was no easy way to get a tape level around the trunk. I ran the pole out to nearly its total length, and soon had the tape where I wanted it. Circumference at breast height was 22 feet 8 inches.
The pole methodís not as fast as just shooting two triangles, but I think the improved accuracy is worth the trouble.