How To Measure A Tall Tree

In 1963 National Geographic Society funded a survey crew to measure the newly discovered tall trees along Arcata Redwood Company’s Redwood Creek property. At that time, measuring redwoods accurately was a great challenge, even for professional surveyors. Since that time new methods have been developed for measuring tall trees which are easier, faster and more accurate.

The tangent method and textbook way of measuring trees involves trigonometry and triangulation. Typically, a 300’ cloth or fiberglass tape line is pinned to the tree's base and pulled out as straight and far away as possible through the forest clutter to a point where the top can be seen. Tall tree hunters refer to these precious views as windows. From the window, an angle reading device such as an abney level or clinometer is used to measure the angle to the tree’s top. The longer the baseline and the lower the angle the better the accuracy. After the horizontal distance and angle to top are determined, an imaginary right triangle is constructed, with the tape line being the base adjacent to the vertical angle, the opposite leg being the height. The Greek mathematician Pythagoras discovered that for any right triangle, if you know just one angle and one side, or the length of any two sides you can calculate all of the other angles and sides with trigonometric formulas. The tangent of the vertical angle multiplied by the baseline will give the opposite leg, which is the height of the tree. Standard timber cruising equipment such as the abney level and the clinometer, while inexpensive and portable, will never give the required accuracy needed to document tall trees. A transit or theodolite with precision greater than 1/50th a degree is a must for accuracy to within a few inches. If not used properly, even the best surveying equipment will produce disastrous results.

Chris Atkins Measuring "Bomber Tree" With Impulse 200LR® Along Bull Creek, Humboldt Redwoods State Park

By far the most challenging aspect of measuring trees with traditional methods and equipment is determining the position of the tree's top relative the ground. Ron Hildebrant coined the term heighter for this highly important position. I recall many instances where is took only a few minutes to lay out the tape line and to find a window and a vertical angle, yet the rest of the day just to find the heighter. Imagine a leaning tree with a string anchored to the top and a free hanging weight attached descending all the way to the ground. The point at which it hangs perpendicular below the top on the ground is the heighter and this is where the tapeline starting point must be place in order to construct the correct right triangle. What if the heighter lies within the tree trunk? Another challenge of the traditional method. If the top's position is not determined precisely, the results can be off by 20’ or more. In order to calculate this position, at least two and ideally three or more windows must be found. In the dense redwood forests, even finding a single window can be difficult and three or more windows with basal views is often impossible. The 363’ John Muir Tree in the Rockefeller forest is not measurable with traditional techniques as not a single known window to it's top exists. Because of these limitations, I also refer to the traditional way as "The Dinosaur Method". In the really dense stands of redwoods like Rockefeller and Montgomery it is 100% obsolete. In fact, my colleagues and I have found 15 previously unknown 360+ footers in Rockefeller alone because we have diverged from using the traditional method. In the old days it was just too time consuming to measure them so nobody bothered.

The diagram below is another, more complicated variation of the traditional method using two theodolites in tandem and their horizontal angle reading capabilities. The surveyors hired for U.C. Berkelely’s 1966 Humboldt Redwoods State Park survey used this more sophisticated and reliable parallax method. In this method, two height figures are generated from a set of two equations and if they agree closely, the surveyor can be confident that that the result are valid. If one of the angles or baselines was misread, the two heights generated in the equations will not agree. Transit station#1 and station#2 are assumed to be level to each other. To use off-level survey points for this method click the link for "Parallax Method in 3-D space" on the "How to Measure a Tree" page.

Originally developed for the U.S. Military, we now have commercially available laser range finders which have started a revolution in measuring accuracy, speed and ease of use. When measuring a tree with a laser range finder, only one suitable window is needed and measuring trees across a ravine is no problem. Today’s class-I lasers will easily bounce off the top of any tree without a reflector. Depending on the model, the range can be over 1 mile. Like the traditional method, an imaginary right triangle is constructed, however the range finder distance represents the hypotenuse rather than the lower leg of the right triangle. The significance of this is that the position of the heighter has already been computed for the surveyor. See fig 8c. Only a single vertical angle to the top is needed from this window. Instead of the tangent function, the sine function is used. The sine of the vertical angle multiplied by the laser distance (hypotenuse) is the height of the tree above eye level (viewfinder of instrument). An additional amount of height must be added or subtracted to account for ground level difference between measurer and tree and height of their eye level above the average ground level of tree. Some of the newest and more costly tree measuring equipment such as the Laser Technologies Impulse® series have highly accurate built-in inclinometers with computers to calculate everything for you.

This method is very straightforward, but easier said than done. A climber ascends to the top of the tree and drops a weighted sac that is attached to the end of the tape line. The climber carefully checks to make sure the tape line is free of obstruction from branches and that it drops perfectly straight. A person at the base of the tree helps the climber at the top to pull the weighted end of the tape line straight and taught against a tag which was already placed on the tree. This tag is generally placed at 1m above the tree's average ground level. The tag should always is be surveyed seperately before the tape drop. This is the most accurate of all the methods to measure a tall redwood because the top most leaders of these redwoods are often times not visible from the ground. However, getting permits and actually climbing the tall trees can be quite an involved process. At this point in time, I have yet to see a tree that could not be climbed by Steve Sillett or his colleagues. They have adapted this method for tall redwoods, using a standard 300 ft fiberglass tapeline. For tall redwoods, a team of two climbers is generally needed. After getting to the top, a climber drops a tapeline 70-100 feet down into the canopy. Another climber below pulls the line taught and marks its position on the trunk relative to the topmost leader. A leveling device is used to survey a 2nd level point with a clear view to the ground and a point which is directly above the 1m tag previously place at the base of the tree and then a secondary tape drop to the ground is needed to get the overall height. By using this method you can accurately measure a 360”+ tree with only a 300’ tape and by breaking the measurement into 2 parts, you can get unobstructed drops by using over hanging branches. When the topmost leader is too spindly get close to in fear of it snapping off, a telescoping pole is propped up to the highest point of the tree and the end of the pole position is marked. Ideally, a hillside observer or person climbing a nearby tree will be in radio contact with the climber deploying the pole to assist with leveling the position relative to the topmost leader and straight up and down (90°). The pole has a known length, which is added to the initial 70-100 ft measurement. If done carefully, you will get to within a few inches accuracy using this technique. Another advantage to the climber deployed tape drop is that you can measure trees like John Muir and Redwood Creek Giant. These trees cannot be measured by the ground based observer because there aren’t any windows to their tops (or at least not that are within the Impulse® laser’s 1200’ range).

Steve Sillett preparing telescope pole and tape line to measure 85m+ 1926 re-growth eucalyptus regnans of Victoria.Climber in adjacent tree confirms pole is perpendicular and level with top