The role of the American Forests National Cadre is set to expand, although this is not yet a done deal. However, we are moving toward establishing stronger ties to the state big tree program coordinators. There will not be enough Cadre members to do all the certifying of candidates to the National Register, so state certification will continue to play a big role. This means that certifiers will often be urban and country foresters answering directly to the state coordinator. A few may own lasers, but the majority will continue measuring tree height using tape and clinometer. So, how will the Cadre assist the state coordinators minimize the kinds of tangent-based errors that we so commonly see?
We have plenty of diagrams showing the source of tangent-based errors and formulas for calculating their magnitudes. We also have two methods for minimizing tangent-based errors: (1) crown-point cross-triangulation, and (2) positioning so that the line of sight to the trunk is at 90 degrees to the vertical plane containing the top and end of the baseline. But neither of these techniques is easy to apply in uneven terrain for trees with large crowns. The methods are easier to talk about from the relative comfort in front of one's computer. This is the voice of experience speaking. Will Blozan and I measured many, many trees using cross-triangulation in the days before the infrared laser rangefinder. It is kind of fun to measure height by cross-triangulating, but you need an assistant and room to maneuver.
What else can we do to help state people who will continue using tape and clinometer? One way is to develop rules for minimizing the impact of instrument error. An oft repeated rule for clinometer measurements is to get far enough back that the angle from eye to top does not exceed 45 degrees. How good is this advice? I set out to investigate and came up with some surprising results. Here is a summary.
1. If there is angle error, but no distance error, then height error due to the angle error is minimized at exactly 45 degrees - no more, no less. But remember, there can't be any error in the distance measurement for this to be correct.
2. If there is distance error, but no angle error, then height error due to distance error steadily decreases with greater distance, and consequently, diminishing angle.
3. If there is both a distance error and an angle error and they are both over or under (in the same direction), then their combined impact on height error is minimized somewhere between 30 and 45 degrees. As imprecise as this rule is, it is better than simply applying the 45-degree prescription.
4. If there is both a distance error and an angle error and they are in opposite directions, it's Katie bar the door. The combined impact on height error is all over the board. All bets are off.
To help people analyze specific measuring scenarios, the attached Excel spreadsheet is attached. The green cells are for data entry. After entering a tree height and angle and distance errors, you can see the sweet spot, i.e. angle and corresponding distance for a tree of the specified height where the height error is minimized.
As a spinoff of this kind of analysis, we might investigate offsetting errors. For example, if we specify a height, distance to the tree, and distance error, what angle error would offset the distance error. Conversely, we could specify height, distance, and angle error to get the offsetting distance error. This kind of analysis admittedly has limited value. But where a measurer has unknowingly made offsetting errors and argues that his/her technique is flawless, we need to be able to explain the intricacies of the interactions.
As a final point, I realize that this discussion and the spreadsheet go far beyond what will hold the attention of casual tree measurers. They not need concern themselves with this discussion or look at the spreadsheet, but for state-level tree certifiers, there is no justification for tolerating the continuation of incompetence. Measurers who consistently mis-measure nominated trees by significant amounts and refuses to upgrade their skills is incompetent. These people need to be weeded out even if that leaves certain, otherwise, deserving trees from being certified. The credibility of state big tree programs is at stake, and sense the state programs will continue to feed the national one for at least several years to come, this is a problem that has to be solved.
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Robert T. Leverett
Co-founder and Executive Director
Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest