Measurement Basics

The Native Tree Society has developed tree measuring guidelines for use in our data collection efforts.  These guidelines are  "The Tree Measuring Guideline of the Eastern Native Tree Society" by Will Blozan.  This document outlines the techniques we use to measure tree heights, girths, and crown spread.  In addition it touches on volume measurements, and some site characterization techniques. We are continuing to push the envelope, to use a cliché, on experimenting with and developing new measurement techniques to better characterize individual trees and forested localities.  Many of these are discussed here on the website and within ongoing discussions taking place on our BBS

Dr. Lee Frelich, Dr. Don C. Bragg, and Dr. Robert Van Pelt and Will Blozan,Robert Leverett and Michael Taylor are writing a book on tree measurement expanding on the techniques described in our measurement guidelines along with additional techniques. The book will be entitled "Dendromorphometry - the Art and Science of Measuring Trees in the Field."  It is intended to serve as a credible book reference for measurement techniques.  In the meantime new articles, essays, and diagrams will be posted in the Bulletin of the Eastern Native Tree Society and to the NTS BBS

The two biggest problem areas in basic tree measurement process involve errors in height measurements and invalid girth measurements.  These nature of the problems problems and solutions are briefly outlined below.  For more detailed discussions see the links cited above.


Height Measurements

There have been dozens of techniques developed to quickly measure tree heights, from Biltmore Sticks, to string-stick-tape measurements, and tape and clinometer measurements to name the most prominent.  Why is there a need to develop and promote these laser rangefinder - clinometer tree height measurement methodologies?  Frankly, these previous techniques do not produce height measurement that are accurate enough to be useful for most scientific purposes.  In spite of the gross inadequacies of the height data produced by these inaccurate methods, the data has been used in numerous publications and distributed across the web.  If the height data is bad, then the conclusions that are drawn based upon that inaccurate data are also bad.  There are methods of obtaining good tree heights using a total station and other formal surveying techniques, however these are time consuming and not practical to use when measuring a large number of trees on a site.  The laser rangefinder - clinometer techniques allows trees to be measured quickly with a high degree of accuracy. Typically the heights obtained using this technique is within one foot or less of the actual height of the tree.

When using standard baseline tangent techniques to estimate tree height, first a distance is measured from the base of the tree to point of measurement using a tape. Then the angle to the top of the tree is measured using a clinometer.  the tree height is equal to the tangent of the angle to the top x the distance to the base of the tree. There are two main sources of error in these techniques used to calculate tree heights.  The top of the tree may be offset horizontally from the base of the tree and the top of the tree may be misidentified.  For example the magnitude of error, with a 100-foot baseline to the trunk of a tree and shoot to a crown point at an angle of 45 degrees, if the top is at a horizontal distance from you of 80 feet, you are going to make a measurement error of 20 feet height.

To put the needed emphasis on the point, if the spot in the crown that you are shooting has a different horizontal component of distance as compared to the length of your baseline, you ARE going to make an error in height.  A preliminary analysis of a set of data from about 1500 trees found the average distance the top of the tree was offset from the base was over 13 feet.  The offsets for broad-topped broadleaf trees are often larger. If the true top of the tree is misidentified then the potential errors in height measurements is even greater.  A listing of some of the height errors that made it through at least a minimal vetting onto various big tree lists can be found here:  Mismeasured Trees (2005).

Diagram by Jeff LaCoy, 2009
The ENTS Method

The sine-top/sine bottom or ENTS Method is a fairly straight forward process. Essentially the straight-line distance to the top of the tree is measured using a laser rangefinder. Then the angle to the top of the tree is measured with a clinometer. The height above eye level is calculated using a pocket calculator to be the trigonometric sine of the clinometer reading x the distance measured to the top of the tree. The same process is used to measure how far the base of the tree extends vertically above or below eye level. Then the number for the height of the base of the tree above or
below eye level is added or subtracted from the number for the top of the tree to get the total height of the tree. In the ENTS technique it does not matter if the top is directly over the base of the tree as the height above eye level of the top and the height of the base above or below eye level are calculated as independent triangles above or below a horizontal plain. In addition the tree can be scanned using the laser to better identify the true top of the tree.  An expanded guide to basic tree height measurements can be found here: The Really, Really Basics of Laser Rangefinder/Clinometer Tree Height Measurements  by Edward Frank

There are other techniques that will produce tree heights that are acceptable for our usage.  These include pole measurements, a climber deployed tapeline, and various professional surveying techniques.  Michael Taylor has developed a three station Triangle Method that allows the height of a tree to be measured from three ground points using a transit to an accuracy of less than an inch. There are other methods of cross-triangulation using hand held instruments that potentially could achieve fairly good accuracies, but there also are  potential errors involved that still call to question the results obtained through these techniques.


Girth Measurements

Girth measurements are fairly straight forward and it would at first pass seem to be something that would be difficult to get wrong.  A distance of 4.5 feet (1.5 meters in countries using metric measures) is measured upward from the base of the tree.  At that point a tape is wrapped around the tree perpendicular to its trunk and that taped distance is the girth of the tree.  This works well for the vast majority of trees.  There must be special considerations for trees that are low branching, are slanted over, or are growing on steep slopes. What to do in these situations is described in the Tree Measuring Guidelines.  The big problem with many champion tree lists is that those compiling the lists, or those people submitting trees to the list fail to distinguish multitrunk trees from those with single trunks.  Multiple tree trunks may grow together at the base to forma single large mass, but If the tree has more than one pith at ground level it is a multiple-stemmed tree. Mutlitrunk trees are worthwhile documenting and may be impressive, but they should be not be intermixed on champion tree lists with single trunk trees.  Some ideas on how to best document multitrunk trees are presented here Multitrunk Trees and Other Forms.

These quick examples discuss only two measurement parameters.  Further discussions on these and other measurement parameters and procedures are found at the links above on this website and within ongoing discussions on the BBS  in the Measurement and Dendromorphometry forum and other forums.

The Trees Database is an experimental project by Steve and Mitch Galehouse, with utilization by the Native Tree Society in mind.   This site allows you to review, search, sort, add, and download information concerning native (and exotic) tree species, including height, girth dimension, geographic location, and site specifics. The intent is to include as many tree species from as many locations as possible to make it possible to compare tree species' size potentials across their native ranges.


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