Trees have a wide variety of sizes and shapes and growth habits. Specimens may grow as individual trunks, multitrunk masses, coppices, clonal colonies, or even more exotic tree complexes. Most champion tree programs focus finding and measuring the largest single-trunk example of each species. There are three basic parameters commonly measured to characterize the size of a single trunk tree: tree height measurement, tree girth measurement, and tree crown measurement. Foresters also perform tree volume measurements.
Various groups involved in documenting big trees around the world use different methods. Other parameters also measured include trunk and branch volume, canopy structure, canopy volume, and overall tree shape.Leverett, Robert T. 2007. A new look at tree trunk modeling: old formulae and new. Bulletin of the Eastern Native Tree Society, Volume 2, Issue 4, Summer 2007, pp. 5-11.Leverett, Robert T.; Blozan, Will; and Beluzo, Gary. 2008. Modeling tree trunks: approaches and Formula. Bulletin of the Eastern Native Tree Society, Volume 3, Issue 2, Spring 2008, pp. 3–13. The appropriate measurement protocols for multi-trunk trees and other more exotic forms are less well-defined.
Height
Tree height is the vertical distance between the base of the tree and the highest sprig at the top of the tree. The base of the tree is measured for both height and girth as being the elevation at which the pith of the tree intersects the ground surface beneath, or "where the acorn sprouted." On a slope this is considered as halfway between the ground level at the upper and lower sides of the tree. Tree height can be measured in a number of ways with varying degrees of accuracy.Tree height measurement There are direct measurements. Shorter trees can be measured using a long pole extended vertically to the top of the tree. Larger trees can be climbed and a tape measurement made from the highest point of the climb to the base of the tree. The distance to the top of the tree can be measured from that point, if needed, using a pole. Historically the most direct method of finding a tree's height was to cut it down and measure it prostrate on the ground.
Tree heights can also be measured remotely from the ground. The most basic remote height methodologies are all variations of the stick measurement.Champion Trees of Pennsylvania, Measurement. http://www.pabigtrees.com/Measure.aspx Accessed March 4, 2013.Champion Trees of Pennsylvania, Measurement Notes. http://www.pabigtrees.com/measure_notes.aspx Accessed March 4, 2013. The height is calculated using the principle of similar triangles. A short stick is held out pointing vertically at arm's length by its base pointing vertically. The surveyor moves in and out toward a tree until the base of the stick above the lower hand aligns with the base of the tree and the top of the stick aligns with the top of the tree. The distance from the lower hand to the surveyor's eye is measured, the distance from the lower hand to the top of the stick is measured, and the distance from the eye to the base of the tree is measured with a tape. The ratio of distance from the eye to the hand is to the distance from eye to the base of the tree, as is equal to the ratio of the length of the stick to the height of the tree provided that the top of the tree is positioned vertically over the base.
A second method uses a clinometer and a tape and is commonly employed in the forestry industry. This process applies the trigonometric tangent function. In this process the horizontal distance is measured to the trunk of a tree from a sighting position. The angle to the top of the tree is measured with the clinometer.
If the tree extends below eye level the same process is used to determine the length below eye level and that is added to height above eye level to determine total tree height. Different clinometers have different reading scales, but all apply the same function. Calculations are as above if it reads in degrees. If it has a percentage scale, then the percentage is multiplied by the distance to the tree to determine a height or extension above and below eye level. Others have a 66' scale where if used at a distance of 66 feet from the tree the height above or below eye level can be directly read from the scale. Errors frequently accompany this type of measuring. The process assumes that the top of the tree is directly over the base of the tree. The tree top may be offset significantly from a point directly over the base (or level point on the trunk). Typical errors from this effect are often on the order of 10 to 20 feet. A bigger problem is misidentifying a forward leaning branch for the actual top of the tree. Errors associated with this mistake may yield measurements that are off many tens of feet and some errors of over forty feet and more have made it onto various champion tree lists, and in at least two cases, the errors have exceeded 60 feet.Rucker, Colby. 2008. Great Eastern Trees, Past and Present. Bulletin of the Eastern Native Tree Society, Volume 3, Issue 4, Fall 2008, pp. 6–40.
A third remote method is called the sine-height method or ENTS method.Blozan, Will. 2004, 2008. The Tree Measuring Guidelines of the Eastern Native Tree Society. Accessed March 4, 2013.Blozan, Will. 2006. Tree Measuring Guidelines of the Eastern Native Tree Society. Bulletin of the Eastern Native Tree Society, Volume 1, Number 1, Summer 2006. pp. 3–10.Frank, Edward Forrest. January 12, 2010. The Really, Really Basics of Laser Rangefinder/Clinometer Tree Height Measurements. http://www.nativetreesociety.org/measure/really_basic_3a.pdf Accessed March 4, 2013. It requires the use of a laser rangefinder and clinometer. In this method the distance to the top of a tree is directly measured by the surveyor using the laser rangefinder. The angle to the top is measured with the clinometer. The height of the tree top above eye level: [height = sine Θ of angle x distance to the top] the same process is used to measure the extension of the base of the tree below or even above eye level. Since the measurement is along the hypotenuse of the right triangle and both the top and bottom triangles are independent, it doesn't matter if the tree top is offset from the base as this does not affect the calculation. In addition the top branches of the tree can be scanned with the laser rangefinder to find which top is actually the tallest and the major error of misidentifying the top can be avoided. If the true top of the tree is misidentified the height measurement for the tree will simply be short by some amount, and will not be exaggerated. The height will be correct for the target being measured. With calibration, multiple shots, and a technique to deal with scales that read only to the nearest yard or meter, tree heights can typically be measured to within one foot using this methodology. Other measurement techniques include surveying with a transit and a total station, extended baseline method, the parallax method, and the three verticals method.
Girth
Girth is a measurement of the distance around the trunk of a tree measured perpendicular to the axis of the trunk.Tree girth measurement Use of girth to arrive at an equivalent diameter is an older forestry measurement that is still used. In the United States girth is measured at a height of 4.5 feet above ground level. Elsewhere in the world it is measured at a height of 1.3 meters, 1.4 meters, or 1.5 meters.
Tree girth measurement is commonly performed by wrapping a tape around the trunk at the correct height. Tree girth may also be measured remotely using a monocular with reticle, through photographic interpretation, or by some electronic surveying instruments. In these remote methods a diameter perpendicular to the surveyor is what is actually being measured and that is converted to girth by multiplying that number by pi. Many trees flare outward at their base. The standard height up the trunk for determining girth is easily measurable and in most trees it is above the majority of the basal flare and gives a fair approximation of the size of the trunk. For the largest trees, or those with a broad basal flare far up the trunk, it would be appropriate to also measure a second girth above the flare and to note this height.
If there are significant low branches that emerge below this height, ignoring any minor epicormic sprouts and dead branches, then the girth should be measured at the narrowest point below the lowest branch and that height noted. If there is a burl or protuberance at the measurement height, then the girth should be measured immediately above the protuberance or at the narrowest point of the trunk below the protuberance and that height noted.
If the tree is growing on sloping ground the base of the tree should be considered to be where the center or pith of the tree intersects the supporting surface below, typically at mid-slope along the side of the tree. If the tree is large and this measurement would place one portion of the girth loop below ground level, then the measurement should be made at 4.5 feet above ground level at the high side of the slope.
It should always be noted whether or not the tree being measured is single or multitrunk. A single trunk tree is one which would only have a single pith at ground level. If the tree would have more than one pith at ground level, it should be listed as a multitrunk tree and the number of trunks included in the girth measurement noted. If the flare at the base of the tree extends above this default girth height, then ideally a second girth measurement should be collected where possible above the basal flare and this height noted.
Crown spread
Crown spread is a measure of the footprint or plan area of the crown of the tree expressed as a diameter. The most basic crown spread measurement is the average length of two lines across the crown area. The first measurement is made along the longest axis of the crown from one edge to the opposite edge. A second measurement is taken perpendicular to the first line through the central mass of the crown. The two values are averaged to calculate crown spread. A second method is to take a series of four or more spokes running from the side of the center of the trunk to the edge of the crown. The more spokes measured the better the crown size is represented. The crown spread is twice the average of all of the spokes. For trees in open areas, crown spreads can also be measured from aerial imagery such as Google Earth. There are length measurements tools built into the program that will allow multiple diameters across the crown to be measured or averaged. Alternatively there are add-on software packages available that can allow an area to be outlined on the Google Earth image and the area enclosed calculated. This can then be converted to crown spread. has also provided four options for measuring the crown area through compass and clinometer surveys around the outer edge of the crown or through a combination of measurements from the edge of the crown and to the trunk, and those around the crown perimeter. Maximum crown spread and maximum limb length measurements can also be measured if needed.