The diameter and height of a tree must be known before you can manage a forest full of trees or determine their value for forest products. A tree diameter measurement, also called dbh measurement, is always done on the upside of standing trees and demands exact measurements at a specific point on the tree.
Two instruments are often used to measure tree diameter - a steel diameter tape (d-tape) or a tree caliper, a popular steel tape widely used by foresters is the Lufkin Artisan which will accurately measure the most trees in North America to one-tenth of an inch. It is a 3/8" wide steel tape with a length of twenty feet housed in a tough vinyl-covered steel case.
Why Determine a Tree's Diameter
Foresters use tree diameter measurements (along with heights of trees using hypsometers) when determining usable wood volume in standing trees. The tree's diameter is important to determine volume when trees are sold for pulp, lumber or hundreds of other volume determinations. A steel d-tape carried in a forester's vest makes for fast, efficient and accurate dbh measurements.
A tree's diameter can be taken in several ways depending on the required degree of accuracy necessary. The most accurate tool used in making a diameter measurement is a tree caliper and is used most often in exacting tree studies. They are too cumbersome for fast field estimations of tree volume.
A third method in measuring dbh is using a Biltmore stick. This "cruiser's stick" is a scaled "ruler" that is held at arm's length (25 inches from the eye) and horizontal to the tree's dbh. The left end of the stick is aligned with the outer tree edge and the reading is taken where the opposite edge intersects the stick. This is the least accurate method of the three and should be used only for rough estimates.
The Diameter Tape and Volume Tables
Tree volume tables are developed to provide an estimated volume of wood in a standing tree for a certain product by simply measuring the diameter and height. Tables are typically developed with diameters listed along the right side of the matrix and heights along the top. Running the diameter row to the correct height column will give you the estimated wood volume.
Tools used to measure the heights of trees are called hypsometers. Clinometers are the height tool of choice for foresters and Suunto makes one of the best.
The traditional measurement is taken at diameter breast height (dbh) or 4.5 feet above level ground.
Using a Tree Diameter Tape
A diameter tape has an inch scale and a diameter scale printed on a steel tape. The diameter scale side is determined by the formula, circumference divided by pi or 3.1416. You wrap the tape level around a tree's trunk at 4.5 feet dbh and read the diameter side of the tape for the tree diameter determination.