Girard form class

Girard form class is a form quotient calculated as the ratio of diameter inside bark at the top of the first 16 foot log to the diameter outside bark at breast height (DBH). Girard form class is the primary expression of tree form in the United States.

To allow for log trimming and a 1-foot stump height the diameter inside bark is measured at 17.3 feet above the ground. The closer the form class value to 100 the closer the log resembles a cylinder, where a value of 100 means the log is nearly a cylinder.

Volume tables created with Girard form class use the assumption that trees of the same diameter and merchantable height have similar rates of taper above the first log. This implies that volume differences in trees with the same diameter and merchantable height can be mostly attributed to taper variations in the first log. Volume tables using Girard form class are composite tables and are created independently of tree species.

To calculate GFC, you need several things: DIB (Diameter inside bark) @ 17.3', DOB (diameter outside bark) @ 17.3', DBH (diameter @ 4.5'), and [single] bark thickness (which will be used to calculate double bark thickness).

Example:
Bark Thickness = .56"
Double Bark Thickness = 1.12"
DOB @ 17.3' = 18.2"
DIB @ 17.3' = 17.08" (18.2" - 1.12")
DBH = 21.9"

((17.08")/(21.9")) * 100 = 77.99%