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).
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 very nearly a cylinder.
The volume tables may be corrected where systemic errors are identified,[2] and modified GFC volume tables are available for conifers in the northeast, outside of the original study area.
[3] Girard form class assumes a large minimum merchantable top diameter (40% for large trees, such that a 40” tree would have a top diameter of 16”), and the tables will overestimate if additional logs are counted to smaller diameters[4] commonly utilized by modern mills.