https://www.forestry.gov.uk/PDF/FCBK049.pdf/$FILE/FCBK049.pdf

Measurement Conventions

Introduction
Diameter
Length
Volume

A. Introduction

B. Diameter

Tree and log diameters will normally be measured overbark by girthing tape, rounding down to the nearest whole centimetre.
The zero point in these tapes is the extreme edge of the brass rectangel, or the point of the hook, according to fitment.

i. Diameter at Breast Height (DBH)

Enusre the trees are girthed at a point on the tree 1.3 m above ground level, which is the breast height point.

The diameter tape must be taut and at right angles to the stem.

on sloping ground always measure the diamter at 1.3m from the ground level on the upper side of the tree.

ii. Mid section diamter

iii. Top diameter

C. Length

Length measurement will not normally include materail beyond the timber point( 7cm diamter), or beyond the point at which no stem is distinguishable.
Special Length conventions apply to pitwood (pitwood is timber used chiefly for roof props in mines.)

D. Volume

The minimum DBH of a tree considered to have measurable volume is 7cm.
Where trees are forked below breast height, each limb is treated as a separate tree and will be girthed as such.
The volume of the tree is therefore the volume of the appropriate limb.
The volume of the butt piece is ignored, its length being excluded in determining the volume of the tree.