Diameter at breast height
Tree trunks are measured at the height of an adult's breast. However, this is defined differently in different countries and situations. In continental Europe, the UK, and Canada the diameter is measured at 1.3 metres above ground. In the US, Australia, New Zealand, Burma, India, Malaysia, and South Africa, breast height diameter is measured at a height of 1.4 metres. Previously 4.5 ft (1.37 m) was used. In many cases the height makes little difference to the measured diameter. Ornamental trees are usually measured at 1.5 metres above ground.
However, some authors maintain that the term DBH should be abolished precisely because the heights at which the diameter is measured are so variable and because it may strongly influence forestry calculations such as biomass. Instead Dx was proposed whereby the x denotes the exact height above the floor (and along the stem) at which the diameter is measured. For instance D130 denotes a diameter measured at 130 cm above the floor and along the stem.
On sloping ground, the "above ground" reference point is usually taken as the highest point on the ground touching the trunk, but some use the average between the highest and lowest points of ground. If the DBH point falls on a swelling in the trunk it is customary to measure the girth below the swelling at the point where the diameter is smallest. Other ambiguous settings for determining the exact place where to measure the diameter is given in Dahdouh-Guebas & Koedam (2006).
Treetec, a company based in Melbourne, has created a free online calculator to determine the DBH for multi-stemmed trees as per AS 4970-2009 Protection of Trees on Development Sites. 
The two most common instruments used to measure DBH are a girthing (or diameter) tape and calipers.
A girthing tape actually measures the girth (circumference) of the tree; the girthing tape is calibrated in divisions of π centimetres (3.14159 cm). The measure assumes the trunk has a circular cross-section and gives a directly converted reading of the diameter. It is accurate for most plantation trees.
Calipers consist of two parallel arms one of which is fixed and the other able to slide along a scale. Calipers are held at right-angles to the trunk with the arms on either side of the trunk. Precision can be improved on non-circular stems by averaging two caliper measurements taken at right-angles. Electronic calipers are also available enabling highly accurate measurements to be taken and stored for further analysis.
DBH is used in estimating the amount of timber volume in a single tree or stand of trees utilising the allometric correlation between stem diameter, tree height and timber volume, (Mackie, 2006). It can also be used in the estimation of the age of veteran trees, given that diameter increment is the only, "constant non-reversible feature of tree growth", (White, 1998).
- Cris Brack, PhD (UBC) Standard point on tree bole for measurement. Forest Measurement and Modelling. Retrieved 2009-04-18.
- Russell M. Burns and Barbara H. Honkala Silvics Manual, Volume 2, Glossary (USDA Forest Service)
- Brokaw, N. & J. Thompson, 2000. The H for DBH. Forest Ecology and Management 129: 89-91.
- Brokaw, N. and Thompson, J. (2000). The H for DBH. Forest Ecology and Management 129: 89-91.
- Hamilton, G. J. (1975). Forest Mensuration Handbook. Forestry Commission Booklet 39. HMSO, London.
- Mackie, E. D. and Matthews, R. W. (2006). Forest Mensuration, a handbook for practitioners. HMSO, Edinburgh. ISBN 0-85538-621-5
- White, J. (1998). Estimating the Age of Large and Veteran Trees in Britain. Forestry Commission Information Note 12. Surrey.