The taper step method divides the tree stem into sections of variable length, and the volume of each section is calculated using Smalians formula. Unlike the standard sectional method, the length of the section is related to the taper, i.e. the difference between the diameter at the base of the section and the section top. Whyte (1971) claims that the taper step method gives a more reproducible and realistic index of tree stem volume than the standard sectional method.

The taper step method involves measurement at representative and repeatable points along a stem.

Regulating the drop in diameter (i.e. taper steps) minimises errors in the basic assumptions regarding shape and improves the estimate of volume. Measurement is made at mid-internodes, the lengths of sections being determined by taper steps (big-end minus small-end diameter). Whyte suggests the following taper steps for radiata pine:

Note: A tolerance of +/- 1 cm is allowed in determining actual points of measurement using taper steps. Do not attempt to have steps exactly 2.5 cm, 5.0 cm, etc. as this will increase the cost of measurement unnecessarily.

Bias of method

Theoretically, volume determined using this method invariably is less (negative bias) than that determined by traditional methods, but precision is improved and between tree variance is reduced. Whyte argues that the negative bias is of no consequence as total stem volume is simply a standard or index from which recoverable and merchantable out-turns are deduced.

Precision of method

Positions of measurement on the stem are not fixed so different operators may select different points on any given tree leading to some loss of precision.

However, some gain in precision relative to the standard sectional technique may result because unrepresentative points can be avoided.

Detection of errors

The method does not allow any simple visual check on measurements, therefore one must be alert for:

1. mistakes in measurement,
2. conversion errors (diameter to sectional area),
3. recording errors,
4. arithmetic errors.

Mistakes in measurement can often be detected by a check to see that DOB, DUB and 2BT decrease with height in a sensible manner.

Practical aspects

One valid criticism of the taper step method is that the frequency of measurement is proportional to the taper of the tree and that measurements tend to be concentrated along the upper stem due to changing the taper steps (from 5 cm to 2.5 cm) when the large-end diameter decreases to below 25 cm.

Goulding (1979) estimated that 50% of the taper step measurements on a sample of radiata pine trees in New Zealand were used to calculate less than 5% of the total volume. He recommended a modification of the taper step method, viz:-

• take measurements at stump height (approx. 0.2 m), half-breast height (approx 0.65 m), breast height and about 3 m.
• over the remainder of the stem, take measurements at regular intervals corresponding to the level of accuracy required.
• where a sharp reduction in diameter occurs, measure both above and below the point of sudden change in taper.
• if true (= biological) volume is required, do not confine measurements to mid-internodes but avoid points with a severe abnormality in cross-section such as deep scars and local large swellings.