Tree volume estimation: graphical method©

This method involves measuring diameter at various positions along the stem, plotting the data, and joining the points by curve lines giving a diametral outline of the stem - called a taper curve or stem profile.

Stem volume is then calculated in one of several ways:

Read from the taper curve the diameter at selected positions along the stem and calculate stem volume by summing the volumes of sections using either Smalian's or Huber's formula (Not strictly the graphical method, more like a detailed standard sectional or taper step).
If sectional area rather than diameter is plotted against height, volume is represented directly by the area under the curve and can be determined by planimeter or square counting.
The most practical and accurate way of determining volume from graphical data is by computer. A program calculates the volume of the section between each set of successive points and accumulates the sum to any diameter limit.

Advantages

Flexibility;
Applicable to all types of trees, i.e. excurrent and deliquescent;
Measurements can be made at any odd and convenient interval along the stem without complicating volume calculations;
Plotting provides a visual check on measurement - any sudden bulge or indentation prompts a check;
The method makes no assumptions about overall stem shape. Calculating volume from the area under the curve can be used with equal accuracy for all degrees of taper.
- Note: If points are joined by straight lines, some assumptions about shape are being made For example, if plotting sectional area then you are assuming each section between the points is a second degree paraboloid.
The graph provides a permanent and convenient record of the stem which has many uses, e.g. total volume or volume to any diameter limit (OB or UB) can be read off; volume by assortments can be determined;
Simple to program by computer.

Disadvantages

Impractical for standing trees.
If many measurements are taken, the method is time consuming (to realise the full advantages of the method, many measurements should be taken).

Bias

The method is liable to bias because the points of measurement are chosen subjectively. However, the large number of measurements tends to control this.

Precision

The more measurements taken, the more precise will be the estimate for the same operator and between operators. Note that:

the more measurements taken, the closer will the volume estimate approximate biological volume.
the graphical method is used as the standard for comparing volume as determined by other methods.

Other measurement methods

http://online.anu.edu.au/Forestry/mensuration/GRAPHIC.HTM
Cris.Brack@anu.edu.au
Mon, 29 Apr. 1997