This volume table is based on the average volume of trees growing over a large geographical area. Applicable to wider range of distribution. Generally based on two variable i.e. dbh and tree height. This table shows volume of trees by diameter classes and in each diameter class by height classes. It has limited direct application. It can be application to prepare local volume table.

A. Graphical method
B. Regression equation method or method of least squares

A. Graphical method

1. Selection of trees

• Trees with typical height and developed need to be selected
• Evenly distribution of selected trees over the entire area
• Defected trees should not be selected
• Select smaller number of trees rather much larger number of improper selection –best result
• Care: not to select trees above the average
• Number of trees depends on:
  • The grouping adopted
  • Precision required
  • Deviation of individual trees volume from the mean
• Regression method requires lesser number than Graphical method
• Special fellings are rarely carried out for preparation of volume tables

2. Measurements

• Measurement techniques vary with the kind of general volume table
• For example, standard volume tables require measuring standard timber and small wood while commercial volume table require only commercial timber volume.
• These standard timber, standard small wood, etc of a felled tree are measured by dividing the total length into sections of 3 m length, odd lengths being included in the last section, which should not exceed 1.5 m measuring the underbark diameter at the middle of each timber section and overbark diameter at the middle of each small wood section , computing their volumes separately and adding them to give the volume of the tree.
• The data of individual trees is then summarized and finally grouped into height and diameter classes in a table called table of basic averages of which a specimen is given below:

An example of General Volume Table

3. Fitting the curve

• Step-1:
  • Average diameter is plotted against corresponding average volume separately,
  • Smooth curves will be drawn
  • Based on smooth curve, read volume and tabulated
• Step-1a:
  • Average heights is plotted against corresponding average diameter class
  • Smooth curves will be drawn
  • Based on smooth curve, read average heights and tabulated
• Step-2:
  • Corresponding Average volume tabulated from step1 against average heights tabulated from step1a will be plotted
  • Smooth curves again drawn
  • Volumes against average height class will be read and tabulated
• Step-3:
  • Volumes tabulated from step2 again plotted against the middle of diameter classes for each height class.
  • Smooth curve drawn
  • Final volumes are read from these curves against middle of diameter classes and tabulated

Plotting in graph paper

4. Checks

• Aggregate check: the actual volume of trees measured should be checked against the total volume read from the curve, should not exceed 1%
• Height diameter class check: aggregate check for each diameter and height class, not more than 5%, minimum 20 trees
• Relative check: two or more tables which were prepared from the same data, should be checked against each other, should not exceed 3%
• Average deviation check: the average deviation of the actual individual tree volumes from those read from the curve is computed and should be as low as possible.

B. Regression Equation Method

In this method, while the basic data essentially remains the same as in the graphical method (i.e. ht. and dbh), relationships between volume as dependent variable and dbh, ht and form, etc as independent variables are given mathematical expressions by a regression equation. Various workers have developed various equations or models.

Some of them are given as below:

• V = a + bD² + cD²H + dH² + eDH² (Meyor modified)
• V = a + bD² + cH + dD²H (Austrian)
• V = a + bD²H (Combined variable)
• V = aD²H (Constant Form Factor)
• log V = log a + b log D + c log H (Schumacher)
• log V = a + b log D + (3-b) log H (Dwight)
• log V = log a + b log (D²H) (Logarithmic)

where, a, b, c, d, e are regression constant and coefficients, D = Diameter at breast ht, H = Tree height (m), V = Volume (m³)