Why measure
ANU

 Why measure
Forest Measurement and Modelling.

When you measure an object, essentially all you are doing is counting the number of standard pieces it takes to be the same size as the object. For example, the length of an Olympic swimming pool is 50 m, because 50 one metre standard lengths laid end-to-end would be exactly the same length.

The same principle works when we deal with something that is too small to be conveniently handled by straight counting of standard pieces. For example, the heights of people or of trees are not usually an exact number of meters. So, we simply say that the standard meter is actually a group of smaller standard lengths - centimetres. Each centimetre can also be a group of still smaller objects - millimetres. So we can describe or measure anything simply by counting the number of standard or groups.

We may define measurement as:
the determination of size in relation to some observed standard, e.g. metre, kilogram, second, ampere, degree Kelvin, candela, mole, or some unit derived from these seven basic units.
Why measure? Measurements are made for a number of reasons:

  1. To learn something. The famous British scientist, Lord Kelvin, said:
    "When you measure what you are speaking about and express it in numbers, you know something about it, but when you cannot (or do not) measure it, when you cannot (or do not) express it in numbers, then your knowledge is of a meagre and unsatisfactory kind."

  2. To meet legal and political obligations. Lund (1998) outlines a number of International Treaties and Agreements that bind governments (and some individuals) to provide measurements about the land and resources.

  3. To help managers make appropriate decisions. Systematic collection of information about forests often begins as a management reaction to perceived shortages of forest resources available (Brack, 1997). Managers collect this data to help them allocate these scarce resources.
Standard units Australia, as a signatory to the Metric Convention, has made a commitment to use the International System of Units (SI for short). These units are precisely defined so that, for example, 1 metre in Australia can be exactly compared with 1 metre in Europe.

The standard units are:
  • metre (m) - length
  • kilogram (kg) - mass
  • second (s) - time
  • ampere (A) - electric current
  • degree Kelvin (K) - thermodynamic temperature
  • candela (cd) - luminous intensity
  • mole (mol) - amount of substance

Other units can be derived through the inclusion of decimal prefixes (e.g. deci, kilo). Some units are not included in the SI, but are nevertheless commonly used and accepted:
  • hectare (ha) - area
  • day (d) - time interval
  • hour (h) - time interval
  • minute (min) - time interval
  • tonne (t) - mass

Standards also extend to the way measurements are recorded. These standards include the way symbols are written and large and small numbers are reported. The Code of Forest Mensuration Practice (RWG#2, 1999) at http://www.anu.edu.au/Forestry/mensuration/rwg2/code/1-1.htm provides a detailed list of these standards for writing.

Throughout history, many standards have been developed and used in measurement, including:
  • yard - length from the nose to the tip of an out-stretched arm (in England).
  • yard - length of a yardstick kept by the King in medieval France.
  • rod - combined length of the left foot of 16 Englishmen.
  • cubit - length of a human forearm.
  • inch - length of 3 barley corns.
  • Hoppus measure - tree or log volume that approximates the true volume (without the need for calculating the value PI).
    • Volume in super feet Hoppus was calculated as: (G/4)^2*L/12
      where G denotes girth (inches) and L is log length in feet.
    • Volume in cubic feet Hoppus was calculated as: (G/4)^2*L/144



[measure.htm] Revision: 6/1999
Cris.Brack@anu.edu.au