(1) Definitions diameter
In mathematics, the diameter is defined as a straight line through the center point of a circle. Based on the understanding that the measurement of the diameter in the forestry sector, especially in geometry timber is carried on a tree stand, part of the tree is cut and branches. Diameter measurement is important because it is a dimension that can be directly measured and the diameter can be determined from the cross-sectional area of trees / basal area and volume of the tree.
Anonymous (2011) provide an understanding of the diameter as a fundamental dimension of the circle. Thus the trunk diameter can be defined as the length of the line between two points on a circle around the stem through the center (axis) of the stem. Diameter rod into the dimensions of the tree stands are most easily measured because the measurement is performed at the bottom of the tree. However, the shape of the tree trunk generally has narrowed to the top end (taper), then from a tree can be obtained infinite number of values stem diameter corresponding number of dots on the base of the stem to the tip of the rod.
Therefore the need to set the location of stem diameter measurements will characterize the characteristics of a tree. On that basis ditetapkanlah diameter at breast height or dbh (diameter at breast height) as a measurement standard trunk diameter. There are at least three reasons why the diameter measured at breast height elevation, namely:
(A) The reasons of practicality and convenience when measuring, the measurement is done without having to bend or tiptoe.
(B) In most trees at breast height elevation free from the influence of the buttresses.
(C) The diameter at breast height (dbh) generally have a fairly close relationship with variables (dimensions) other trees.
In practice dbh measurements, there are differences among some countries about chest-high altitude, namely:
(A) State the metric system of measurement, dbh = 1.30 m above ground level (dat).
(B) USA and Canada, dbh = 4 ft 6 in = 1.37 m above ground level (dat).
(C) the United Kingdom and some Commonwealth (British system of measurement), dbh = 4 ft 3 in = 1.29 m above ground level (dat).
(D) Japan, dbh = 4 ft 1.2 in = 1.25 m above ground level (dat).
(2) Area of the base plane (LBDs)
Ash’ari et al. (2012) in his book had explained the formula or calculation of basal area (cross-sectional area) tree. Formulator or for an area basis in principle refers to the size of the diameter, but the reality on the ground is generally the calculation of basal area using girth.