Terms used in point sampling

• Diopter : This is measure value of an angle expressed as its sine. One diopter is approximately equivalent to an angle of 0.57° or 34.36 minutes. Since sin 0.57° = 1/100 = 1 diopter
• Basal Area Factor (BAF): It is the multiplying factor associated with any instrument. If the instrument gives basal area in square meter, then, BAF= 2500K² = 10000sin²θ/2 as K= 2 sinθ.
• Plot Radius Factor (PRF) : This factor defines for each tree radius of circle within which the tree. If the radius of the plot circle are measured in the same unit, then plot radius factor is equal to the reciprocal of the ratio defining critical angle in diopter. The PRF is useful to check the doubtful trees.
• Tree factor : Tree factor is the number of tree per ha represented by each tree tallied. This is obtained by dividing the associated plot area into the square meter area of one ha. F = 10000/(πR²) = 10000/(πD²/k²) = 10000k²/πD² = 4*(2500k²)/πD^2. BAF/BA (where BA is basal area a particular tree)
• Calibration distance factor: This factor indicates the distance at which a given angle will exactly cover or prism will exactly deviate, a rectangular target which is one meter wide. Calibration distance factor is 100 times the plot radius factorIf the calibration distance factor is C, then BAF=(10000/(1+C)), Plot radius factor in meter per cm of dia=C/100, Diopter value of an instrument =100/C

Point sampling (PS) is a method of selecting trees to be tallied on the basis of their sizes rather than by their frequency of occurrence. (Developed by Walter Bitterlich in 1948)

Counting from a random point the no. of trees whose breast-height cross-section exceeds a certain critical angle when multiplied by a factor gives an unbiased estimate of basal area/ha. This technique is called : angle count cruising, point sampling, variable plot cruising, PPS (Probability proportional to size) sampling. horizontal sampling has been widely used.

The probability of tallying a given tree depends on its cross-sectional area and the sighting angle used. The smaller the angle, the more stems will be included in the sample.

• PS does not require direct measurements of either plot areas or tree diameters. A predetermined basal area factor (BAF) is established in advance of sampling and resulting tree tallies can be easily converted to basal area /unit area.
• BA conversion factors are dependent or the sighting angle or critical angle arbitrarily selected. Even though all trees are of same basal area, some are counted while others are not because of being far away from sampling points and they do not form an angle bigger than critical angle. Inclusion of trees in tally for a given angle depends upon
  • sizes of trees
  • their distance from the sampling point

There are two types of sampling units. They are fixed and points. Point sampling is based on latter one. Two methods of point sampling,

1. vertical point sampling
2. horizontal point sampling

Generally horizontal point sampling is generally used in field.