Units of measurement, symbols, significant digits and rounding off 1.1 Units of measurement and symbols 1.2 Significant digits 1.3 Rounding off 1.4 Bias, accuracy and precision Individual Trees and Logs 2.1 Bole characteristics 2.1.1 Diameter 2.1.2 Height 2.1.3 Bark thickness 2.1.4 Volume 2.1.5 Stem form and taper 2.2 Log characteristics 2.2.1 Diameter 2.2.2 Length 2.2.3 Volume 2.2.4 Weight 2.2.5 Allowance for defect 2.3 Crown characteristics 2.3.1 Width 2.3.2 Depth 2.3.3 Surface area 2.3.4 Volume 2.3.5 Biomass 2.4 Stem analysis Groups of Trees (Stands) 3.1 Number of trees 3.2 Diameter 3.3 Basal area 3.3.1 Fixed-area plots 3.3.2 Angle count sampling 3.3.3 Advantages and disadvantages of angle count sampling 3.4 Height 3.4.1 Mean height 3.4.2 Predominant height, top height, dominant height 3.4.3 Stand height curve 3.5 Volume 3.6 Crown closure 3.7 Crown biomass 3.8 Growth and increment References Appendix 1: Checklist of equipment and materials |
[RWG#2] [Copyright] [Title Page] [Next Page] [Last Page] 3.3.2 DERIVING G - USING ANGLE COUNT SAMPLING The principle of PPS sampling has been applied to various forest sampling problems but most notably to estimating G by angle counting where inclusion of a tree in the count depends on the basal area of the tree and its proximity to the sample point. The procedure is simple, quick and cheap to apply, but applying it properly requires considerable skill and a thorough understanding of the many sources of error likely to affect the estimate. Collectively, the instruments used in ACS are termed angle gauges, the best known and most widely applied of which are the Spiegel Relaskop and wedge prism. Because of its exceptional versatility and other features, the Relaskop is particularly appropriate for forest inventory, whereas the low cost of the wedge prism makes it ideal for controlling silvicultural operations, e.g. monitoring the basal area that is both removed in thinning and left after it. Apart from cost, the wedge prism has two advantages over the Relaskop. Firstly, only two lines need to be aligned to decide whether a tree is 'in' or 'out' and, secondly, slight movement of the tree or instrument does not interfere with the alignment - the tree and its image remain in the same relative position. Thus, the wedge prism is sometimes claimed to be faster and more accurate than the Spiegel Relaskop. In dense stands however, a problem may be experienced with the prism in matching the images to the trees from which they derive! This problem is easily overcome by rotating the prism through 90°ree; in the vertical plane which causes each image to 'return' to 'its' tree . Controlling the many sources of error likely to affect estimates derived from ACS requires careful attention to the procedures described in Section 3.3.2.1. 3.3.2.1. Controlling Error in Angle Count Sampling The procedures for controlling error in angle count sampling are grouped below into three categories related to (a) general matters, (b) the wedge prism, and (c) the Spiegel Relaskop . (a) General
The following comments relate specifically to the standard metric Relaskop.
Generally speaking, it is unnecessary to calibrate or check the calibration of either manufactured wedge prisms, which today are made from lenses of 'eyeglass' quality, or of the bands and band combinations of a Relaskop. However, if the prisms are cut locally, e.g. in a scientific workshop on a university campus, the user will need to calibrate each instrument. Calibration is best done in the laboratory . Recommended procedure is:
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January, 1999.