Calibration of Angle Count Instruments ©

Sight on either a flat target (preferable) or a target of circular cross section (tree), and move backwards or forwards until the target is perfectly covered (thumb, stick, Relaskop) or the image is perfectly displaced (wedge prism). For precise calibration, use a target with straight, clearly defined borders.

Measure distance (D) from eye to target (thumb, stick, Relaskop) or from instrument to target (wedge) and width of target (d). With the Relaskop, also measure slope angle and correct slope distance to the horizontal equivalent. Then apply the appropriate calibration formula, namely:

FLAT TARGET

Thumb, stick, Relaskop

Basal Area Factor (BAF) = 10 000 / (1+4(D/d)^2)
where D and d are both in metres.

Wedge prism

Basal Area Factor (BAF) =10 000 { PI [1 + (d/D)^2]  - 1} / 2 PI [1 + (d/D)^2 ]

For a calibration of high accuracy, use a telescopic sight to determine the borderline situation (wedges).

CIRCULAR TARGET

All angle count instruments

BAF (m^2ha^-1) = 2500 (d/D)^2, or = 10 000 Sin^2Q,

where the prism angle is 2 Q.degrees.

References for Formulae

Calibration factors for a given angle gauge may vary slightly with individuals because of physiological and psychological differences.

Bay (1960) - N.Z.J. For. 8 (2): 231-7.

Beers and Miller (1964)-Res. Bull. 786, Ag. Expt. Sta., Purdue Univ.

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Document URLhttp://online.anu.edu.au/Forestry/mensuration/BAF_CALI.HTM
Editor Cris Brack ©
Last Modified DateFri, 9 Feb 1996