Optical prisms (wedge prisms) for measuring stand basal area ©

The optical properties of wedge shaped prisms are particularly suited to angle count sampling, and since the early 1960s, the OPTICAL WEDGE or WEDGE PRISM has been used extensively as an angle gauge for basal area estimates.

The prism is a wedge shaped piece of glass which refracts light rays, thus establishing a critical angle. A tree viewed through the prism is displaced through an angle depending on the DIOPTRE STRENGTH of the prism:

One prism dioptre is equivalent to the right angle displacement of an object by 1 unit per 100 units of distance.

In sweeping an area, each tree is counted whose lateral displacement of the image is less than its d (DBHOB).

The wedge prism has two major advantages over instruments like the relaskop :

only TWO lines have to be aligned
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, some problems may be experienced in matching the image to the tree! This problem can be overcome by turning the prism through 90 degrees which causes each image to 'return' to the tree from which it derives.

Calibrated wedge prisms can be purchased for $35 to $45.

Calibration of the wedge

Effect of Slope on Estimates of G (Wedge Prisms)

To reduce the standing basal area estimate to a horizontal area equivalent, a correction must be made for slope. This is done by multiplying the basal area estimate by Sec Q, where Q is the maximum angle of slope at the angle count spot. Slope can be corrected for in other ways (Barrett and Nevers, 1967) but all have the same result. The method outlined above is the one generally recommended.

Correction can be ignored for slopes less than 5 as the error is less than 0.5%

         Slope   Secant    Error (%)
           1     1.000     0.0
           2     1.000     0.0
           3     1.001     0.1
           4     1.002     0.2
           5     1.004     0.4
           6     1.005     0.5
           7     1.008     0.8
           8     1.010     1.0
           9     1.012     1.2
          10     1.015     1.5
          15     1.035     3.5
          20     1.064     6.4
          25     1.103    10.3
          30     1.155    15.5

Index    Help    Authors

Document URL	http://online.anu.edu.au/Forestry/mensuration/PRISMS.HTM
Editor	Cris Brack ©
Last Modified Date	Fri, 9 Feb 1996