Prisms

[SwampDonkey]

sunflower4dayz , hope you found your answer. 

Welcome to the forum anyway. 

[SwampDonkey]

Tom here is a way to demonstrate what Ron talked about when using a prism and determining "in" or "out" trees. The yellow circles are trees, the blue circles are limiting distances of trees of different diameters. The red dot in the middle is the point centre where the prism is held for the circular sweep. If you didn't have a prism, you could calculate manually the limiting distance from a tree centre in order to count it. So, if the blue line does not capture the point centre then it's not an "in" tree. The bottom tree is marked borderline because it's just on the outer reach of the point centre.

Ignore the fact I miss labeled the prism point as the "plot centre". My circles are a little wobbly.

[SwampDonkey]

From further reading in Husch, Miller and Beers (1982), beenthere's prisms are termed uncalibrated prisms. They were either home made or done by a lens shop that might make prescription lenses. A calibrated prism would be one manufactured for 10, 12, 14 etc ..ft²/acre and won't have fractions like 9.6 etc... Nothing wrong with them, as long as you calibrate them as the gentlemen obviously did to arrive at those BAF's. The prisms are usually cheaper to get made uncalibrated. The procedure I discussed earlier.