Questions from the Field ...

“What is the BAF when prisms are combined?”

If you want to try the “Big BAF method”, but do not have a large prism, such as 100 or 200 BAF, how can you combine several smaller prisms?

It’s not too hard, actually. The combined BAF for two prisms that are joined to make one large one can be compute with this formula :

For a BAF 30 and BAF 20 combined, you would get a BAF of 98.99, close enough to a BAF 100 for practical purposes, and you can get the Plot Radius Factor for checking borderline trees if you want to use a 100 exactly. Issue #28 of the Newsletter, October 1994, has the details for Plot Radius Factor (see the table below for various BAF combinations).

You can also compute the size for a target on the end of a short stick if you do not want to tape prisms together. For a 12 inch stick, a target of 1.63 inches in width would produce a 200 BAF angle gauge.

The formula is :

Where W is the width of the crossbar, and D is the length of the stick. Both distances are in inches or centimeters.
This and other computations are easy to do with a spreadsheet. The next time you order a Relascope, consider getting a “wide scale”, which has quite a number of the same size bands and is ideal for large BAFs.
For those who are interested, Kim Iles has upgraded his very comprehensive Excel STAR_BAR program to do these calculations as well as the traditional ones like computing the optimum number of count and measure plots. You can order a copy by email from

Originally published January 2003

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