Calculating how much wood there might be in a forest is a bit like comparison shopping, says professional forester Linda Armstrong.

Foresters don't count and measure every tree in the forest. That would simply take too long. Instead, they sample.

"We use sampling principles every day in our own lives," says Armstrong. If we're shopping for a stereo, she explains, we can't always check every price in every store. Instead, we sample a few brands and stores to get an estimate of what's available.

Similarly, foresters measure a few bits of the forest and, on the basis of those bits, estimate what the whole forest contains. Many of the techniques they use involve little more than careful measuring and some high-school mathematics.

The first step is to choose which bits to sample. It's important, Armstrong says, to avoid picking samples that will give a false picture of the forest. The solution is to choose the sample plots randomly.

You could just throw darts at a map and sample where the darts land, says Armstrong, but affordable access is important. If your darts land well beyond the reach of roads, flying costs will soon eat up the sampling budget.

Personal bias can be avoided by selecting locations on a map before going out in the field, rather than just walking through the forest and choosing good-looking trees, Armstrong says. But once the locations are chosen, you have to stick with them, no matter what you find when you actually visit them.

"If one of your plots is in a clearing with very few trees, it's awfully tempting to move it to an area with more trees. But you have to remember, that clearing represents lots of other clearings in the forest."

For biological studies, the most common approach to estimating the amount of wood is called a fixed area plot. The plots can be any shape, but all plots within a study must have exactly the same shape and dimensions.

"For estimating tree volume, fixed area plot size is chosen with the aim of including 12 to 20 trees," says Armstrong.

The next step is to measure the trees within the plot. Although the goal is to estimate volume, it's not easy to measure that directly without destroying the tree. Instead, you measure the tree's height and its diameter and use those two numbers to calculate the volume.

You can determine the tree's height by using trigonometry. If you measure the horizontal distance between yourself and the tree, and measure the angles leading to the tree's top and base, you have enough information to calculate the tree's height.

In most fieldwork, a device called a clinometer is used to measure the angles as percentages, says Armstrong. Using these numbers and the horizontal distance to the tree, you can estimate the tree height.

The second measurement, the diameter, is taken at a standard height of 1.3 metres, called "breast height." That's simply a convenient height for measuring, Armstrong says -- high enough to avoid the widening at the base of the tree but low enough to be reached easily.

"Then you plug these two numbers into a formula that gives you a volume."

The formula isn't quite that simple, she says. People have spent years working on the equations that relate height and diameter to volume. However, the results of all those years of work are available in simple tables that can provide a quick estimate of the volume of wood represented by a tree of known height and diameter.

"Estimate" is the key word to remember, Armstrong says. The measurements provide an estimate of the volume of wood in the plot, and the plots are used to estimate the volume of wood in the forest. But the numbers aren't exact.

"The thing to keep in mind is that everything is a ball-park figure."

For more information about forestry techniques, contact Forest Resources, Department of Indian Affairs and Northern Development, or call the Canadian Institute of Forestry in Whitehorse at (867) 668-5506.