Set up a spreadsheet. It’ll make this a lot easier.

You’ve got a bunch of trees in your points, with dbh and number of logs. So you need to figure out how much volume per acre is represented by each of those dbh-log combinations. So a 16 inch, 2 log tree represents a certain amount of volume per acre, a 22 inch, 1 log tree represents a different amount of volume per acre, etc.

To do this, you’ve got to figure out the tree factor for each dbh class. It’s easier to do this by 2 inch classes (half the work), but you can do it for 1 inch classes, too.

The tree factor for a certain dbh class is

43560

PI(dbh x 2.75)2

So you get a separate tree factor for each dbh class.

For example, the tree factor for 10 inch trees is about 18.33, 12 inch trees is about 12.73, 20 inch trees is about 4.58, etc., etc.

Figure these out for every dbh class you might come across, say from 10-40 inches. (See why 2 inch classes is easier.)

Once you’ve got the tree factors figures, you take the volume per tree you get from a volume table (Doyle, Intl ¼) and multiply it by the tree factor. This gives you a volume per acre figure represented by that tree.

For example, say you’ve got a 22 inch, 1 log black oak. You’ve figured out the tree factor for 22 inch trees is about 3.79. And you look at the Doyle log rule (form class 78) and see that a 22 inch, 1 log tree has 174 BF. So you take the tree factor (3.79) times the tree volume (174 BF), and get 659. That 659 means that each 22 inch, 1 log tree represents 659 BF/acre.

Do that for every tree in your point. Then add up the BF/acre figures for each tree to get a total BF/acre volume for the point.

So then you’ll have an estimate of volume per acre for each point of your cruise. For the cruise I just did, these ranged from 0 to 15000 BF/acre.

Then take these volumes per point and add them up, and divide by the number of points. That’ll give you a mean volume per acre for the whole tract.

For standard deviation, get the spreadsheet to do a standard deviation on the volumes per point for each point you’ve done so far. Or, if you really hate yourself, do it by hand. On the cruise I just did, the standard deviation was around 3000 BF. That’s pretty typical around here. Might be lower or higher where you all are, depending on how even the topography and the timber are.

Then you’ve got the standard deviation figure you need to plug into the formula on page 38 to find out how many points you need to do. You can also use the formula on page 39 with the CV (coefficient of variation) instead. CV is just the standard deviation divided by the mean, times 100 (see page 19).

The rule of thumb for number of points is 40 + (0.25 * number of acres). So if you’re doing 211 acres, that’s 93 points. Plugging the numbers into the formulas should be fairly close to this, unless the timber you’ve sampled so far is really variable (then it’ll be higher), or really even (then it’ll be lower).