A Very Efficient Sampling Methods for Cruising Timber
by Doug Corrin, Malaspina University-College
This paper assumes the reader is familiar with the basic concepts of prism cruising; also values given are based on metric units (i.e. m3 and hectares (ha)). For the American readers, sorry about that, eh!
I once heard a seemingly lone, but determined voice say, "We measure too many trees in British Columbia." This simple statement is insightful and significant.
I am, of course, referring to cruising timber in BC. And yes, we do use a prism for point sampling. However, we are not being as efficient as we could be. Our measure of acceptability is:
When only full measure plots are used, sampling error is based on the variation (i.e. CV) of volume/ha between the plots. This CV is typically between 30-50%. Life is simple but not efficient. Given a CV of 40%, approximately 30 plots would have to be established to meet a 15% sampling error @ 95% confidence.
Now we do have the option of sampling with full measure plots (all trees fully measured) AND count plots (where simply a tree count is taken). This breaks the calculation of volume/ha into two parts: BA/ha (from tree counts) and VBAR (volume/BA ratio based on measure trees). The CVs for BA/ha and VBAR are approximately 45-55% and 15-20%, respectively. Given that we actually do know the typical CVs for these two variables, we should optimize our sampling. Unfortunately, we do not.
A rough approximation indicates an optimal ratio of one measure tree for every 3½ count plots. Under the current procedures, for every full measure plot a maximum of three count plots can be established. As there is a minimum of 4 trees/plot (on average), the best sampling ratio we can hope for is 4 measure trees for 4 prism sweeps (1 full measure + 3 count plots). Simplified this works out to 1:1 sampling ratio; a far cry from the optimal 1:3½ ratio!
The following is a description of a project carried out by the second year class of the Forest Resources Technology Program at Malaspina University-College.
This paper presents only method 3, Big BAF.
Eight full measure plots are required to fulfill Ministry of Forests sampling requirements (i.e. sampling intensity = 1 plot/ha). Note that a minimum of 32 measure trees is required, although more are likely to be measured. The objectives of this project were to:
What follows is a description of the Big BAF method followed by a summary of our findings.
Big BAF Principle
Objective: take many sample points as BA/ha has a considerable CV (~55%) but measure relatively few trees as the CV for VBAR is small (~15%). A second advantage of this method is that the measure trees are not clumped in a few plots but are instead spread throughout the block. Volume/ha is calculated based on the following:
volume/ha = BA/ha * VBAR
The volume/ha determination is essentially the same as prism cruising with count plots, except two different prisms are used: one for tree count (i.e. BA/ha) and a second for measure tree selection.
In the end, more sample points are established than "traditional cruising" but fewer trees are measured. In addition to sampling efficiency and better dispersal of measure trees, a third advantage of this method is a superior estimation of species composition. This is because the BA is more intensively sampled.
Sampling error is based on the variation of BA/ha among the count plots and of VBAR among the measure trees. To get the combined SE%, determine each SE% on its own and then combine with
It is important to note that, unlike prism cruising with count plots, BA/ha is based solely on the count plots (i.e. the tree counts from the "Big BAF" plots are not used). This is because there will be great variation in BA/ha based on the big BAF typically no trees in many plots and few plots with one or two trees (with each tree representing a substantial basal area).
Mechanics of the Cruise
Note: it is important to establish the sample point without bias. Specifically to ensure that if the sample point lands inside a tree, THAT is where the sample point is. There is often a bias to establish the sample point where we can physically stand. As the plot radius factor is very small for a BAF of 165 (or 718 BF/acre), it is important to establish plot centers properly. The prime reason is not for BA determination, as it is less affected by a bias of a few centimeters (or inches) in plot location. Instead, if there were an unconscious bias to move the plot centers from the inside of trees, it could result in selecting too few measure trees.
Also, note as few trees are measured, it is important to not miss measuring less common species. This is easily handled by measuring the first tree of a "new" species when it occurs in a count plot, i.e., it becomes an enhanced count plot. If you are a purist, trees selected this way could later be dropped from the compilation if additional trees of that species are selected by the big BAF as measure trees.
The proper way to compile this cruise is to determine BA/ha solely from the sweeps using the "regular" BAF (in our case BAF = 11, or BAF = 48 BF/acre) and to determine VBAR solely from the sweeps using the big, fat prism (in our case BAF = 165, or BAF = 718 BF/acre).
Determination of sampling error was already described under "Statistics."
Quick Summary of Results
Credit Where Credit is Due!
Originally published July 1998
Back to Guest Index