Chemainus River 3P (probability proportional to prediction) Timber Cruising Trial: comparing net volume from Variable Plot (VP) & 3P cruising to scaled volumes

by Bruce Markstrom, BC Ministry of Forests


This project was initiated in response to one of the recommendations of a report by the Provincial Cruising Advisory Committee which stated that 3P cruising should be implemented for use in appraisal cruising (determining stumpage rates) in British Columbia (BC). Currently 3P cruising is not approved for use in BC, although 3P was introduced in 1965, and is widely used in the USA.

The precision of 3P cruising was compared to VP (prism) cruising and the log scale on a 6.0 hectare small business timber sale on Vancouver Island near Duncan, BC.

3P Cruising involves a quick estimate of every tree in the cruise area (usually volume, sometimes related to dbh) and accurate measurement of a few randomly selected trees to determine a correction ratio for the estimates. The correction ratio for each species (stratum) is then applied to the estimates for that species (stratum). This provides very precise volume by species and an excellent stand and stock table. The sampling error for 3P cruising is calculated from the variation between the estimates and the actual tree volumes. On this project we estimated diameter at breast height (dbh) and had the handheld electronic data recorder (EDR) convert it to an estimated volume using the formula:

"Estimated Volume" = {(dbh)2.4 * 0.0002}

This approximation to tree volume was determined from reviewing some trees measured prior to the 3P cruise.

The site chosen was a 70 year old naturally regenerated stand of 75% Douglas-fir and 11% balsam (grand fir) with minor amounts of western hemlock, western redcedar, red alder and broadleaved maple. The road, with a previously logged 25 meter right-of-way, runs through the center of the block on an old railway grade. The silviculture prescription required a 15% volume retention of the dominant trees (162 Douglas-fir, 36 cedar, and 2 maple) which were marked to leave prior to the cruising.

The original VP (prism) cruise had been carried out at an intensity of 2 plots per hectare (75m x 75m grid). We established an additional 14 full measure plots at the mid-way points between plots along the established strips to be able to make comparisons to VP (prism) cruising at 2 plots per hectare and 4 plots per hectare.

The 3P and VP (prism) cruising was carried out by Don Rorison at the Ministry of Forests (MOF) Revenue Branch and Bruce Markstrom (MOF) Vancouver Forest Region). We had both taken training in 3P Cruising from Rich Holmboe and Dr. Kim Iles but had never actually carried out a 3P cruise outside of the training.

A Hewlett Packard 200 handheld electronic data recorder (EDR) with DR Systems Incoporated’s 3P Cruising program was used to conduct the 3P cruise.

Planning to determine the number of sample (measure) trees required was based on the results of the original VP (prism) cruise (12 BAF averaging 3 trees per plot) and the expertise of Dr. Kim Iles. The predicted number of sample (measure) trees required to achieve the target sampling error of 5% was 30 trees but 60 trees were chosen as the target number to review the effect of the number of sample (measure) trees on the results.

See the table below for the effect on the sampling error (SE) of the number of sample (measure) trees and the number of prism plots required to meet the 3P sampling errors.


# Measured Trees SE % #prism plots req’d to meet 3P SE’s




(11.2 /6.1 )*26 =89 plots (633 trees)




(11.2 /8.2 )*26=49 plots (348 trees)




(11.2 /9.5 )*26=36 plots (256 trees)

BAF 6 - 13 plots




BAF 6 - 26 plots




The logs were 100% stick scaled at the Nanoose Forest Products dryland sort and the timber sale was closely monitored to help ensure accurate scale information. The licensee was informed that the project was being carried out.

The volumes, by species and grade were used with the appraisal data from the timber sale to determine the stand selling price for the different VP (prism) cruises, the 3P cruise and the scale. CG/NF = Call Grading/Net Factoring, Comp = Computer Grades (Algorithm) Loss Factor Tables. VP = Variable Plots using Computer Grades and Loss Factor Tables.



SE %

Diff(m3 & %)

Selling Price

Diff($ & %)



3P (CG/NF)



90 m3 = 2.6%


$6,000 = 1.8%

3P (Comp)



46 m3 = 1.3%


$-24,000 = -7.2%




-675 m3 = -19.6%


$-68,000 = -20.4%




-594 m3 = -17.2%


$-64,000 = -19.2%

We are unable to explain the magnitude of the difference in volume between the 3P and VP (prism) cruises but all the VP (prism) cruises, including those with 4 plots per hectare, show less stems per hectare than the 3P cruise. VP (prism) cruising showed less stems per hectare in all diameter classes up to 45cm but above that showed slightly more stems per hectare than the 3P. The fact that a very large amount of the block is affected by edge effect may have contributed to the difference.

Computer generated grades (algorithm) averaged 5% less value than the scale. The cruiser called grades showed as 3% higher in value, likely due to the growth rates at the top of some logs being too fast to meet grading rules. This would reduce the actual value of those logs slightly since those growth rates cannot be measured by the cruiser.

Tree taper was also reviewed. Twenty-one (21) of the 3P sample trees were measured for taper to check the validity of the MOF taper equations. Adjusting the taper based on the measurements of those 21 trees increased the overall volume by approximately 250 cubic meters or 7%. The MOF taper equations underestimated the volume of the Douglas-fir by 13% and overestimated the volume of balsam fir by 17%.

Our costs of conducting the 3P cruise were approximately twice as much as establishing 2 full measure plots per hectare (MOF policy requirement for selection logging) but it produced a much better result. This difference would be reduced if the 3P cruising was done by an experienced 3P crew.

All VP (prism) plots were cruised with BAF 6, 8, 10 and 12 prisms to check the effect on overall volume and volume by species due to the different average tree count per plot. The volumes had a range of approximately 400 m3 with the closest volume to the log scale coming from the BAF 10 although only the BAF 6 met the ministry policy of an average of 6 trees per plot. The volumes for Douglas-fir and balsam fir had a range of 6% but there was no pattern. As expected, increasing the average tree count did not equal increased volume.

The VP (prism) cruise closest in volume to the scale was the BAF 10 (13 plot) cruise that was taper adjusted and it was still 333 m3 (55m3/ha) less than the scale volume. All other VP cruises including those with 4 plots per hectare showed even less volume.

The closest stand value among the VP (prism) cruises was also the BAF 10 cruise (13 plots) at $291,000 which was $43,000 (12.9%) less than the estimated selling price of the scaled wood ($334,000). All other VP (prism) cruises were at least $63,000 (18.9%) less than the estimated selling price of the scaled wood. The BAF 6 cruise with 26 plots at $270,000 (19.2%) was the closest in stand value among the BAF 6 cruises (which were the only cruises to meet the ministry standard of an average of 6 trees per plot).

We started out with the intention of just comparing 3P cruising to VP (prism) cruising but complicated the project by looking at average number of trees per plot (4 different prism sizes) and 2 plots per hectare and 4 plots per hectare. We also compared using all full measure plots to various ratios of count plots to measure plots. This comparison is not discussed here but is covered in our final report. A similar project on a 10 hectare patch cut in the Interior of BC is close to completion.

As expected the 3P cruises, whether using Call Grading/Net Factoring or the ministry’s loss factor tables, provided very accurate volumes by species and stand value when compared to the scaled volumes.

Difference from Scaled Wood:

  • 3P - Call Grading/Net Factoring = 2.6% volume and 1.8% stand value
  • 3P - Loss Factor Tables = 1.3% volume and -7.2% stand value

This project confirmed the problems of getting accurate information when using VP (prism) cruising on small blocks. Even establishing 4 plots per hectare did not provide information as accurate as the 3P cruising. 3P cruising is an excellent method for cruising small blocks, areas with high value timber and when more accurate information is required. We are recommending that 3P Cruising be implemented as soon as possible for use in appraisal cruising in British Columbia.

Originally published July 1999

Return to Home
Back to
Guest Index