3P Sampling

3P Cruising in Region 5, USFS

by David D. Johns
Check Cruiser
R5 Central Quality Control Area

Published January 1997

In recent years, the U.S. Forest Service in Region 5, as well as other western Regions, has changed the way it manages timber stands due primarily to wildlife concerns. This shift in policy has brought about cutting prescriptions that often force the timber markers to visit every cut tree. If every tree in a given stratum is to be visited, it is advantageous to 3P cruise that stratum. This, of course, involves estimating something about each tree that will be consistent with or parallel the volume of that tree. In Region 5, the net volume of the tree is most often used as the estimation parameter or KPI for that tree. The method for arriving at the net volume estimation for any given tree does vary from Forest to Forest and among cruise designers.





Volume (BF) KPI

(Dec C)

Ave. Tree Size
























































If available volume tables approximate actual net volume or parallel actual net volume, they are useful tools for estimation and are relatively easy to use. In the past, when sales have most often dealt with large trees, DBH2 was used as the KPI parameter. However, DBH2 does not work well for a large range of tree diameters or small trees and seems to create a higher coefficient of variation than does using different exponents that increase with DBH. In either case, the estimation approximates Scribner Board Feet Decimal C by rounding the result to the nearest ten and then dropping the zero. This is done in Region 5 in order to prevent the SKPI's from exceeding five characters or 99999. If this happens on the Corvallis MicroTech MC-V field data recorder, data will be lost because the SKPI field will clear itself. The table provided here demonstrates the relationships between DBH, exponent, KPI, and average tree size that correlates with KPI.

By running the pre-cruise data through NATCRS (National Cruise Program) and requesting report R6, the cruise designer can get a good idea of the number of trees by one inch size class. The SKPI’s and the KZ for the stratum can be calculated. DBH2 has worked well in most parts of Region 5 for trees with DBH’s between twenty and thirty inches. Exponents greater than 2 should work for trees over thirty inches DBH. Experiment with this on your calculator.

Since net volume is desirable in most cases, the KPI for any given tree can be modified by reducing it in proportion to a QUICK estimate of the percent defect in the tree in question. Quick estimates of defect must be stressed when applying this technique. Here is an example of this method:

The KPI for a 20" DBH tree would be 40; the marker/estimator does a quick estimate of cull at 20%; a quick calculation can be done by multiplying 40 (KPI) by 0.8 (decimal complement of 20%) to yield a KPI of 32 for this tree. Likewise, the KPI for any given tree can be increased or decreased if the tree being estimated is exceptionally taller or shorter than the height correlated with DBH in the KPI chart shown above. The chart supplied to the marker/estimators should include only the DBH, KPI, and average tree height associated with that DBH. The main idea here is to supply a tool that makes estimation of any tree quick, easy and consistent. Also, the cruise designer must keep KPI’s, SKPI’s, and KZ in like units.

On some (rare) occasions, it may be desirable to cruise a biomass or pulpwood stratum utilizing the classic 3P method. This should only be considered when the trees to be cruised are scattered or clumped so as to make plot cruising inappropriate because of the high coefficient of variation of volume per acre in the stratum. This can casue some logistical problems such as numbering sample trees which are quite small and having to visit a large number of trees. The cruise designer should consider these problems before using 3P in this situation. If 3P is chosen for biomass cruising, the KPI’s should be in tenths of cubic feet rather than cubic feet. The Corvallis MicroTech MC-V field data recorder Next Column


used in Region 5 will not accept a KPI less than one and small diameter trees typically contain less than one cubic foot. So, to keep the coefficient of variation as low as possible, tenths of cubic feet has been used as the KPI parameter for biomass 3P cruising. This will yield an R ratio of somewhere around ten, but, as long as this is consistent, the CV should stay relatively low. The cruise designer can run a list of small trees covering the size range of anticipated biomass trees through NATCRS and generate cubic foot volumes in report A5. These volumes can be multiplied by ten to reflect tenths of cubic feet for each tree size class.

3P plot cruising has been used for biomass cruising in Region 5. It should be noted that this method will not produce a volume for limbs of included trees. This method does require a pre-cruise that will generate an estimate of the cubic feet per acre for the stratum, the CV of cubic feet per acre, and the Basal Area Factor necessary to produce an average of four to eight "in" (cut) trees per plot. This method also requires traversing the stratum acres which the classic 3P method does not. By knowing the CV of cubic feet per acre and the allowable error for the stratum, the cruise designer can calculate the number of plots necessary to meet the allowable error. Then, to calculate the SKPI’s, the number of plots is multiplied by the average cubic feet per acre. KZ can then be calculated. Cubic feet per acre is the KPI parameter for this system and can be quickly estimated by multiplying the number of "in" (cut) trees (at any given plot) by the BAF and then multiplying that product by one-third of the average tree height of the "in" trees. This estimate of cubic feet per acre is compared to an internal random number generator in the Corvallis MicroTech MC-V field data recorder which will generate samples based on KZ. Sample (measure) plots are carefully measured, including limiting distance for borderline trees. Borderline tree limiting distance is not measured on estimated plots. This method can also be used with fixed plots, but the KPI is calculated differently: where r = tree radius in inches. Both KPI parameters are based on the formula for solving the volume of a cone.

Mapping of any 3P samples, either trees or plots, is very important. If the stratum is to be check cruised, the check cruiser will need to know the location of all samples. Potential purchasers may well want to locate samples in order to generate their own ratios for the stratum. GPS has been used to map the samples, but a sketch map will suffice if topographic features are included and the mapper has the aptitude for mapping. Sample trees and plots should be uniquely identified on the ground; e.g. double striped trees and bright flagging.

The optimum crew size seems to be four; three marker/estimators and one data entry person. If more than three estimators are used, confusion may occur when estimates are being called out to the data entry person. Each estimate must be confirmed by the data entry person before more estimates are accepted. The more consistent the crew is in estimating, the lower the CV for that stratum will be. It has been the experience in Region 5 that crew CV's will be close to 40%; sometimes 35% and sometimes 45%.

Insurance samples (plots or trees) are generated, numbered, and mapped in Region 5 during the cruising process. Corvallis MicroTech MC-V field data recorders will automatically halve the KZ when requested to generate insurance samples and then identify half of the samples as insurance samples. These insurance samples are only considered for measurement when the allowable sampling error of the cruise is exceeded.

P3P, or point 3P cruising has been used in Region 5. The cruise designer must remember to stipulate tree height as the KPI parameter for each "in" tree. For SKPI’s and KZ calculation, the sum of tree heights of "in" trees (in plots only) must be used, not the estimated volume or sum of tree heights for the entire stratum.

Avoid sure-to-measure (STM) trees whenever possible. Use STM for incidental species only and, then, only when there are very few of these trees. In Region 5, STM species can be identified during the field data recorder set-up or by manual input during the data entry process.

In Region 5, certified Qualified Cruisers are not necessary for the estimation phase of the cruise. It is mandatory for certified Qualified Cruisers to cruise the samples for tree measurement sale preparation. Consistency should be the focus of the estimators.

When the defect percent is expected to equal or exceed 25% for a given species or stratum, fall, buck and scale should be done on a sub-sample of the species or stratum in question. This introduces a new error term in the sampling error calculations. However, the CV for this part of the calculations should be relatively low and not require many more samples to be taken during the initial phase of cruising.

 Also see our article: "How do I get DBH to a power to Equal Tree Volume?", Issue 35, July 1996.

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