3P Sampling

How to Examine a 3P Cruise

Published October 1988

There are good reasons to know more about 3P sampling. Some outfits are using it now, but not very efficiently or cleverly. 3P, or something very much like it, is the next big improvement to forest sampling after prism cruising. Those who see it coming and learn how to do it right will save a lot of sweat and money. The basic points of view are from the buyer (who wants to know if the answer is right), and the seller (who wants to know if he is doing a cost-effective job of cruising as well as getting the right answer.

There are two major processes going on in a 3P cruise. First, you are visiting every tree in the area to make an estimate of its volume. We will concentrate on the classical method where every tree is visited. You virtually cannot do anything wrong in this stage of the work. Any estimates you make will be corrected by the second process. This is not quite true of course, since you could cruise the wrong sale entirely, write down the wrong numbers, miss an entire group of trees, etc., but these are blunders you could make with any system and ordinary care will eliminate them. Visiting each tree is a costly endeavor which ordinarily will not be repeated by the buyer, and which the seller does quickly to minimize cost.

The second stage is to measure a few trees for accurate answers in whatever units are desired. The measurement can be in different units than the estimates. We will just consider volume here. The volumes measured are divided by the estimated tree volumes to get an "average correction ratio".Next Column


This correction ratio stage is where errors can occur, and this is what the buyer must repeat to his own satisfaction. Choose a random or systematic sample of the "carefully measured" trees to resample, then replace the seller's measured values with your own. The rest of the math is identical. There is no need to go to the expense and effort of repeating all the estimates. The seller's estimates may be used as they are, if you can just assume that they really did visit each tree in the area.

This ability to assign your own true values to the correction ratios makes 3P an attractive system to the buyer, who can make use of all the estimation effort already done, and if the sample trees are felled they can get good net volume measurements. Good maps or trails to the sample trees are of real, practical importance.

The sellers must check a few additional items to see if they are doing an efficient job. Look at the correction ratios. These should be as consistent as possible. Identify the ones that vary, and if you can't do a better job of estimating that kind of tree then try to stratify them. Small trees often present this problem in 3P cruising.

Worrying about getting the exact sample size, the "proper K value" and estimating tree DBH correctly are all minor matters, particularly after the fact. You must examine the ratios. They alone tell you about the efficiency of your cruise. If you aren't getting them on your cruise output then request them!

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