Economic Impacts of Projecting Horizontal Angles to the Wrong Height When Conducting Point Sampling

Abstract

Point-sampling selects sample trees by projecting horizontal angles. In many inventories, angles are to be projected to where diameter at breast height (d.b.h.) is measured (4.5 feet above the ground), but due to user error, angles are often projected to diameters at other heights. Thus, errors are made as to which trees should be sampled, probabilities of sampling individual trees are incorrect, and the basal area estimate does not truly correspond to d.b.h. The objective of this paper is to determine the potential economic impacts of projecting angles at heights other than d.b.h. when d.b.h. is the desired height. Projections at age 15 years for 200 surviving trees per acre on a low quality site (site index 50 feet at base age 25) and for 400 and 700 trees per acre on a higher quality site (site index 70 feet at base age 25) were examined. On all three sites, two ages (20 and 30) were used to establish virtual plantations and sampling was conducted using 10 and 20 basal area factor prisms by projecting horizontal angles to four heights: 4.5 feet, 5.0 feet, 5.5 feet, and 6.0 feet. A taper equation was used to estimate changes in diameter along the stem. For the stand conditions examined, incorrectly projecting angles to heights other than d.b.h. reduced timber appraisals by as much as 7.4 percent and up to $170 per acre. Across many acres and stands, this type of nonsampling error can result in serious errors in valuing stumpage.