Personal Bias in Forest Measurements

Abstract

The assumption that measurement errors are negligible during a forest inventory was tested using forestry students. The magnitude of personal bias in measuring tree diameter at breast height (dbh), total height, and basal area Rer hectare was obtained. Bias was negligible in dbh measurements and the among-crew coefficient of variation (CV) was 8.16%. Heights were significantly underestimated and the among-crew CV was 21.86%. The percentage error in basal area Der hectare determination was 4.09% at the measurement biases are summarized by Loetsch et al. (1973); and for dbh, the most notable is the work of Myers (1961).It has been suggested that if the measurement errors occur randomly, their effect approaches zero. In this paper, the assumption that measurement errors are negligible during a forest inventory was tested. Specifically, the magnitude of personal bias and variation in measuring tree dbh, tree total height, and stand basal area was determined. 95% probability level, using a prism of basal area factor 6. Measurement errors are., in general, not negligible. Their rnagn~tude Method of Data Collectionshould be estimated per inventory and included in the total error of the inventory estimate.Data for the study were collected at the University of British Columbia Research Forest, Maple Ridge, B.C., during the third year forestry students' field school in mensuration. ROsum6Sixty sampling points were systematically set at about 80-m intervals in a 70-year-old second growth stand containinaLe fait que les erreurs de mesurage s'avbrent n6gligeables.lors d'inventaire forestier a BtB verifie avec des Btudiants en foresterie. On calcula I'importance de I'influence du personnel mesurant le diametre de I'arbre a hauteur de poitrine (1,30 m), la hauteur totale et la surface terriere a I'hectare. Celle-ci fut donc negligeable pour les mesures du diametre et le coefficient de la variation (CW entre les membres d'une mbme Bquipe se chiffra 8.16%. Par contre les prises de la hauteur furent fortement sous-estim6es et le coefficient de la variation (CV) entre les membres d'une mbme equipe se fixa a 21.86%. Le pourcentage d'erreur enregistre pour le calcul de la suface terriere a I'hectare fut de 4.09% suivant le taux de probabilite de 95% et utilisant un prisme facteur 6 pour cet item. En general, les erreurs de mesurage ne sont pas negligeables. Leur importance devrait btre estimee pour chaque inventaire en plus d'6tre incluse dans l'erreur totale de I'inventaire.

Assessing Precision in Conventional Field Measurements of Individual Tree Attributes

“…Dbh is the most fundamental of tree measurements and is defined as the outside-bark-stem diameter of a tree at a point on the stem that is 1.3 m above the ground from the base of the tree [1,3]. In forestry, the word "diameter" implies that the trees would between several re-measurements of dbh and height have been reported for example in [32][33][34][35][36][37][38]. The results of these studies have been reported using various means (e.g., mean, absolute and relative standard deviation, and bias) and straightforward comparisons of the resulting precision are not possible in all cases.…”

“…For example, in [37], the author's own measurements of dbh and height were used as the true value in the study and were assumed to be error free when compared to measurements conducted by others, whereas Hyppönen and Roiko-Jokela [33] admitted that measurement bias could not be calculated since it was not possible to obtain the exact dbh. Furthermore, Kitahara et al [39] have noted that in tree height measurements, educated and experienced mensurationists provide more precise results than beginners with only basic knowledge.…”

Developing an Improved Parameter Estimation Method for the Segmented Taper Equation through Combination of Constrained Two-Dimensional Optimum Seeking and Least Square Regression

“…For example, when the range of / h H was from 0.00 to 0. Measuring tree diameter and height may be subject to errors, even though stand and tree variables are commonly assumed to be measured without error [42,43]. The measurement errors made by field crew or faulty instrument or both might be substantial [42].…”

“…It is well known that the violation of the second assumption may lead to biased estimation of parameters and (or) of the standard errors of the parameters, and consequently to misleading the hypothesis test [44,45]. If the predictor variables in Equation (1) Measuring tree diameter and height may be subject to errors, even though stand and tree variables are commonly assumed to be measured without error [42,43]. The measurement errors made by field crew or faulty instrument or both might be substantial [42].…”

“…If the predictor variables in Equation (1) Measuring tree diameter and height may be subject to errors, even though stand and tree variables are commonly assumed to be measured without error [42,43]. The measurement errors made by field crew or faulty instrument or both might be substantial [42]. In all existing taper equations including Equation (1), it is assumed that (i) the dependent variable is a random variable; and (ii) the independent variables are fixed and measured without errors.…”