Use of the correlation coefficient between plots in order to improve the accuracy of forest inventories

Abstract

Forest inventories are usually compiled without taking into account the existing correlations between sampling units, which is debatable particularly where the calculations involve environmental variables. When the potential correlations between sampling units are overlooked, the accuracy of such inventories becomes distorted in terms of the confidence interval range for the variable of interest, which is volume in cubic meters. The magnitude and form of such distortion will vary according to the correlation intensity between sampling units. This study aimed to present an analysis of the addition of the correlation coefficient to the calculation of the variance of the mean in a systematic sampling procedure of a native forest population or area, as well as its impact on the accuracy of the resulting estimates, with the assumption of independence between sampling units and with the addition of a correlation between sampling units as suggested by Cochran. Results revealed that, where the correlation coefficient was added to the variance of the mean formula, it increased inventory accuracy by about 14.3%, leading to the conclusion that such an effect will occur in any forest inventory being compiled for any forest population or area of interest.Key words: Sampling, variance of the mean, native forest. O USO DO COEFICIENTE DE CORRELAÇÃO ENTRE PARCELAS VISANDO AUMENTO DE PRECISÃO EM INVENTÁRIOS FLORESTAIS RESUMO: Os inventários florestais têm sido realizados sem levar em consideração a relação existente entre as unidades amostrais. Esse fato se torna altamente questionável, principalmente quando nos cálculos estão envolvidas variáveis de natureza ambiental. Ao serem desprezadas as possíveis relações entre as unidades amostrais, a precisão advinda desses inventários florestais se torna distorcida em termos de amplitude do intervalo de confiança da variável estudada, sendo esta o volume em metros cúbicos. A magnitude e a forma dessa distorção variam conforme a intensidade da correlação existente entre unidades amostrais. Dessa forma, no presente trabalho, objetivou-se apresentar uma análise da incorporação do coeficiente de correlação no cálculo da variância