Abstract
In cut-to-length logging, the harvester operator adjusts the bucking in accordance with visible defects on processed stems. Some of the defects, such as a sweep on the bottom of the stem, decrease the yield and quality of sawn products and are difficult for the operator to notice. Detecting the defects with improved sensors would support the operator in his qualitative decision-making and increase value recovery of logging. Predicting the maximum bow height of the bottom log in Norway spruce (Picea abies (L.) Karst.) with log end face image and stem taper was investigated with two modelling approaches. A total of 101 stems were selected from five clear-cut stands in southern Finland. The stems were crosscut and taper measured, and the butt ends of the bottom logs were photographed. The stem diameter, out-of-roundness, and pith eccentricity were measured from the images while the max. bow height was measured by a 3D log scanner at a sawmill. The bottom logs with an eccentric pith had higher max. bow height. In addition, a highly conical bottom part of the stem was more common on the bottom logs with a large max. bow height. Applying both log end face image and stem taper measurements gave the best model fit and detection accuracy (76%) for bottom logs with a large max. bow height. The results indicate that the log end face image and stem taper measurements can be utilised to aid harvester operator in deciding an optimised length for logs according to the bow height.
Highlights
Bucking of tree stems bounds the forest resources to meet the quality criteria of end users
A low correlation was found between pith eccentricity and dbh (r = 0.17, p < 0.1 ), but no correlation was found between dbh and out-of-roundness
The possibility of estimating the max. bow height of the bottom log of Norway spruce with a log end face image and stem taper was studied with two modelling approaches, in which the max. bow height was first predicted by fitting a linear regression with two sets of explanatory variables
Summary
Bucking of tree stems bounds the forest resources to meet the quality criteria of end users. It is performed in cut-tolength harvesters based on price and demand matrices, which define the desired log dimensions and their values, while defects are observed by the operator. The sweep (i.e. crookedness) refers to a steady deviation from a straight line along the stem (Richter 2015). A basal sweep indicates the vertical position of the maximum deviation. This deviation is often referred to as “bow height” (Pfeifer 1982; Gjerdrum and Warensjö 2001; Rune and Warensjö 2002). Basal sweep affects the most valuable part of the stem and decreases value recovery through
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