Method and device for the determination of a thickness of a crosssection (S) of a tree trunk (30) received in a harvesting assembly (10) comprising a pair of opposite feed wheels (14) engaging with the tree trunk and a delimbing knife (18) abutting against the tree trunk. The mutual position of the feed wheels is detected as a measure of a transverse dimension (D1) of the cross-section of the tree trunk. The position of the delimbing knife (18) in relation to an abutment surface (22) of the tree trunk opposite thereto is detected as a measure of an additional transverse dimension (D2) of the cross-section (S) of the tree trunk, which additional transverse dimension forms an essentially right angle with the first-mentioned transverse dimension. The thickness of the cross-section (S) of the tree trunk is calculated based on the two measures of the transverse dimensions (D1, D2).
