Disclosed is a timber-working device, such as a harvester head (8), for processing and measuring mulitple tree stems simultaneously. The device includes a frame (200), a drive system (comprised of 206a, 206b, 208a, 208b, 210a, 210b), at least two distance-measuring devices (such as 214) and at least one processor (not shown). The frame (200) includes a feed axis (212) having first and second sides relative to the frame (200). The drive system is configured to independently feed stems along the feed axis (212) and includes a first rotary drive (210a) having a first wheel (208a) configured to act against a first stem on the first side of the feed axis (212) and a second rotary drive (210b) having a second wheel (208b) configured to act against a second stem on the second side of the feed axis (212). Each side of the feed axis (212) has an associated one of the distance measurement devices, each configured to generate measurement signals indicative of the length of stems fed along the feed axis. At least one of the distance measurement devices (not shown) is associated with one of the rotary drives (210a, 210b) and its measurement signal is indicative of rotation of the drive. The at least one processor is configured to receive the respective measurement signals and, for each of the first and second stems, determine the length of at least a portion of the stem fed by the drive system based at least in part on the respective measurement signals. A method of determining lengths of stem portions using this device and a computer program for performing the method are also disclosed.
