Development of low-cost, root collar diameter measurement devices for pine seedlings

To assess the growth stage of pine seedlings in the field, nurseries annually measure the root collar diameter (RCD) of a large number of seedlings. Traditionally this is done manually using a slide micrometer and registration on paper, a tedious and error-prone practice. As an alternative, two generic semi-automatic diameter measurement devices were developed, based on an optical time-of-flight principle. Here the diameter was obtained using the timing information generated by the shadow of an object passing a pair of receivers. In the first method named the ‘sliding-edge device’, the operator manually slides the object along a guiding edge after which the diameter is measured and recorded automatically. In the second method named the ‘counterbalanced device’ the operator places the object on a table after which the device measures and records the diameter at the push of a button. Both devices were tested using cylindrical objects with known diameters. The sliding-edge device produced a maximum systematic error of 0.3 mm (9.5%), most likely owing to inaccurate model assumptions and a maximum variability of 0.1 mm. The counterbalanced device proved superior with a maximum systematic error of 0.1 mm (1%), virtually independent of operator skill, and a maximum variability of 0.1 mm.