The implications of log taper on the expected difference of JAS scaling to 3D scale in New Zealand.

Abstract

As a result of the strong price and demand trends observed in the export market, the importance of value recovery from log exports is emphasised by suppliers/owners.

Two methods of analysis were used for the duration of this study. The first was using real log measurements, utilising SED, LED and Length of logs. The second was through modelling of volume, estimated using the same three variables as the above. This data was presented in a matrix that predicts taper and estimation differences based on the log measurement inputs.

Analysis was performed utilising two datasets; one from the North Island (East Coast) and one from the South Island (Southern South Island). The taper ranges for each study area were calculated as base data. The North Island on average had taper of 1.45cm/m, whereas the South Island data showed average taper of 1.83cm/m. The logs were separated into their respective grades; overall the logs that displayed the most taper were South Island Pulp and K grade logs.

The overarching question of the study was to determine the volume logs measured with a JAS scale and then compare that to the volume predicted by a 3D scale (assumed to be as close to the true volume as possible for this study). The analysis showed that for the South Island estimation differences of; -15.8%, -4.7%, 3.7% and 6% for Pulp, K, A and Pruned grades respectively. The North Island dataset showed estimation differences of; 2.7%, 8.7%, 6.4%, 3.5% for Pulp, KI, A and Pruned grades respectively. The main trend that is observed through these estimations of difference is that the smaller SED logs; South Island Pulp grade (average SED of 21.4cm) and South Island K grade (average SED of 27.5cm) are more likely to be underestimated. The remaining log grades for the North and South Island all have average SED values over 30cm, therefore it can be assumed that when the SED is over 30cm, the difference of JAS/m³ will be overestimated. Using the prediction matrix created, the taper that is required for underestimation to occur for small SED logs (10-20cm) which are seen as the most susceptible to differences are; 0.276 – 0.925cm/m for 5.8m logs and 0.421 – 1.447cm/m for 3.8m logs. The tapers required for larger SED logs are larger and tend not to be underestimated using JAS.

The study highlights the tapers and associated differences of JAS/m³. Knowledge of the causal factors of this difference is valuable for exporters in order to adjust prices etc.