slash pile copy

A woodland owner has a choice of time tested measuring techniques to predict how much lumber can be produced from trees. Accurate volume estimates can help forest managers plan for the number and type of trees to harvest and the appropriate workforce and equipment needed.

NARA provides technical assistance to an emerging industry that, instead of using the bole wood (trunk) for lumber, uses the wood residuals (limbs, tops, stumps and low grade-wood) left over after timber harvest or forest thinning to produce chemical products like biojet fuel. Here as with harvesting trees for lumber production, knowing how much feedstock, in this case forest residuals in slash piles, is available on site is helpful to plan processing and transport options.

Methods are available to estimate the amount of wood residuals attached to a standing tree, however, this volume does not necessarily represent the volume available in slash piles. First, a percentage of those residuals are left on the forest floor and second, some of those residuals may be removed to produce higher value products like pulp. With a new use for the forest residuals that normally would be burned, methods to determine the amount available need to be evaluated.

Terrestrial LiDAR to establish baseline

To evaluate methods used to estimate the volume of forest residuals in slash piles, NARA researchers Justin Long and Kevin Boston at Oregon State University tested two methods for 1) ease of use and 2) accuracy: one method relies on a geometric base and the other relies on a laser rangefinder. The results from their analysis are published in the journal Forest Science.

To read “An Evaluation of Alternative Measurement Techniques for Estimating the Volume of Logging Residues”, view here.

To test the accuracy of these two methods, volume baselines were determined on 33 slash piles of different shapes and sizes using terrestrial LiDAR analysis. Terrestrial LiDAR is considered a very accurate tool to measure 3-dimentional shapes with sub-centimeter precision and is recognized as the best option to measure slash pile volumes; however, the hardware and software costs, plus the technical skill required for data analysis, make this method an unlikely choice for harvest operators due to high costs.

The geometric method

To use the “geometric method”, the operator first visually classifies the slash pile as having one of seven geometric shapes and then calculates the volume based on the shape. For instance, if the slash pile shape is similar to a half-sphere, then the parameters measured for the volume would be the height and width. Slash piles with complex shapes may require multiple geometric shapes to be applied and added together. This method is fast and can be performed without specialized equipment; however, as the slash piles get bigger and more irregular in shape, the volume measurements stray further from the terrestrial LiDAR measurements. In addition, the operator determines which geometric shape(s) to use, so measurement variability can be increased when multiple people are taking measurements.

The laser rangefinder

The laser rangefinder measures distance by bouncing a laser beam off an object and calculating the distance based on time of flight. In this study, an average of 173 measurement points were determined by laser rangefinder for each slash pile, and the total surface volume was then calculated via specialized software. According to the study, “the laser rangefinder produced results that were closer to the LiDAR-generated estimates in more than two-thirds of the samples measured in this study”.  In addition, measurements are reproducible when using multiple users. However, since this technique requires “at least three instrument setups to ensure adequate coverage of the entire pile”, it is more time consuming than using the geometric method.

Measuring the volume of raw materials such as coal, sand and gravel piles is not new. What is unique about measuring slash piles is applying measurements to their irregular shape. When the geometric method and a laser rangefinder are compared for accuracy and ease of use, the authors of this study recommend the use of a laser rangefinder.