Using low-cost drones to estimate woody residues

Harvest residues play a role in sustaining forest site productivity and providing wildlife habitat, leading states and other entities to develop biomass harvesting guidelines that commonly recommend specific quantities or proportions of residues to be retained on site. However, estimating the amount of harvest residue biomass following operational harvesting can be challenging.

With support from NCASI, researchers at the University of Georgia are developing a low-cost approach to rapidly and reliably estimate post-harvest woody residue biomass. Professors Tripp Lowe and Larry Morris, along with graduate student Brian Davis, are using small, unmanned aerial drones fitted with high-resolution cameras to image recently harvested sites. The research could lead to a system that replaces the time consuming and inaccurate line-transect field methods currently used to estimate residue volumes. 

When using the drone, flight boundaries are established using mission planning software installed on a cell phone or tablet, and flight lines and photo point locations are transferred to the aircraft, which carries out the mission autonomously. Images are processed to create ultra-high resolution orthophotos and three-dimensional terrain models.

The researchers are developing methods to combine visible and model-derived data to separate woody debris from background such as mineral soil, vegetation, and forest floor litter. After isolating residues in the images, the volume and mass of individual slash piles are calculated.

Initial comparisons have found quantities of residues estimated by the drone method exceeded estimates from a 100% field inventory of 10m x 10m plots by about 15%. Oblique imagery after flight lines have been completed will be used to correct overestimates and reach the goal of achieving an error of less than 10%. When the project is completed later this year, the researchers will be able to provide the methodology needed to fly, image, and estimate woody residues.


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