Estimating DBH with iPad Pro LiDAR in Boreal Forests: methodological considerations and a case study in natural boreal forests

Abstract

Diameter at Breast Height (DBH) is the measure of the diameter of a tree stem 1.3 meters above the ground. DBH is a key variable measured in Forest Resource Inventories (FRIs) and is traditionally measured manually, which is labour-intensive. The 2020 Apple iPad Pro 12th Generation is a lightweight, consumer-level tablet with an integrated LiDAR scanner with a maximum range of 5 m and a positional accuracy of ±1 cm. The overall objective was to examine the feasibility of estimating DBH in boreal forests with iPad Pro LiDAR. A scoping study was conducted in a plantation forest (48.37°N, 89.39°W) near Thunder Bay, ON, Canada, with the specific objective of determining an optimal method for DBH estimation with the iPad Pro. Different combinations of scanning methods (i.e., circular, figure-8, transect), numbers of stem cross-sections (i.e., one or five), sizes of stem cross-sections (i.e., 4 or 10 cm), and curvefitting formulas (i.e., Pratt’s circle fit, Taubin’s circle fit, Taubin’s ellipse fit, Szpak’s ellipse fit) were tested to identify the combination producing the most accurate estimates of DBH. The optimal method was the circular scanning pattern with a single 4 cm cross-section and a combination of circle- and ellipse-fitting formulas (RMSE = 1.1 cm; 6.2%). The second specific objective was to determine the accuracy of DBH values estimated with the optimal method in natural boreal forests. DBH was estimated for 133 trees on 15 sites in northern Ontario, Canada, representing a range of natural boreal forest site conditions. A secondary objective was to determine if the tested stand- (i.e., species composition, age, density, understory density) or tree-level attributes (i.e., species, actual DBH) significantly impacted the accuracy of estimated DBH values. An RMSE of 1.5 cm (8.6%) was achieved. Estimated DBH was within 1 cm of actual DBH for 78 of 133 (59%) measured trees. Stand age had a large effect (> 0.15) on the accuracy of estimated DBH values, while density, understory density, and actual DBH had moderate effects (0.05-0.15). In both studies, Inertial Measurement Unit (IMU) and positional accuracy errors with the iPad Pro LiDAR scanner limited the accuracy of DBH estimates. Future studies should incorporate a greater number of natural boreal forest sites to better understand the impacts of different stand and tree attributes on the accuracy of estimated DBH values. Future studies should also compare the accuracy of DBH values estimated from the iPad Pro and those estimated from traditional MLS and TLS for the same sites to identify the trade-off between device cost, device size, and accuracy. However, the scanning range of the device limits the variables that can be estimated from LiDAR data, rendering it unsuitable for use in FRIs until the scanning range is improved.