Abstract
The measurement of tree height has long been an important tree attribute for the purpose of calculating tree growth, volume, and biomass, which in turn deliver important ecological and economical information to decision makers. Tree height has traditionally been measured by indirect field-based techniques, however these methods are rarely contested. With recent advances in Unmanned Aerial Vehicle (UAV) remote sensing technologies, the possibility to acquire accurate tree heights semi-automatically has become a reality. In this study, photogrammetric and field-based tree height measurements of a Scots Pine stand were validated using destructive methods. The intensive forest monitoring site implemented for the study was configured with permanent ground control points (GCPs) measured with a Total Station (TS). Field-based tree height measurements resulted in a similar level of error to that of the photogrammetric measurements, with root mean square error (RMSE) values of 0.304 m (1.82%) and 0.34 m (2.07%), respectively (n = 34). A conflicting bias was, however, discovered where field measurements tended to overestimate tree heights and photogrammetric measurements were underestimated. The photogrammetric tree height measurements of all trees (n = 285) were validated against the field-based measurements and resulted in a RMSE of 0.479 m (2.78%). Additionally, two separate photogrammetric tree height datasets were compared (n = 251), and a very low amount of error was observed with a RMSE of 0.138 m (0.79%), suggesting a high potential for repeatability. This study shows that UAV photogrammetric tree height measurements are a viable option for intensive forest monitoring plots and that the possibility to acquire within-season tree growth measurements merits further study. Additionally, it was shown that negative and positive biases evident in field-based and UAV-based photogrammetric tree height measurements could potentially lead to misinterpretation of results when field-based measurements are used as validation.
Highlights
Tree height is an important parameter required to quantify timber resources and is essential in evaluating the economic and ecological value of a forest stand
In terms of intensive forest monitoring plots, in particular at research stations (Level III—core plots), the methodology introduced in this study shows that Unmanned Aerial Vehicle Photogrammetry (UAVP) could prove an invaluable addition to on-site continuous measurements of not just the highly temporal and rapid acquisition of tree heights and the measurement of tree crown diameter, as well as the quantification of phenological observations
We showed that photogrammetric measurements can attain similar accuracies to that of indirect field measurements in an even-aged Scots Pine stand when destructive direct measurements are used as validation
Summary
Tree height is an important parameter required to quantify timber resources and is essential in evaluating the economic and ecological value of a forest stand. As a typical measurement parameter for forest inventory and monitoring programs, tree height (h) is measured in the field by means of the direct or indirect measurement of the distance between the base (ground-level) and the tip (apical meristem) of a tree [8,9,10]. Direct measurements can be carried out with destructive methods, where trees are required to be harvested and the length determined along the ground with a measuring tape. On the other hand, would be that of non-destructive geometric or trigonometric methods [2] carried out via field measurements with a hypsometer [11], laser device [12], or Total Station (TS) [4]. Indirect tree measurement is possible using remote sensing techniques, as in traditional photogrammetric measurements derived from analog aerial imagery [13,14], digital aerial photogrammetry (DAP) [15,16], active sensor remote sensing techniques, such as Light Detection and Ranging (LiDAR) [4,17,18,19], or Interferometric Synthetic Aperture Radar (InSAR) [20,21]
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