Abstract
Background and Objectives: The recent use of Structure-from-Motion with Multi-View Stereo photogrammetry (SfM-MVS) in forestry has underscored its robustness in tree mensuration. This study evaluated the differences in tree metrics resulting from various related SfM-MVS photogrammetric image acquisition scenarios. Materials and Methods: Scaled tri-dimensional models of 30 savanna trees belonging to five species were built from photographs acquired in a factorial design with shooting distance (d = 1, 2, 3, 4 and 5 m away from tree) and angular shift (α = 15°, 30°, 45° and 60°; nested in d). Tree stem circumference at 1.3 m and bole volume were estimated using models resulting from each of the 20 scenarios/tree. Mean absolute percent error (MAPE) was computed for both metrics in order to compare the performance of each scenario in relation to reference data collected using a measuring tape. Results: An assessment of the effect of species identity (s), shooting distance and angular shift showed that photographic point cloud density was dependent on α and s, and optimal for 15° and 30°. MAPEs calculated on stem circumferences and volumes significantly differed with d and α, respectively. There was a significant interaction between α and s for both circumference and volume MAPEs, which varied widely (1.6 ± 0.4%–20.8 ± 23.7% and 2.0 ± 0.6%–36.5 ± 48.7% respectively), and were consistently lower for smaller values of d and α. Conclusion: The accuracy of photogrammetric estimation of individual tree attributes depended on image-capture approach. Acquiring images 2 m away and with 30° intervals around trees produced reliable estimates of stem circumference and bole volume. Research Highlights: This study indicates that the accuracy of photogrammetric estimations of individual tree attributes is species-dependent. Camera positions in relation to the subject substantially influence the level of uncertainty in measurements.
Highlights
A comprehensive and precise three-dimensional representation of individual trees is important for accurate appraisal of their biophysical attributes, such as morphology and growth
There has been an increase in the use of software that combine the computer vision techniques of structure-from-motion and multi-view stereoscopy (SfM-MVS) as a photogrammetric tool in forest research [2,3,4,5,6]
This study showed that image acquisition scenario for Structure from Motion (SfM)-MVS photogrammetry influences the quality of the 3-D model reconstruction of standing trees
Summary
A comprehensive and precise three-dimensional representation of individual trees is important for accurate appraisal of their biophysical attributes, such as morphology and growth. The potential of this technique to accurately resolve individual tree geometry in relation to terrestrial laser scanning methods has been highlighted [2,8,9]. In their plot-based comparative study, Liang et al [2] reported stem count accuracies of 60%–84% and 92%–100% for data derived from handheld camera and terrestrial laser scanners, respectively. The recent use of Structure-from-Motion with Multi-View Stereo photogrammetry (SfM-MVS) in forestry has underscored its robustness in tree mensuration. This study evaluated the differences in tree metrics resulting from various related SfM-MVS photogrammetric image acquisition scenarios. There was a significant interaction between α and s for both circumference and volume MAPEs, which varied widely
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