In unmanned aerial vehicle (UAV) photogrammetric surveys, the camera can be pre-calibrated or can be calibrated "on-the-job" using structure-from-motion and a self-calibrating bundle adjustment. This study investigates the impact on mapping accuracy of UAV photogrammetric survey blocks, the bundle adjustment and the 3D reconstruction process under a range of typical operating scenarios for centimetre-scale natural landform mapping (in this case, a coastal cliff). We demonstrate the sensitivity of the process to calibration procedures and the need for careful accuracy assessment. For this investigation, vertical (nadir or near-nadir) and oblique photography were collected with 80%-90% overlap and with accurately-surveyed (σ ≤ 2 mm) and densely-distributed ground control. This allowed various scenarios to be tested and the impact on mapping accuracy to be assessed. This paper presents the results of that investigation and provides guidelines that will assist with operational decisions regarding camera calibration and ground control for UAV photogrammetry. The results indicate that the use of either a robust pre-calibration or a robust self-calibration results in accurate model creation from vertical-only photography, and additional oblique photography may improve the results. The results indicate that if a dense array of high accuracy ground control points are deployed and the UAV photography includes both vertical and oblique images, then either a pre-calibration or an on-the-job self-calibration will yield reliable models (pre-calibration RMSE XY = 7.1 mm and on-the-job self-calibration RMSE XY = 3.2 mm). When oblique photography was Remote Sens. 2015, 7 11934 excluded from the on-the-job self-calibration solution, the accuracy of the model deteriorated (by 3.3 mm horizontally and 4.7 mm vertically). When the accuracy of the ground control was then degraded to replicate typical operational practice (σ = 22 mm), the accuracy of the model further deteriorated (e.g., on-the-job self-calibration RMSE XY went from 3.2-7.0 mm). Additionally, when the density of the ground control was reduced, the model accuracy also further deteriorated (e.g., on-the-job self-calibration RMSE XY went from 7.0-7.3 mm). However, our results do indicate that loss of accuracy due to sparse ground control can be mitigated by including oblique imagery.
The three‐dimensional morphology and surface area of organisms such as reef‐building corals is central to their biology. Consequently, being able to detect and measure this aspect of corals is critical to understanding their interactions with the surrounding environment. This study explores six different methods of three‐dimensional shape and surface area measurements using the range of morphology associated with the Scleractinian corals: Goniopora tenuidens, Acropora intermedia, and Porites cylindrica. Wax dipping; foil wrapping; multi‐station convergent photogrammetry that used the naturally occurring optical texture for conjugate point matching; stereo photogrammetry that used projected light to provide optical texture; a handheld laser scanner that employed two cameras and a structured light source; and X‐ray computer tomography (CT) scanning were applied to each coral skeleton to determine the spatial resolution of surface detection as well as the accuracy of surface area estimate of each method. Compared with X‐ray CT, wax dipping provided the best estimate of the surface area of coral skeletons that had external corallites, regardless of morphological complexity. Foil wrapping consistently showed a large degree of error on all coral morphologies. The photogrammetry and laserscanning solutions were effective only on corals with simple morphologies. The two techniques that used projected lighting were both subject to skeletal light scattering, caused by both gross morphology and meso‐coral architecture and which degraded signal triangulation, but otherwise provided solutions with good spatial resolution. X‐ray CT scanning provided the highest resolution surface area estimates, detecting surface features smaller than 1000 µm2.
Constructing realistic energy budgets for Antarctic krill, Euphausia superba, is hampered by the lack of data on the metabolic costs associated with swimming. In this study respiration rates and pleopod beating rates were measured at six current speeds. Pleopod beating rates increased linearly with current speed, reaching a maximum of 6 beats s À1 at 17 cm s À1 . There was a concomitant linear increase in respiration rate, from 1.8 mg O 2 g D À1 h À1 at 3 cm s À1 to 8.0 mg O 2 g D À1 h À1 at 17 cm s À1 . The size of the group tested (50, 100 and 300 krill) did not have a significant effect on pleopod beating rates or oxygen consumption (ANCOVA, F=0.264; P>0.05). The cost of transport reached a maximum of 75 J g À1 km À1 at 5 cm s À1 , and then decreased with increasing current speed to 29 J g À1 km À1 . When considered in light of energy budgets for E. superba, these data indicate that the cost of swimming could account for up to 73% of total daily metabolic expenditure during early summer.
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