Data and code from: Repeated drone photogrammetry surveys demonstrate that reconstructed canopy heights are sensitive to wind speed but relatively insensitive to illumination conditions

Unoccupied aerial vehicle (UAV) based structure-from-motion (SfM)
photogrammetry surveys are becoming a standard tool for ecologists to
measure plant structure and biomass in non-forest ecosystems. The
reproducibility of SfM survey results under different operational
conditions, namely wind speed, sun elevation and cloud condition, is
poorly understood. It is also unclear to what extent commonly applied
point-to-grid interpolation of derived point clouds affects inference of
vegetation structure. These knowledge gaps limit the use of these methods
for measuring and monitoring detection. We captured 61 UAV SfM surveys at
the same study area under a range of wind/sun/cloud conditions over a
24-day period during 2021 and used generalised linear mixed effects models
to test how the structural reconstructions varied with environmental
conditions. Wind speed significantly influenced the canopy height
reconstructions, with greater wind speeds reducing mean canopy height.
Different plant species exhibited varying sensitivities to wind that are
likely related to leaf attributes (size, structure, and density), growth
form of the canopy, and physical properties such as limb flexibility. The
movement of plants can reduce canopy height estimates derived from
photogrammetric surveys, even under relatively low wind speeds.
Reconstructed canopy heights were comparatively insensitive to solar
elevation variations. Cloud conditions and illumination by direct sunlight
had a weak, non-significant effect on reconstructed canopy height, with
sunny conditions (generating shadows) resulting in a measurable but
marginal reduction in mean canopy heights. When comparing canopy heights
of interpolated and discontinuous derived canopy height models the results
highlighted that these were insensitive to interpolation in this specific
setting. We recommend measuring mean wind speeds throughout surveys where
comparisons are to be made between different drone based SfM surveys of
vegetation over either time or space. Care should be taken to ensure that
the effects of wind are controlled for so that inferences are valid.