Measuring Tree Sway Frequency with Videos for Ecohydrologic
Applications: Assessing the Efficacy of Eulerian Processing Algorithms
Abstract
Measurements of biophysical tree properties and hydrologic fluxes are
necessary for improving models and monitoring the impact of
disturbances. Prior research has demonstrated that measurements of tree
sway frequency can be used to quantify important ecohydrologic
processes, such as drought stress and snow interception, that otherwise
require expensive measurement techniques. However, existing instruments
used to measure tree sway lack spatial scalability. We investigate
whether the virtual vision sensor and multilevel binary thresholding
video processing algorithms can be used to accurately extract tree sway
frequency at multiple points in a video camera field of view and enable
scalable measurements of ecohydrologic processes. Comparing sway
frequencies extracted from video and accelerometer data at two sites, we
show that for 30-60 s videos, the video processing algorithms can
reproduce accelerometer sway frequencies with ±0.03 Hz accuracy. The
results suggest that video processing algorithms may be suitable for
applications where changes in sway frequency are on the order of tenths
of hertz or larger, for example the measurement of snow in trees.
Further work is needed to clarify the accuracy of the algorithms when
applied to longer videos, which may be required to monitor processes
with more subtle changes in sway frequency, such as diurnal changes in
tree water content.