Reducing the operator effect in LSPIV image-based discharge measurements

<p align="justify"><span lang="en-US">The operator effect is a prominent error source in image-based velocimetry methods. The LSPIV </span><span lang="en-US">method </span><span lang="en-US">is known to be sensitive to the parameters and choices of the user, </span><span lang="en-US">as shown in the literature and emphasized by the results of a video gauging intercomparison, the Video Globe Challenge 2020 (VGC2020) (Le Coz et al., 2021). </span><span lang="en-US">The intercomparison </span><span lang="en-US">was carried out </span><span lang="en-US">during the COVID-19 lockdown of spring 2020 </span><span lang="en-US">and</span><span lang="en-US"> involved 15 to 23 participants using the LSPIV method </span><span lang="en-US">among other techniques</span><span lang="en-US"> on 8 videos </span><span lang="en-US">representative of the diversity of river gauging conditions and imaging viewpoints</span><span lang="en-US">. Each video came with a discharge reference and associated uncertainty. </span></p> <p align="justify"><span lang="en-US">An in depth investigation of the </span><span lang="en-US">intercomparison results has been carried out to identify the most sensitive parameter(s) for each video and also to review the common setting mistakes (cf. Bodart et al., 2022). The investigation highlight</span><span lang="en-US">ed</span><span lang="en-US"> the strong impact of the </span><span lang="en-US">image </span><span lang="en-US">temporal sampling (extraction framerate) and </span><span lang="en-US">of</span><span lang="en-US"> the velocity filtering on the discharge errors. The ortho-rectification and the surface coefficient were also found to be impacting in given cases. </span></p> <p align="justify"><span lang="en-US">Based on these observations, s</span><span lang="en-US">everal assistance tools and automated filters are proposed to reduce the operator effect. They are evaluated on the intercomparison dataset. The assistance tools use available information (e.g. transect data) or basic user inputs (e.g. manual spotting of </span><span lang="en-US">some</span><span lang="en-US"> velocities) to determine the </span><span lang="en-US">optimal </span><span lang="en-US">extraction framerate, grid points and searching area (SA) for LSPIV computation. The sequence of automated filters is buil</span><span lang="en-US">t</span><span lang="en-US"> for the specific context of discharge measurement: spatial coherency of the velocities in a local neighborhood and temporal coherency of the velocities </span><span lang="en-US">computed </span><span lang="en-US">at a point. The</span><span lang="en-US">se</span> <span lang="en-US">velocity </span><span lang="en-US">filters are systematic and do not require any input from the user. </span></p> <p align="justify"><span lang="en-US">The application of the assistance tools and automated filters to the intercomparison dataset leads to a significant improvement of the results. </span><span lang="en-US">On the eight videos, the</span><span lang="en-US"> mean interquartile range of the percent error initially at 17% </span><span lang="en-US">is</span><span lang="en-US"> reduced to 2% </span><span lang="en-US">and </span><span lang="en-US">the</span> <span lang="en-US">mean median of the percent error initially at -9% is reduced to 0.6% </span><span lang="en-US">with the assistance tools and filters</span><span lang="en-US">. </span><span lang="en-US">The results are encouraging and </span><span lang="en-US">can be implemented in software tools for</span><span lang="en-US"> the operational deployment of the LSPIV method for discharge measurement.</span></p> <p lang="en-US" align="justify">Le Coz, J., Hauet, A., and Despax, A. (2021). The Video Globe Challenge 2020, a video streamgauging race during the Covid-19 lockdown, EGU General Assembly 2021, online, 19&#8211;30 Apr 2021, EGU21-2116, https://doi.org/10.5194/egusphere-egu21-2116, 2021</p> <p lang="en-US" align="justify">Bodart, G., Le Coz, J., Jodeau, M., and Hauet, A.: Quantifying the operator effect in LSPIV image-based velocity and discharge measurements, EGU General Assembly 2022, Vienna, Austria, 23&#8211;27 May 2022, EGU22-4457, https://doi.org/10.5194/egusphere-egu22-4457, 2022.</p>