Assessing Through-Water Structure-from-Motion Photogrammetry in Gravel-Bed Rivers under Controlled Conditions

Round 1

Reviewer 1 Report

This is an excellent manuscript, well detailed and well written. I do not have any concerns and recommend it for publication as is. It definitely will be useful for the community.

Author Response

Thanks a lot for the comment!

Reviewer 2 Report

This study is a well-organized and conducted experiment to evaluate the possibilities of using photogrammetric survey to study the structure of the bottom surface of water streams. The analysis of the obtained results shows the prospects of development and use of underwater photogrammetry.

As a comment, I would like to note that the laboratory where the experiment was carried out did not take into account the conditions that can affect the photography process in natural conditions (above-water wind gusts, sun glare from the water, etc.). In addition, usually UAVs are equipped with photo cameras, which have less ability to adjust the photographic quality of the images.

Author Response

Thanks a lot for the comment!

We admit that not all the natural conditions can be simulated in the experiment. The situations mentioned by the reviewer will affect photo collection and quality significantly and we would like to examine their effects in the field in future if possible.

Reviewer 3 Report

Th paper is very well written and the aims of the research presented in a very clear and concise way.

100 - 107 The use of GCPs underwater have been used successfully tested and implemented in other sciences successfully. The paper will definitely present a much richer argument if these works are referenced as many of the issues that the paper addresses have been solved before but not necesarily in the field of hydrodynamics. The best example is the case of the archaeological excavations of the underwater site of Ropotamo in the Black Sea where 4D models of either archaeological excavations or sediment transport on underwater sites where created using the same methodology that this paper, but in the field (Pacheco-Ruiz, R., Adams, J. and Pedrotti, F., 2018. 4D modelling of low visibility Underwater Archaeological excavations using multi-source photogrammetry in the Bulgarian Black Sea. Journal of Archaeological Science, 100, pp.120-129.)

Underwater photogrammetry relies heavily on the implementation of lens calibration due to underwater non-linear radial distortion of the optics. It will be valuable to ad to the paper that this was undertaken for the tests. Otherwise the accuracy of the 3D data can be questioned in particular the data capured at the edge of the lens where distortion is greater as this creates a "Bowed" model. (Shortis, M. (2019). Camera Calibration Techniques for Accurate Measurement Underwater. In: McCarthy, J., Benjamin, J., Winton, T., van Duivenvoorde, W. (eds) 3D Recording and Interpretation for Maritime Archaeology. Coastal Research Library, vol 31. Springer, Cham. https://doi.org/10.1007/978-3-030-03635-5_2 and Metashape manual v 1.5 Chpter 4)

622-627 This is problematic as well as the camera calibration issue and the lack of oblique imagery will definitely will create inaccuracies in the model.

These references and additions to the methods as well as the suggested  and changes need to be added/considered for the paper to be published.  However, the camera correction issue must be clearly stated together with the lack of oblique imagery as these are important factors on the results. 

Author Response

Many thanks to the comments!

The underwater reconstruction in the Black Sea was a great work but the underwater GCPs are different from the ones used in this study. The photogrammetry used in their work is totally ‘underwater’, i.e., light only travelling in the water to the camera without going through the surface between water and air. In contrast, we tested through-water photogrammetry in this study, in which the light of the targets (i.e., underwater GCPs and bed surface) had to penetrate the water surface to be captured by the camera. This difference between underwater and through-water photogrammetry makes it difficult to add the suggested reference (Pacheco-Ruiz et al., 2018) here.

As for the issue of camera calibration, indeed it will definitely influence the precision of the outputs of through-water photogrammetry. However, this procedure includes so many parameters and the non-linear algorithm for camera calibration in Metashape is not open-sourced, which would lead to the difficulty in controlling the comparability of the results. So this procedure was not conducted in the SfM workflow. Instead, we only applied the GCPs to refine the SfM models, in which only linear transformation was conducted (Javernick et al., 2014; Metashape manual). This provides a comparable basis for all the results. As suggested by the reviewer, we have added this issue and the suggested reference (Shortis, 2019) in the last point of the limitations in Section 4.4.

Reference

Javernick, L., Brasington, J., & Caruso, B. (2014). Modeling the topography of shallow braided rivers using Structure-from-Motion photogrammetry. Geomorphology, 213, 166-182.