Quantitative Evaluation of Bathymetric LiDAR Sensors and Acquisition Approaches in Lærdal River in Norway

Round 1

Reviewer 1 Report

Dear authors.. Thank you for an interesting study. Lidar bathymetry is an advanced mapping system and applicable to various river studies, we all know that as there are various literature that defines and studies the bathymetric accuracy, Such results are an essential component of hydrographic surveys using Lidar technology and needs to be defined in great detail.

I was interested to read an unbiased comparison of systems. The methodology and data acquisition is good, however, I was not happy with the comparison method and the statistical approach. Histograms are good but they need to support other and more critical charts/results e.g. regression analysis.

I have more comments in the pdf file. Having said that, with its current form, this article does not bring anything new or substantial to the Lidar bathymetry community. I lost my interest in the middle, maybe I was hoping for more scientific and statistical evidences of what's happening with the comparison, which I did not find adequate.

best regards.

Comments for author File: Comments.pdf

Author Response

Dear reviewer. Thank you for your insightful comments. Please find our response to the comments in the attachment. 

Author Response File: Author Response.pdf

Reviewer 2 Report

Review Awadallah et al.

Quantitative evaluation of bathymetric LiDAR sensors and acquisition approaches in Laerdal river Norway

Overall this paper makes a nice contribution to this research area, and will be of interest to the Remote Sensing readership.  Bathymetric LiDAR is becoming increasingly available.  As a fluvial geomorphologist working in shallow gravel-bed rivers, I am aware that some of the early bathy lidar did not perform well in shallow water.  I would like to see in this paper some assessment of the relative performance of the instruments at different water depths.  Rivers have varying turbidity levels also – and this is surely something that needs to be considered when sampling. How does this affect performance of the sensors.

We also need a detailed bathymetric map of the test reach.

Sections 2.2 and 2.3 contain information on methods- so should these not be contained within section 3?

Other points:

Line 47, I disagree, I don’t think traditional approaches are necessarily and health and safety risk, rather it very much depends on the environment. The key limitation is the spatial coverage point density and much longer timespan to collect data

Lines 57, red-wavelength topographic lidar can also be airborne, and blue-green bathymetric lidar ;likewise could potentially be deployed form a terrestrial platform.

Line 62,  Not sure I fully agree. Red-wavelength lidar has been deployed in many rivers studies from aircraft (Charlton et al., 2003), terrestrial (Milan et al., 2007), and boat platforms (Kasvi et al., 2013).  There are dozens of river studies that have deployed red wavelength lidar – but usually on shallow gravel bed rivers with exposed bar surfaces above the water.

Line 112, I would like to know what the main limitations of using bathy LiDAR in rivers? How well do they work in shallow gravel-bed river systems for example?  As far as I was aware their performance in shallow water <0.5 was not so good

Line 145, Were all the sensors deployed on the same date and discharge conditions?  If not then we need to know the discharge. It would also be beneficial to know about turbidity – this must surely influence the penetration capabilities.  More discussion is needed on this.  I think we also need to know more about the depth variability of the test river.  The paper focuses on bathymetry but nowhere are we provided with a bathymetric map – we need to see one!

Line 200, what dates were TLS and MBES data collected relative to the bathy lidar? what of differences in flow discharge, depth and turbidity also?

Line 222, Can the authors cite references to support approaches - and why are 2 approaches being employed in the first place?

Line 236, There has been some research on the spatial variability in error in DEMs (e.g. Wheaton et al., 2010; Milan et al., 2011) derived from point clouds. This does have relevance and should be cited.

Table 1 – The table is useful but could also contain information on the performance of sensors in different turbidity’s, and accuracy at different water depths, and information on the change in footprint with distance from the sensor.

Figure 3 What should be an important figure seems very poor quality.  Overall it is difficult to relate the images to the river reach shown in Fig 1.  The Fig could be clearer, and we need to see actual bathymetry from MBES alongside.  The elevation/depth units need marking on scale. 

Figure 4 units on scale. text colour difficult to see. need length scale. we need to see the actual bathymetry as well as the differences. what causes the spatial patterns - is it reduced sensitivity with depth, or is it morphological complexity?

Suggested references

Charlton, M.E., Large, A.R. and Fuller, I.C., 2003. Application of airborne LiDAR in river environments: the River Coquet, Northumberland, UK. Earth Surface Processes and Landforms: The Journal of the British Geomorphological Research Group, 28(3), pp.299-306.

Kasvi, E., Vaaja, M., Alho, P., Hyyppä, H., Hyyppä, J., Kaartinen, H. and Kukko, A., 2013. Morphological changes on meander point bars associated with flow structure at different discharges. Earth Surface Processes and Landforms, 38(6), pp.577-590.

Wheaton, J.M., Brasington, J., Darby, S.E. and Sear, D.A., 2010. Accounting for uncertainty in DEMs from repeat topographic surveys: improved sediment budgets. Earth surface processes and landforms, 35(2), pp.136-156.

Milan, D.J., Heritage, G.L., Large, A.R. and Fuller, I.C., 2011. Filtering spatial error from DEMs: Implications for morphological change estimation. Geomorphology, 125(1), pp.160-171.

Author Response

Dear reviewer. Thank you for your insightful comments. Please find our response to the comments in the attachment. 

Author Response File: Author Response.pdf

Reviewer 3 Report

The manuscript submitted by Mahmoud Awadallah et al. is a well written and an interesting article dealing with quantitative analysis of bathymetric LiDAR sensors and acquisition approaches. With the current changes in the climate systems, the article topic is of great interest for readers and scientists working in coastal areas and for lakes and rivers level fluctuations analyses. The results are well presented, and the discussion provided is supported by the results. However, the authors need to improve the methodology provided.

Below are some comments and suggestions to improve the overall quality of the manuscript:

Lines 23-24, 121-122: Please indicate the comparison methodology adopted in evaluating the performance of three bathymetric LiDAR sensors, CZMIL Supernova, Riegl VQ880-G, and Riegl VQ840-G against a multibeam echosounder (MBES).

Lines 41-42: Please adopt the citation style of the journal as indicated in the instructions for authors. Please check for the whole manuscript.

Lines 49-51: Please also provide references of the recent studies conducted in the bathymetry acquisition and studies using remote sensing, multi-spectral satellites images for coastal areas and rivers studies. Please refer to: (1) Muzirafuti, A.; Barreca, G.; Crupi, A.; Faina, G.; Paltrinieri, D.; Lanza, S.; Randazzo, G. The Contribution of Multispectral Satellite Image to Shallow Water Bathymetry Mapping on the Coast of Misano Adriatico, Italy. J. Mar. Sci. Eng. 2020, 8, 126. https://doi.org/10.3390/jmse8020126; (2) Ma, Y.; Xu, N.; Liu, Z.; Yang, B.; Yang, F.; Wang, X.H.; Li, S. Satellite-Derived Bathymetry Using the ICESat-2 Lidar and Sentinel-2 Imagery Datasets. Remote Sens. Environ. 2020, 250, 112047.

Lines 205-211: The evaluation of the point clouds conducted using the differences (residuals) between the two-point clouds is not efficient as the data could have been acquired with different spatial references. Please provide the coordinate system and the projection settings used during acquisition data both for Lidar and MBES datasets.

Lines 221-222: please indicate how these RASTER images were created. With which interpolation method? Did you filter the points before creating these RASTER images?

Line 236: Please provide more details about the estimation of the spatial distribution, the median, and the root mean squares (RMS). Please provide their equations.

Author Response

Dear reviewer. Thank you for your insightful comments. Please find our response to the comments in the attachment. 

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

no further comments