|Review of the manuscript:|
Combination of UAV and terrestrial photogrammetry to assess rapid glacier evolution and conditions of glacier hazards. Fugazza et al.
In this manuscript, the authors describe and analyse geomorphological features on the tongue of a hazard-prone glacier in the Italian alps with the help of three different (close-range) remote sensing methods. They found that UAV- and terrestrial photogrammetry are the best surveying techniques to assess ice thickness change and map glacier hazards.
Compared to the first manuscript, the authors reduced significantly the length of the manuscript which was preconized by both reviewers, set the aims more clearly and re-organized the different sections in a much better way. However, there are still several points of the paper that require substantial improvements, such as:
1) This version of the paper in my opinion, is poorly written. I had large difficulties understanding several sections of the manuscripts, due to the use of unconventional words and wrong sentence construction. As this is not my mother-tongue either, I could not give suggestions for every case and did not highlighted all of them. A English proof reading is surely needed before considering this paper for publication.
2) The individual methods and comparison of the methods used as suggested in this version of the manuscript are not new. The only aspect that could have been interesting and relatively new, is the merging of different datasets. However, the authors only merged the terrestrial and UAV photogrammetry point clouds, without giving a quantitative explanation of how this merged point cloud is much better than the terrestrial one alone. Looking at Figure 6, and the very sparse point cloud they obtained from UAV photogrammetry compared to terrestrial photogrammetry, I doubt that the merging of both point clouds did make a big difference. It would however be good if the authors could give more information on this point. Moreover in this study, this merged point cloud is only used to map the hazards (along with the orthophoto), which I guess, could have also been done with the point cloud from one method only (the terrestrial photogrammetry). So I am not sure if this merged point cloud was really necessary. I think the authors should emphasis more on the scientific value of this study.
3) I think that the authors wrote a lot about the differences between the methods they used, but very little refer to other literature. There are a lot of papers (below only a few for example) comparing point clouds and DEMs generated from different surveying methods.
a. Baltsavias, 1999: A comparison between photogrammetry and laser scanning
b. Rayburg et al. 2009: A comparison of digital elevation models generated from different data sources
c. Naumann et al. 2013: Accuracy comparison of digital surface models created by unmanned aerial systems imagery and terrestrial laser scanner
In most sections of the results and discussion, the authors report results that have already been found in other studies. For instance in the discussion section, where the advantages and drawbacks from all methods are explained, the authors cite publications that found the same results (but several years ago). As several papers are already stating these, I think this information should not be discussed anymore, but taken as granted. I suggest the authors to integer older and newer publications related to the comparison of point clouds and DEM specifically.
4) To my point of view, the authors are often making statements such as: “The UAV-based DEMs hold the potential to become a standard tool to investigate geodetic mass balance”, but the authors did not try to do this and the publications that succeeded to do it are very rare. Another one: “The final accuracy of our UAV photogrammetric products was nevertheless adequate to investigate ice thickness change over two years,…”. The reader don’t know for what it is “adequate”, as the authors found DEM errors over two meters (for these two years). They do not give any percentage error that would mean on the total melt. Moreover, the authors do not state why they need ice thickness data. We only know that ice thickness change is related to an increase of natural hazards.
I think in general, the manuscript needs to be more carefully written, with results presented in a more quantitative robust way.
I think the manuscript requires again major revision before being considered for publication in NHESS. More specific and short comments are reported in the supplementary material as .pdf.
Comments on Figures and Tables:
- I would zoom in more as there is a lot of space under the reference area, and eventually show the positions of the terrestrial pictures.
- It’s hard to make the difference between the green triangle and the green points. Can you change the colour? If you do this, I think you can also remove the “ (in 2016 two different…)” in the caption, because we can see it on the map.
- Is the number after +/- based on 1 standard deviation? Or 2?
- It is very hard for the reader to have an idea about the size of the feature. Could you insert a scale?
- This Figure shows the hazardous features on the glacier but these are not the ones that you survey and studied. So I am wondering what kind of information the reader gets out of this Figure. I would maybe recommend to put them on the right side of Fig.1 where there is some space left before the caption’s end, and delete the Figure 2.
- Very clean Figure
- I would only centre the b in the white case
- I think this figure does not give much information to the reader. My suggestion is that you either put the location of the images on Fig 1., or that you merge a and b.
- Caption: Small question in (a). In the text you say that you took 134 pictures. Are they all displayed here? I have the feeling that they don’t, so did they all align in the software?
- The background image is very dark. It would be nice to see more lighter colours
- Also here on the image it is hard to see how big these features are. As you don’t have much space, I suggest you make a similar scale everywhere and that you add it on top of the 100 scale bar and you show the number here.
- Caption: Maybe you could mention that the scale bars don’t have the same scale?
- Caption: “L” in Location
- In the caption and in the main text you use orthomosaic and orthophotos. Please stick to one term.
- I would add after (a) and (b), situation in 2014 (situation in 2016) or something similar, so that you don’t start with a year.
- Maybe consider to change the total ice thickness change! In the manuscript you are stating values to -30 and -50m that we don’t see on the map! The reader needs to calculate if he/she has the yearly values.
- In the caption, it would be nice if you could state something like:” DEM 2007 from aerial multispectral survey, DEM2014 and DEM 2016 from UAV photogrammetry.” So that the reader do not need to go back to the text to remember which DEM is which.
- This table definitely need some adjustments, because it’s not easy to read.
1. The “sample window” text could be rotated 90°, and use all space above the numbers (merge cells).
2. The meaning of k should be explained in the caption
3. You could choose 3 abbreviations (UAV, TP and TLS) in the table and explain them in the caption, so that the text is less squeezed.
4. Watch that you use capital letter at the beginning or you text everywhere.
5. The numbers with 1645+/- 54 need to be in one line! Otherwise the reader asks: what is the number below it? There seems to be a bit of space left on the right side of your table to enlarge it (till the level of your caption right).
1. The “sample window” text could be rotated 90°, and use all space above the numbers.
2. Same comment as above with the abbreviations
3. Explain what is “–“ and Ref. in the caption?
4. The caption could be a bit more elaborated!
- The text could be all set on left side of the cell
- Caption: Could you explain what is sigma?