Latest Geodetic Changes of Austre Lovénbreen and Pedersenbreen, Svalbard

Round 1

Reviewer 1 Report

General comments.

This is a very interesting manuscript about the use of RTK-GPS to derive geodetic glacier changes in Svalbard glaciers, dealing with a new original method to better quantify them and to produce accurate and reliable DEMs. The manuscript includes a very important discussion on the performance of the different interpolation methods which in my opinion it’s not only important for this kind of studies, but it can be useful for the production of palaeoglacier DEMs from glacier modelling along flowlines.

The results are appropriate, well presented and assessed. However, the manuscript lacks a more extensive discussion of them considering those coming for other areas using similar methodologies. This is important to make of this manuscript a robust article.

In the following lines, I invite the authors to consider some minor comments, and I suggest a minor revision of the manuscript.

Title.

The title is OK. No comments.

Abstract.

The content of the abstract is OK, very complete and synthetic, including the objectives, methods, results and conclusions. No major comments.

Keywords.

OK.

The introduction is brief, clear, well-written and concise. However, I would like the authors to expand a little bit some information about the recent glacier trends in the Arctic, to frame the aims of this paper. This information, which can be added at the end of the section, would enrich the introduction and the problem stating.

Comments:

L40-42: I suggest to move this sentence to the “Methods” section.

L52-55: please, add some previous results to know the recent glacier trends at a glance.

I think that it’s not appropriate to mix the study area and data as they are different information. What the authors mean with data, the previous results published elsewhere on the literature or the data used in this study? Pleas, clarify it. On the other hand, I miss some information regarding to the climate in the area, at least in terms of mean annual/summer temperature, annual precipitation.

Comments:

L58: please, remove “Figure” as it’s repeated.

L64-65: what is the impact of this debris-cover on the mass balance of the glaciers? Have the authors observed a correlation between melting and presence of debris in these areas despite belonging them to the accumulation areas? I invite the authors, if possible, to include some brief discussion on this issue in Results/Discussion.

L74: please, remove “Figure” as it’s repeated.

L74-75: the purpose should have been mentioned in the “Introduction” section.

L69-82: this information concerns to a classic “Methods and materials” section, and thus, I suggest the authors to move this paragraph to the “Methods section”.

The content in this section is appropriate, and the different interpolation methods are well-presented with very clear explanations. However, some specific references on this issue should be provided. Moreover, after reading the interpolation methods, I miss other kind of interpolation to generate DEMs, i.e. the ArcGIS “Topo to raster” command. Why the authors did not consider it in the exploration and discussion?

I invite the authors to consider this reference when discussing Kriging interpolation and other methods:

Oliver, M.A., Webster, R., 2014. A tutorial guide to geostatistics: Computing and modelling variograms and kriging. CATENA 113, 56–69. https://doi.org/10.1016/J.CATENA.2013.09.006

Pellitero, R., Rea, B.R., Spagnolo, M., Bakke, J., Ivy-Ochs, S., Frew, C.R., Hughes, P., Ribolini, A., Lukas, S., Renssen, H., 2016. GlaRe, a GIS tool to reconstruct the 3D surface of palaeoglaciers. Comput. Geosci. 94, 77–85. https://doi.org/10.1016/j.cageo.2016.06.008

Comments:

L98: what the authors mean with “structural components? Please, clarify it.

L101: please, substitute “interpolation” by “approach” in order to avoid the repetition.

L112: how was the “optimal interpolation method” identified?

L115: I guess that the RTK-GPS track and points are not the same in the different years. What is the impact of this fact when calculating the differences in the crossover points? Any estimation of error percentage?

4.1. Comparisons of different interpolation resolutions and methods.

In my opinion, the results from this section are well presented and very easy to follow, and the Tables 1 and 2 illustrate appropriately them as a support to the text. Thus, I have no major concerns except for a very few minor comments that I’d like the authors to consider and clarify when necessary.

Comments:

L122: please, remove “Table” as it’s repeated.

L122-124: maybe I’m wrong, but the sentence appears to me a bit contradictory: “RMSE is smaller with a higher resolution, which means that the elevation extracted near the RTK-GPS tracks is more accurate at a higher resolution. All the 123 mean errors are close to zero but do not seem to be related to the resolution”. Please, could you clarify it?

L133: please, remove “Table” as it’s repeated.

L140: please, remove “Figure” as it’s repeated. And also, I suggest removing “interpolation” after “OK, NN”.

L141: what the authors mean with “smooth”? How have they measured/calculated it? Please, explain it.

L146-147: please, could the authors explain this with more detail? This is a very interesting issue, but the sentence appears to me a bit vague. The explanation might be useful for the potential readers of this paper dealing with similar tasks regarding to DEM production.

L147-151: this discussion on the examples of different applications may be moved to the discussion may be as “Results” section should be focused only on the presentation of the results obtained by the authors.

4.2. Glacier surface elevation changes.

As in the previous sub-section. The results are well presented and well-written. However, although I am aware that there are maps for the representation of DEM differences, I would like to see a synthesis of the results in a table with some statistics of the differences (maximum, minimum, average, median, etc.). This strengthen the results avoiding the only support of the maps description.

Comments:

L157: please, remove “Figure” as it’s repeated.

L161/164: please, remove “Figure” as it’s repeated.

L162: please, explain what you mean with “anomalous”.

L165-167: this is an interpretation of the error source, and maybe it can be moved to the “Discussion” section in order to keep only the raw results in the current section.

L190-191: “altitude has an important impact on elevation changes”: this sentence appears to me quite obvious. What do you mean with it? Can you expand a little bit what you mean? The spatial pattern of the negative/positive values?

L196: please, remove “Figure” as it’s repeated.

L208: please, remove “Figure” as it’s repeated.

L209-211: an ablation region? Later, the authors say that it increased the elevation. To me, it appears somewhat contradictory? Please, could you clarify this sentence?

L212: please, remove “Figure” as it’s repeated.

L221: please, remove “Figure” as it’s repeated.

4.3. Geodetic glacier mass balances.

The results are clear and concise. No major concerns on it except for some minor comments:

L227-234: this explanation is a methodological one, so I suggest the authors to re-locate it in the “Methods” section.

L236-237: please, remove “Table” as it’s repeated. I don’t understand the sentence. Can you clarify it, please?

L248-256: I think that this is a methodological content so maybe it should rather been located in “Methods” section.

L262: maybe I’m wrong or I’m misunderstanding the sentence but the authors said earlier in the manuscript that the error was at the millimeter level. Please, clarify it.

L267: please, remove “Figure” as it’s repeated.

L273: please, remove “Table” as it’s repeated.

L282: and, what is the long-term ELA in these glaciers? Could you provide it?

L293: please, remove “Figure” as it’s repeated.

L296: why 500 km/m3? What reference or measurements are the authors basing on to assume this?

L299: please, remove “Figure” as it’s repeated.

This section presents a thorough and interesting discussion and interpretation of the results conducting to the validation of the proposed methodology. However, the discussion of the results is very local, only comparing with data from the same area. I miss a more critical auto evaluation of the methodology in the light of more extensive literature covering the same techniques in other areas and glacier types. By this way, the authors could achieve robust ground to make of this method a versatile reference for further studies elsewhere. I invite the authors to expand this section far beyond the results of the study area.

Comments:

L307: be careful, the number of this section should be 5 instead of 4.

L312-313: could you provide the accumulated hypsographic curves of both glaciers to see it at a glance and verify it?

L322: please, remove “Table” as it’s repeated.

L330-332: I agree with this reasoning, but could you provide it other examples from other areas to fully validate this method. We don’t know if this method could work in other study areas. It would give a strong support to the conclusions and would strengthen the paper. In this current state, the manuscript appears to me quite local instead of more global. I invite the authors to think about it.

L333-343: this paragraph may be re-located in the “Methods” section.

L343: ok, what interpolation? The author should explain it.

L354: please, remove “Figure” as it’s repeated.

L364: please, remove “Figure” as it’s repeated.

L369: density of 350 kg/m3 appears from nothing? Where does this value come from?

L369-371: this is an information more appropriate for the methods section. Please, think on re-locate it.

L371: please, remove “Figure” as it’s repeated.

L371-373: could the authors explain it in more detail, please? The sentence is a bit vague.

L374/375: “TableError! Reference source not found”. Please, correct it.

The conclusions section is appropriate summarizing the main findings of the manuscript. I don’t have major concerns or any comment

Figures.

The figures, both maps and charts are well drawn and very informative, which make them very informative. I only suggest the authors to add the north arrow to the maps in the Figure 2 and to consider adding the surface hillshade to the generated DEMs in order to better see the smoothness or roughness of the surfaces in each interpolation method.

Tables.

The number and content of the tables is appropriate to illustrate the results, but consider adding one more according to my previous comment in the “Results” section. I only have one small suggestion to the Tables 1 and 6: keep the same letter style in the heading.

Author Response

Dear reviewer and editor,

Thanks for your comments and suggestions.

We have revised the manuscript according to the reviewers’ comments. Our responses to the comments are provided in a pdf document. Please view the detailed response in the document.

By the way, our manuscript has undergone English language editing by MDPI in this revision.

Best wishes,

Songtao Ai et al.

23rd Nov. 2019

Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript reports on the changes of two glaciers in Svalbard using RTK-GPS measurements over 3-year period of time between 2013-2015. The work is a demonstration of how precise geodetic positioning serves different research communities. It fits nicely in the scope of the special issue that authors aim for.

The manuscript consists of five sections and a list of references spread over 16 pages. Twelve figures and six tables are used to visualize and summarize the research results. They are of good quality and include self explanatory captions. Most of the reported work refers to the creation of the digital elevation model and how to interpret the changes in the glaciers using the derived model. The methodology is based on widely used methods. It is somehow ambiguous how this work differentiate from existing publications.

General comments
The quality of the RTK surveys is significant as the entire work, including interpolation procedure and data interpretation, relies on the foundation of the RTK-derived positions. Considering the title it should have been discussed more thoroughly in the manuscript. Alternatively, it is suggested to reformulate the title to better match the content of the reported work. Moreover, some of the contradictory or anomalous results may found explanations by answering to the following questions which could not be found in the manuscript:
What is the quality of the RTK surveyed tracks in reality? ... not the one reported by the hardware manufacturer or in the books.
Where was the base station located? Was its location kept fixed from year to year?
How the year-to-year comparison was conducted: based on the coordinates at the epoch of measurement or coordinates at a fixed epoch in time?
How the ellipsoidal heights were converted to altitudes?
Why Table 4 and Tables 5 includes different units although they refer to the same parameter: mass balance?

Specific comments
Check the insertion of the Figure/Table labels. All of them are duplicated. Pay special attention to line 374-375.
Clarify lines 217-222 where year 2019 appears in the text and also Figure 9. This is confusing since the time interval was mentioned to be 2013-2015.
Improve consistency in specifying the unit. Use Table 3 as example.
Explain what w.e. stands for.
Reorganize the Discussion section to better emphasize the key points of the work. Putting in balance with other studies will add more value. Make clear the strengths and weaknesses of the study. It is found to lack focus and somehow cumbersome in the current form.

Author Response

Dear reviewer and editor,

Thanks for your comments and suggestions.

We have revised the manuscript according to the reviewers’ comments and suggestions. Our responses to the comments are provided in a pdf document. Please view the detailed response in the document.

By the way, our manuscript has undergone English language editing by MDPI in this revision.

Best wishes,

Songtao Ai et al.

23rd Nov. 2019

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

This is indeed an improved version of the manuscript. Authors, have did a good job addressing reviewers' comments. However, this reviewer thinks that several aspects remain still ambiguous. First, the title does not reflect the content of the work. The content is focused on comparing and analysing different interpolation methods to generate DEM. RTK was only the field data collection method. It has little contribution to the content. This is not an RTK paper. Moreover, it is still unclear how authors dealt with the dynamic earth properties  for their three-year time data. During 2013-2015, Ny Ålesund has moved about 5.5 cm in the horizontal plane and about 2 cm in the vertical plane (http://itrf.ensg.ign.fr/site_info_and_select/site.php?domesnum=10317M003) Thus, the assumption that points have same coordinates from one year to another it is incorrect. All the surveys have to be brought to the same epoch in time for a consistent and reliable comparison. Coordinate velocities should be applied before doing any further DEM generation. Furthermore, there is inconsistency and ambiguity in data used in the analyses. The abstract and other parts of the paper refer to 3-year time period. Then in section 4.2 and Figure 9, authors refer to a longer time period 6 years. It is natural to ask, why the authors have not covered the period 2013-2019 for the entire study. No matter what, the authors should be consistent in the data used.

Having this said, this reviewer concludes that the manuscript still requires further clarifications before being considered for publication. Nevertheless, the final decision belongs to editor in charge.

Author Response

Dear Reviewer and editor,

First of all, we would like to take this opportunity to thank the reviewer for your constructive comments and relevant questions.
We have revised our manuscript, and the responses to the comments are provided in the pdf document.

Best regards,
Songtao Ai et al.

2019-12-02

Author Response File: Author Response.pdf