|This is my first review of this study by Dudek and Petlicki. I carefully reviewed the revised manuscript as well as the previous reviews and replies by the authors. I unfortunately could not find a way to access the original manuscript to assess the progress since the review.|
Overall, this study is relatively well written, the methods are sound and the results seem satisfactory. The study is interesting as most studies working with archival topographic maps, that I am aware of, usually address the issues related to this data in a very synthetic way (e.g. Ye et al., 2015; Nuimura et al., 2012). However, I share the impression of the previous reviewers that the method is not extremely novel (see main comment below).
- Methods and methods description:
I stand with R1 and R2 who highlight that the methods need to be clearly presented and novel enough for this study to be a benchmark for other studies analyzing topographic maps. I also do not find the methods extremely novel, but the results seem satisfactory anyway. There are two ways of improvements though, that have already been brought up by the previous reviewers that could be considered:
- Interpolate the elevation difference map (DoD) rather than interpolate the contour lines with TIN. This was brought up by R1 and considered to be addressed by the authors, but I noticed that the latter method is still used in this version. I quickly checked the data available on the Zenodo repository, and the slope derived from the provided 1961 DEM shows interpolation artifacts. This could probably be improved with R1’s suggestion.
- There are still relatively large and systematic elevation differences in stable terrain (e.g., in the west of the study area on Figure 20). It looks like the elevation difference shows a step-like pattern at the junction of two sheets. I see at lines 313-314 that you correct for a mean bias, but if I understand correctly, you calculate a single bias for the DEM mosaic of all sheets at once? If so, I would recommend estimating this bias for each sheet individually. If this does not work, I would encourage you to follow the suggestion of R2 to apply a blockwise coregistration. I understand your preference to use the topographic point for the horizontal alignment, but you could still apply this method to correct for a smooth vertical offset.
I would also include in the text some parts of your replies to the reviewers, especially concerning 1) your unsuccessful attempt to process low quality scans of the images unsuccessfully 2) your attempt of applying the Nuth & Kaab (2010) method to all stable terrain and the issue of non-rigid transformation (i.e., a shift alone is not enough).
Finally, I believe that the methods would need to be more detailed to make this work fully reproducible. Figure 7 provides a nice list of the different steps and commands used in ArcGIS. I have never worked with ArcGIS, however, I assume that there are several tools e.g., in the “ARC SCAN/VECTORISATION” box and several options for each tools. It would be extremely useful to provide the exact tools and options that were used (e.g., in a supplementary table if this takes too much space) and whenever possible, provide a clear description of the algorithms, so that this could be reproduced with open-source alternatives. For example, what are the steps in the “RASTER CLEANUP” or “FEATURE CLEANUP”?
It is a shame that the authors spend so much effort in generating a DEM from the archival map, but the only results that are presented are only vague numbers of maximum thinning and a map. I don’t think it would be much more effort to calculate an actual glacier-wide mass balance or at least a mean elevation change for each glacier and for each period. This could then be compared to existing estimates in the literature for this area of the whole of Svalbard and make this study more valuable.
I also wondered why you did not use the information on the 1984 map to generate glacier contours for 1984. You said that the elevation contour lines were not updated, but the color were changed to reflect glacier area changes. With this information, you could include a fourth period in your tables.
- Overall structure
Although the manuscript reads relatively well, there are still some parts that could be better structured to be more easily followed, or some sections that are not completely appropriate.
The introduction lacks acknowledgment of recent studies using archival terrestrial and aerial images: Girod et al. (2018), Holmlund et al (2021), Geyman et al. (2022). The first two are briefly mentioned in the discussions, but definitely belongs to the introduction as they are part of the state-of-the-art. The last one is very recent and not yet referenced but would deserve to be acknowledged as well.
The Results and Discussion sections are not so well structured. The use of subsections could help guiding the reader. There sometimes seem to be repetitions. Some of it stems from the fact that you first describe land-terminating glaciers then marine terminating glaciers, but this is not always very clear.
The conclusions should summarize the main findings and methods. Instead, the current conclusions enter too quickly in very specific details of the study, such as the map date. It also does not summarize the findings on area changes. Hence, I suggest rewriting the conclusions.
- l 30: you could add references to the studies by Girod et al. (2018) or Geyman et al. (2022).
- L 52-54: Mannerfelt et al. (2022), already referenced elsewhere, is another good example of use of historical terrestrial images in Svalbard.
- L 85: Maybe the currents that you discuss could be represented on Figure 2?
- Table 1, line 1936. You could include the references to Girod et al. (2018) and Geyman et al. (2022) in the list of references.
- L 96: the reference does not have the right format
- Figure 2: This figure should probably appear first and be referenced at the first mention of the Sorkapp Land peninsula. Could you please indicate the source of the background map?
- L 139: “analyzes”, do you mean “analysis”?
- L 183: remove “therefore” as it is redundant with “since”.
- Figure 7: The figure could be improved to include the inputs and outputs. Also see my comment on detailing the individual steps a bit more.
- L 239 “The six maps shared 189 points representing the same places” I understand from this sentence that the different sheets have some overlap. What is the size of this overlap (in map units)? Could this not be used to first align the sheets relatively to each other? This would be particularly useful for sheets with little topographic points or stable terrain.
- L 286: “The vector data thus processed was then used to generate a DEM” You may want to briefly re-state how you derive the DEM, or refer to the appropriate subsection. It took me some time to realize it was explained further up.
- Figure 16 and 20: I would adapt the color scale for these figures to better show the residuals. A min/max value of about -50/50 or less would probably be more appropriate.
- L 313-314: “The mean elevation difference (the bias) between the compared models was 2.28 m, with a standard deviation of 3.18 m, indicating that the 1961 model is higher.”. Could this bias correction be done individually for each sheet? Also it is recommended to use the median rather than mean, because it is less sensitive to outliers (Höhle & Höhle, 2009). The 3.18 m standard deviation seem surprisingly low when looking at figure 20. Maybe the change of colour scale would show it is not. Or maybe it would be useful to display the areas masked?
- Figure 21: The legends are too small to be read.
- Table 3: Brackets are lacking in the table (for percentage).
- L 381 “insolation thickness” -> “insolation, thickness”
- L 389-395: this whole paragraph simply repeats what is in the introduction. This should all be moved to the introduction (as the references).
- L 393: There are a lot more recent and appropriate references for the declassified spy satellite studies, such as Maurer et al. (2015); Maurer et al. (2019); Dehecq et al. (2021), Bhattacharya et al. (2021). You do not need to cite them all of course.
- L 395: you could reference Geyman et al. (2022) here.
- L 400: I believe the reference to Zekollari et al. (2020) would be more appropriate here.
- L 430-432: This is an example where the discussions seem to repeat itself. The argument of elevation and shadow was already brought up a few paragraphs above. I then realized this part was about land-terminating glacier while the other focused on marine terminating glaciers. There is some lack of logical flow in the discussions that makes it hard to follow.
- Supplementary: In the supplementary table, you should describe the table content (it should be understandable without searching in the main text) and the meaning of each column, as it is not very clear.
Bhattacharya, A., Bolch, T., Mukherjee, K., King, O., Menounos, B., Kapitsa, V., Neckel, N., Yang, W., Yao, T., 2021. High Mountain Asian glacier response to climate revealed by multi-temporal satellite observations since the 1960s. Nat Commun 12, 4133. https://doi.org/10.1038/s41467-021-24180-y
Dehecq, A., Gardner, A.S., Alexandrov, O., McMichael, S., Hugonnet, R., Shean, D., Marty, M., 2020. Automated Processing of Declassified KH-9 Hexagon Satellite Images for Global Elevation Change Analysis Since the 1970s. Front. Earth Sci. 8. https://doi.org/10.3389/feart.2020.566802
Geyman, E.C., J. J. van Pelt, W., Maloof, A.C., Aas, H.F., Kohler, J., 2022. Historical glacier change on Svalbard predicts doubling of mass loss by 2100. Nature 601, 374–379. https://doi.org/10.1038/s41586-021-04314-4
Höhle, J., Höhle, M., 2009. Accuracy assessment of digital elevation models by means of robust statistical methods. ISPRS Journal of Photogrammetry and Remote Sensing 64, 398–406. https://doi.org/10.1016/j.isprsjprs.2009.02.003
Maurer, J., Rupper, S., 2015. Tapping into the Hexagon spy imagery database: A new automated pipeline for geomorphic change detection. ISPRS Journal of Photogrammetry and Remote Sensing 108, 113–127. https://doi.org/10.1016/j.isprsjprs.2015.06.008
Maurer, J.M., Schaefer, J.M., Rupper, S., Corley, A., 2019. Acceleration of ice loss across the Himalayas over the past 40 years. Science Advances 5, eaav7266. https://doi.org/10.1126/sciadv.aav7266
Nuimura, T., Fujita, K., Yamaguchi, S., Sharma, R.R., 2012. Elevation changes of glaciers revealed by multitemporal digital elevation models calibrated by GPS survey in the Khumbu region, Nepal Himalaya, 1992-2008. Journal of Glaciology 58, 648–656. https://doi.org/10.3189/2012JoG11J061
Ye, Q., Bolch, T., Naruse, R., Wang, Y., Zong, J., Wang, Z., Zhao, R., Yang, D., Kang, S., 2015. Glacier mass changes in Rongbuk catchment on Mt. Qomolangma from 1974 to 2006 based on topographic maps and ALOS PRISM data. Journal of Hydrology 530, 273–280. https://doi.org/10.1016/j.jhydrol.2015.09.014
Zekollari, H., Huss, M., Farinotti, D., 2020. On the Imbalance and Response Time of Glaciers in the European Alps. Geophysical Research Letters 47, e2019GL085578. https://doi.org/10.1029/2019GL085578