CO Editor

Suggestions for revision or reasons for rejection

This paper reviews the progress made by the authors in developing and applying rigorous photogrammetric approaches to understanding glaciological problems. The references demonstrate that the authors have an excellent track record in this area, in terms of applying rigorous approaches in demanding conditions, and in developing novel algorithms capable of coping with real-world issues such as shadowing. However, their work is only relatively recently being picked up in the Earth science literature, with their earlier contributions being in more technical-focussed outlets such as ISPRS. Thus, I preface my review by indicating an underlying strong support for a review-style paper within ESurf (particularly for this special issue) to highlight the authors’ work to a highly relevant end-user community. I use the term ‘review-style’, because the paper focusses on elaborating the methods which have been published previously, albeit in a much more abridged fashion. The goal of the paper is not to deliver new glaciological understanding, but to provide an overview of the methods and their applications and could be of value to a wide range of ESurf readers.

Nevertheless, I do think that the paper currently undersells itself by not really presenting any detailed comparisons with similar work, and thus not highlighting the advantages/advances of the authors’ specific approach. Without developing a critical analysis, in some areas the paper becomes more of a general description of time-lapse imaging methods, some of which can be read in glaciological papers going back to the ‘70s. Other aspects, such as the useful classification of measurement setups (p7), are valuable and accessible summaries of information from less accessible domains for the broader readership, such as surveying/photogrammetry. Whilst fully appreciating that some description of the basic aspects of time-lapse methods will be essential prior to entering into detail, I think that increased conciseness in places and providing insightful, quantitative comparisons with other published work would substantially strengthen this paper and increase its impact.

As an example, the introduction provides a number of other scenarios where time-lapse photography has been used, but this is done in a somewhat descriptive list-like manner (para 3, P2). A more critical analysis of these preceding works (and this maybe only really needs to be of the glaciological ones) would help put the authors’ contributions in better context. For example, the basic details of the papers could be collated in a table – e.g. to list process being monitored, stereo/mono, image interval/duration, feature or area matching and algorithm, approximate accuracies achieved etc., – then the key advantages/limitations of these works discussed in a way that highlights how the authors’ advances addresses particular limitations. Image registration/camera stability is an obvious area in which previous authors have done work; a clear recognition of this would enable the quality/flexibility of the approaches covered in the review to be discussed/compared rather than just described. How much better are the authors’ camera registrations than those achieved by others? What do we need to do to improve things?

The Discussion section reads more like an ‘applications’ or ‘case studies’ section than a discussion, and mostly presents the results of specific papers. I would recommend renaming the section to be more appropriate, e.g. ‘Case studies’, and to revise to try and highlight quantitatively how the quality of the work enabled process understanding – i.e. vertical resolutions /precisions of XX m at YY km distance allowed measurements of ZZ m of motion due to ….’.

A revised discussion section (or discussion & conclusions?) could pull together different aspects from throughout the paper, and discuss them in context with prior work. Some insight from the authors in terms of future direction may also make an interesting addition.

Aspects of the Conclusions section read as a general summary of time-lapse imaging, some of which isn’t very exciting or new. Can the authors pick out the key contributions from their work (e.g. dealing with shadows) and focus on these to give a more concise and inspiring summary of their advances?

Finally, I am very happy to see that the software will soon be made available (p28 L8). However, it would really strengthen the paper if the software was provided already, and could be linked directly. This would be an excellent way of providing something substantially new within the work; I encourage the authors to try and do this.

Minor queries/suggestions:

P2 para 3 – volcanological applications could also be relevant, e.g. .papers by Walter’s group on domes: 10.1111/j.1365-246X.2011.05051.x; 10.1002/2016JB013045; 10.1002/jgrb.50066; or USGS: doi.org/10.1016/j.epsl.2009.06.034; or ours on lavas: 10.1007/s00445-011-0513-9; 10.1016/j.isprsjprs.2014.08.011

P2 L32 Should this be James et al (2016)? This work dealt with very difficult imagery, for which fully automated tracking was not possible – an interesting comparison for discussion elsewhere in the paper?

P5 L5 Ensure that photogrammetry terminology is either avoided or explained carefully at first use. Here, maybe rephrase/explain ‘inner accuracy’?

P6 L11 Replace ‘avoid’ with ‘reduce’.

P8 L9 The phrasing could indicate that the algorithms cited are specifically for glacier point tracking, whereas the references are for general image registration. Rephrasing would clarify, e.g. ‘A wide range of algorithms are available for point tracking in image sequences (e.g….’

P10 L 5/19 ‘imported’ → ‘important’

P18 L14 So which variants from P 7 can be employed with PhotoScan?

P18 L19 ‘to warrant up-to-dateness’ → ‘To ensure continued validity’?

Interactive discussion

Peer-review completion