A new digital elevation model of Antarctica derived from  CryoSat-2 altimetry

Slater, Thomas; Shepherd, Andrew; McMillan, Malcolm; Muir, Alan; Gilbert, Lin; Hogg, Anna E.; Konrad, Hannes; Parrinello, Tommaso

doi:https://doi.org/10.5194/tc-12-1551-2018

Articles | Volume 12, issue 4

https://doi.org/10.5194/tc-12-1551-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-12-1551-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 12, issue 4

Research article

|

02 May 2018

Research article |

| 02 May 2018

A new digital elevation model of Antarctica derived from CryoSat-2 altimetry

Thomas Slater, Andrew Shepherd, Malcolm McMillan, Alan Muir, Lin Gilbert, Anna E. Hogg, Hannes Konrad, and Tommaso Parrinello

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Final revised paper (published on 02 May 2018)
Supplement to the final revised paper
Preprint (discussion started on 25 Oct 2017)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Slater et al 2017 - A new DEM of Antarctica using Cryosat_2 altimetry', Anonymous Referee #1, 08 Nov 2017
- AC2: 'Response to Anonymous Referee #1', Thomas Slater, 15 Jan 2018
RC2: 'Review of: A new Digital Elevation Model of Antarctica derived from CryoSat-2 altimetry', Anonymous Referee #2, 25 Nov 2017
- AC1: 'Preliminary response to Anonymous Referee #2', Thomas Slater, 30 Nov 2017
- AC3: 'Response to Anonymous Referee #2', Thomas Slater, 15 Jan 2018
RC3: 'Review of ”A new Digital Elevation Model of Antarctica derived from CryoSat-2 altimetry” by Thomas Slater et al., submitted to The Cryosphere Discussions.', Anonymous Referee #3, 12 Dec 2017
- AC4: 'Response to Anonymous Referee #3', Thomas Slater, 15 Jan 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Thomas Slater on behalf of the Authors (15 Jan 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (26 Jan 2018) by Bert Wouters

RR by Anonymous Referee #2 (13 Feb 2018)

Suggestions for revision or reasons for rejection

I would like to thank the authors for there comprehensive response to all reviewers and the improvement of the paper.

Most of your arguments are reasonable and I can understand your points.
However, I don't see the point why you are not following reviewers 2 suggestion:

"Furthermore, I would suggest to present two figures where the mean difference and respectively the Stddev is plotted against slope (e.g. binned to 0.05) for the whole DEM, observed and interpolated pixel, respectively. In addition, this Figure should include the same analysis for the other 3 DEMs. Such kind of figure would clearly show the difference between observed and interpolated pixels as well as the stated improvement of the new DEM against existing DEMs."

You argue that: "you don't feel that this way will provide a robust indication of the accuracy of a DEM, as the slope distribution of the evaluation dataset is not representative of the slope distribution of the Antarctic ice sheet"
But on the other hand some lines above in your response you mention:
"The analysis with respect to slope presented in Table 3 accounts for 95% of the total ice sheet area, and gives a good depiction of the accuracy of the overall product"

????

I still think that providing such a figure will give useful information, especially in contrast to other DEMs. I think it is important to show where the DEM is better than the others and this seems to be related to the complexity of the topographie and hence slope - and this is worth to show.
Table 3 is more a comparison of the different modes. I agree that a split up in observed to unobserved points is not necessary, due to the high coverage of observed pixels. All your arguments (e.g. we stay with the data resolution and don't introduce interpolation errors) are in your reviewer response - why not include this in form of an additional figure and some text in the manuscript?
If you think to have too many figures than skip Figure 3 and include the subfigures and labelling in Fig2.

At P6 L26 you mention the correction of temporal fluctuations in backscattered power. You don't explain how exactly you apply this correction. It is also not included in your equation. Please add more details in the supplements.

To answer your questions to reviewer2:
For the comparison I used the
preliminary version of the CryoSat-2 DEM available for download to the community and the same references as given in your preliminary response (DiMarzio et al., 2009).
As correctly stated the ICESat DEM has a posting of 500m. For providing the figure I used simply QGIS with its implemented raster calculator.
Doing so, the difference DEM was not resampled.
However, to double check I run the GMT grdsample tool and subtracted both DEMs. Additional I run IDL bilinear interpolation as well to generate the difference map. In all three cases the pixel shift is visible as shown in my review.

It's nice to see that the new version seems to be of better quality and without the pixel shift.

Lot's of people are using this preliminary version already, since this version of your DEM was highly advertised in the media, at conferences and widely distributed to the community via ESA webpages as well as at conferences.

To my opinion this bears some problems, and needs to be clearly addressed in the paper.

Therefore, I would like to see two things in a published paper.

1. Please make very clear in the main text that a preliminary version, which was widely distributed, should not be used anymore and explain the improvements.
2. Please show the difference between preliminary and updated version in the supplements (Difference MAP to point out the pixel shift)
I hope that you can reproduce a similar pattern as I observed. I think this is important for the community.

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ED: Publish subject to minor revisions (review by editor) (27 Feb 2018) by Bert Wouters

Dear Thomas,

Many thanks for submitting your revised manuscript, which addresses most of the comments provided by the referees. As you will see in the report of Referee #2, some concerns remain. I think the manuscript may be suitable for publication after making the following revisions:

1. Mention in the introduction that this version of the DEM supercedes the previously distributed preliminary version
2. Uncertainties are an essential component of any data set, as important as the reported data itself. The uncertainty map in the supplement should be moved to the main manuscript, either as a separate figure, or included in fig. 4. Uncertainties should be provided for the slope map as well. In order to reduce the number of figures, fig. 2 and/or 3 can be moved to the supplement.
3. clearly explain how the backscatter correction was applied.
4. explicitly mention that in the LRM mode, an external DEM was used for the slope correction, and which one. The reader should not have to dig into the referenced document to find this out.
5. In the caption of the uncertainty map, mention that uncertainties due to (remaining) firn penetration of the radar signal are not included.
6. in the supplement, include the difference map between your DEM and the other three DEMs mentioned in the manuscript, as suggested with R2. I agree with you that this is not necessarily an indicator of the quality of the new DEM, but it will give potential users of your DEM an idea of where to expect differences.
7. include a figure showing mean difference/standard deviation against slope, as suggested by R2. I agree that the IceBridge data may not be representative for the entire AIS slope distribution, and it might be good to add a caveat about this in the discussion of the figure.

R2 would like to look further into the pixel shift observed in the preliminary DEM and like to have access to the current DEM. Given that your manuscript is mainly a discussion of a new data set, I think it is a reasonable request for the reviewers to have access to the data. Of course, it will be made clear that the data can only be used for reviewing purposes. Please send me a link to a download location for the new DEM, which I will forward to R2.

Many thanks,
Bert Wouters - TC/TCD Editor

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AR by Thomas Slater on behalf of the Authors (06 Mar 2018) Author's response Manuscript

ED: Publish subject to technical corrections (22 Mar 2018) by Bert Wouters

AR by Thomas Slater on behalf of the Authors (23 Mar 2018) Manuscript

Short summary

We present a new digital elevation model of Antarctica derived from 6 years of elevation measurements acquired by ESA's CryoSat-2 satellite radar altimeter. We compare our elevation model to an independent set of NASA IceBridge airborne laser altimeter measurements and find the overall accuracy to be 9.5 m – a value comparable to or better than that of other models derived from satellite altimetry. The new CryoSat-2 digital elevation model of Antarctica will be made freely available.