the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Nov 2025
ALTICAP: A new global satellite altimetry product for coastal applications
Abstract. Observing sea level and its variations is of great importance for many scientific, societal and economic issues. This data paper presents a new coastal high resolution Sea Level Anomaly (SLA) product, ALTICAP (ALTimetry Innovative Coastal Approach Product), derived from along track satellite altimetry. To enable as many coastal applications as possible, collocated altimetric significant wave height and wind data are also provided, as well as quality flags and the geophysical corrections applied to the SLA. Covering all ocean regions between 0 and 500 km from land, and between 66° S and 66° N, this dataset contains five years (February 2016 to July 2021) of 20 Hz altimetry measurements from the Jason-3 mission. The altimetric standards and geophysical corrections used to compute the SLA have been selected following a round robin study based on 22 of the most recent algorithms available. The processing solution adopted was a compromise between the capability of each algorithm to provide the best sea level solution over the entire strip between 0 and 200 km from the coast and a guarantee of product continuity in the future.
The comparison of ALTICAP and tide gauge SLA time series shows the ability of the ALTICAP product to capture the coastal sea level variability, with average correlation and root mean square deviation values of 0.74 and 9 cm respectively. On global average, altimetry SLA time series remain 80 % complete up to 9 km from the coast after editing. ALTICAP is the first global high resolution altimetry sea level product optimized for coastal applications and ensuring quality continuity up to the open ocean. The complete protocol followed during the round robin study (Birol et al., 2023), as well as all the results (https://www.aviso.altimetry.fr/en/data/products/sea-surface-height-products/global/altimetry-innovative-coastal-approach-product-alticap.html) and the data (LEGOS et al., 2023; doi: 10.24400/527896/a01-2023.020) are freely available online.
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Status: open (until 21 Dec 2025)
- CC1: 'Comment on essd-2025-560', Marcello Passaro, 05 Nov 2025 reply
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RC1: 'Comment on essd-2025-560', Giuseppe M.R. Manzella, 16 Nov 2025
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Comments on
ALTICAP: A New Global Satellite Altimetry Product for Coastal Applications
by Cancet et al.
ESSD-2025-560
The article describes a product derived from altimetry data. Since I'm interested in the coastal zone, its processes, and their diversity from place to place, the title caught my attention. I found the article well-written, but, based on my needs for assessing coastal processes, it was very technical and less oceanographic: the data (and the products derived from it) are used to understand/evaluate the processes.
Based on this topic, I have only one general and one specific comment.
General Comment
The process adopted by the authors is highly professional and correct in all its components. As the authors wrote, it is important to have solutions that represent a compromise between the ability to obtain the best results while also ensuring continuity into the future. This is an interesting and ambitious goal, perhaps too optimistic since the product must be evaluated by intermediate/end users in their own applications.
ALTICAP's application area is the "coastal" zone between 0 and 200 km from the coast. An introduction that addressed not only the technical aspects but also the more oceanographic ones would have been useful. The "coastal zone" considered includes areas with different seabed types and dynamics: from the continental shelf of variable width (possibly less than 1 km) to the open sea with dominant geostrophic phenomena. Therefore, the very concept of "coastal application" should be better specified. ALTICAP's ability to capture sea level variability with a correlation and root-mean-square deviation of 0.74 and 9 cm is certainly better in some areas and less satisfactory in others.
Specific comment
The choice of tide gauges is based on criteria related to satellite operability and data quality requirements. It would have been interesting to discuss the results based on the different sea types to which ALTICAP is applied (enclosed, semi-enclosed, and open seas).
Conclusion
Overall, the article deserves publication, and the authors could add useful information for potential users by highlighting some use cases.
Citation: https://doi.org/10.5194/essd-2025-560-RC1 -
RC2: 'Comment on essd-2025-560', Anonymous Referee #2, 14 Dec 2025
reply
[General Comments]
The oceanographic community welcomes new datasets. Although the ALTICAP itself does not provide "new algorithms," this manuscript includes
- selection of the best combination of algorithms through the round-robin tests (Appendix A)
- importance of the validation flags (Section 3.3)
- comparisons with the X-TRACK dataset (Section 3.4).
I believe these findings are useful to the community, so this manuscript should be eventually accepted.
However, I could not determine whether similar validation flags were applied in the round-robin tests. If the validation filters were not applied to the tests, it should be examined whether they affect the test results, because Section 3.3 shows they significantly control the quality of the data. For example, the "edited ALES" could be better than the "edited Adaptive," although the "raw Adaptive" was superior to the "raw ALES." Meanwhile, if similar flags have already been applied to the tests, Section 3.3 should come earlier in the manuscript.
In Figure 5, the authors explain that the RMS difference of the edited ALTICAP is higher than the X-TRACK due to the 20-Hz sampling. If the authors wish to emphasize the better quality of the ALTICAP dataset, this speculation should be examined by comparing the 20-point averaged ALTICAP data along the track to the X-TRACK data. Otherwise, readers would think that the X-TRACK is better, except for some narrow channels.
[Minor Comment]
The quality of coastal retracking algorithms strongly depends on the complexity of the coastlines, so the authors should better explain how and why only 14 tide gauges were selected. Can the authors show some specific tide gauges with straight or complex coastlines?
Citation: https://doi.org/10.5194/essd-2025-560-RC2
Data sets
ALTimetry Innovative Coastal Approach Product (ALTICAP) LEGOS, CLS, and Noveltis https://doi.org/10.24400/527896/a01-2023.020
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The abstract and the text should be correct. It is a false statement that ALTICAP is the "first global high resolution altimetry sea level product optimized for coastal applications". The "first global high resolution altimetry sea level product optimized for coastal applications" is the ALES dataset available through OpenADB https://openadb.dgfi.tum.de/en/products/adaptive-leading-edge-subwaveform-retracker/ since several years. This product is available at 1-Hz for direct download and 20-Hz data on request, and it is available globally.