Augmented Gravity Field Modelling by Combining EIGEN_6C4 and Topographic Potential Models
Round 1
Reviewer 1 Report
Dear Authors,
I added in the pdf document some comments that I could think of, which may be some starting points to improve the paper. I am also proving an overview here:
I think “The determination” in the title is misleading, as the paper does not actually determine a model, but expands a high-resolution global gravity field model with two different topographic gravity field models using coefficients’ weight information for a transition zone. Part of the analyses presented in the paper are already presented in the literature. I think the paper can still contribute to the literature, but the authors will need to provide more in the content including going through the most recent references as well as provide possibly some additional analysis in different regions or subregions of China or result retrieved from combination of different models (maybe also satellite only or XGM2019).
I find the description of datasets very abstract. More details on the datasets and sources need to be provided in the paper including the preprocessing if included.
The promised future method, combination in the level of normal equation might have been interesting. The method applied in the combination and analyses (validations) presented in the study are partly covered in the literature already and it is not clear to me what is new in the paper. The message given about the enhancement of the models and their contribution w.r.t. the terrestrial records is also already presented by others and they are well known facts. It would be nice to see new analyses that are unique which would make the paper original.
I noticed some recent literature is missing such as Huang et al 2022, Ince et al. 2022. Please refer to previous studies done on the combination of GGMs with TGM and their use in different application areas. For instance Ince et al. 2022 also combined GGMs with TGM and evaluated their models based on different terrestrial datasets, including the DoVs from Colorado. Huang et al. used different set of combined models for the purpose of their use in IHRF which includes SatGRAV.
https://link.springer.com/chapter/10.1007/1345_2022_154
https://link.springer.com/chapter/10.1007/1345_2022_162
In the attached document I have detailed feedback provided.
I hope you will find the comments useful.
Best regards,
Comments for author File: 
Comments.pdf
There are many spelling mistakes which can easily be fixed. In indicated some in the pdf.
I have also noticed some repeating sentences in the text which need to be summarised or revised to improve the flow of the paper.
Author Response
In the attached document, we have provided a point-by-point response.
Author Response File: Author Response.pdf
Reviewer 2 Report
The paper titled "The Determination of Ultra-High Degree Gravity Field Models by Combining EIGEN_6C4 and Topographic Potential Models" presents a methodology for improving the accuracy of gravity field models (GFMs) by combining EIGEN_6C4 with topographic potential models (TPMs). The study utilizes three GFMs, namely EIGEN_6C4, dV_ELL_Earth2014_5480, and ROLI_EllApprox_SphN_3660, and evaluates their accuracy using various datasets.
The authors analyze the spectral characteristics of the GFMs and TPMs, finding that the TPMs can effectively represent high-frequency gravity field signals, which are lacking in the EIGEN_6C4 model. The results show that the signal degree amplitudes of the dV_ELL_Earth2014_5480 and ROLI_EllApprox_SphN_3660 models differ from the EIGEN_6C4 model before degree 900 due to the focus of the topographic potential models on near-surface gravity variations. However, the higher-degree portions of the TPMs exhibit high-frequency signals and show consistency with the EIGEN_6C4 model before degree 2000.
Furthermore, the accuracy of the refined gravity field models is evaluated using GNSS/levelling data, deflection of the vertical (DOV) data, and gravity data from Australia. The GNSS/levelling observations from China and DOV data from both mainland China and Colorado are used to verify the accuracy of the models. The results indicate that the combination of the TPMs and high-degree GFMs improves the accuracy of gravity anomaly determination compared to the EIGEN_6C4 model. The EIGEN_3660 model demonstrates an accuracy improvement of approximately 10.05%, while the EIGEN_5480 model shows an improvement of around 16.5%. This analysis validates the reliability of the determined models in accurately representing the disturbing gravity field.
Overall, the paper successfully presents a methodology for combining GFMs and TPMs to enhance the accuracy of gravity field models. The spectral analysis provides insights into the signal characteristics of the different models, highlighting the complementary nature of the TPMs and high-degree GFMs. The accuracy verification using various datasets strengthens the findings and demonstrates the improved performance of the refined gravity field models.
Questions:
- How do the topographic potential models (TPMs) differ from high-degree gravity field models (GFMs) such as EIGEN_6C4 in terms of spectral characteristics?
- What is the significance of the transition zone and the Hanning function in combining the TPMs with the EIGEN_6C4 model to obtain refined GFMs?
- How does the accuracy of gravity anomaly determination improve when using the combined models compared to the EIGEN_6C4 model, as demonstrated by the GNSS/levelling data, DOV data, and gravity data from Australia?
- Names such ROLI_EllApprox_SphN_3660 are not easy to read. Could you change them to be more elegant?
Author Response
In the attached document, we have provided a point-by-point response.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Dear authors,
Thank you for revising the paper.
I have noticed some of my remarks have been covered, but some have not been addressed, also my recommendation on changing the title.
There are still many typos. Entire manuscript should be checked for any typos left.
I would have put discussions and conclusions together. At the moment, the text is partly repetitive.
In my opinion, the manuscript does not provide any novel contribution to the existing literature. Therefore, I strongly recommend to include additional content such as additional validation results.
Line 56 earth GFMs - Earth GFM
These days many models are assigned DOI #. These can be different than original papers published and need to be cited when the models are used. I could retrieve two of them. You can find how to cite on the related pages. The authors are encouraged to check others via the ICGEM service.
EIGEN-6C4
https://dataservices.gfz-potsdam.de/icgem/showshort.php?id=escidoc:1119897
ROLI_EllApprox_SphN_3660
https://dataservices.gfz-potsdam.de/icgem/showshort.php?id=escidoc:4801903
Line 119 – Germany -> German
Line 120 - Geoscience -> Geosciences
Line 125 - In -> in
Line 137 - verified -> verify ?
Line 151 - "an accuracy" repeating
Line 162 – based on
Line 173 – an -> a
Line 253- find -> conclude
Line 308 and for the rest of the section-> Validation is used commonly and it could be a better option instead of accuracy verification.
Line 353 – can seen -> can be seen
Line 363 – higner -> higher
Line 388 – obaerved -> observed
Line 395 – alos -> also
Regarding the authors’s remarks
Point 1: Still many grammatical issues and typos exist
Point 4: The reviewer’s concerns are only partially addressed
Point 5: I am not sure what the authors’ response refer to here. The figures that are repeating the literature as well as the comparisons in Colorado are still in the revised version with no additional validations results presented
Point 6: The page and line information do not correspond to reviewer’s remarks.
Point 12: The page and line information do not correspond to reviewer’s remarks.
Point 14: The reviewer's remark was not addressed. The question is how and why N2 is 2100. If it is because the literature does so, this should be addressed. The page and line information do not correspond to reviewer’s remarks.
Point 21: Not addressed.
Point 22: Not addressed. The page and line information do not correspond to reviewer’s remarks.
There are many typos, I referred to some of them above.
Author Response
We thank you for your constructive and detailed comments. It has improved the readability, clarity, and quality of our manuscript. The point-by-point response can be found from attachment.
Author Response File: Author Response.pdf
