Analysis of a Relative Offset between the North American and the Global Vertical Datum in Gravity Potential Space

Round 1

Reviewer 1 Report

 It is a very interesting and actual work.

However, some deficiencies were observed which are listed chronologically and should be corrected:

 

1)    It is necessary to check typos because they have been noticed (e.g., "the the" on page 5, chapter 3.2

 

2)    Figure 4 on page 9 is not referenced in the text - fix it!

 

3)    Explain why exactly Shepard's surface fitting method was used for gridding of gravity data (page 9)?

 

4)    Explain how the even distribution of benchmarks (page 9) was obtained, that is, 23961 out of 822301 points were chosen?

 

5)    For the average reader, it remains a bit unclear how Figure 8 was obtained based on Figure 9 - clarify!

 

6)    It is necessary to check the citation of each cited literature, e.g., literature 37 – DTM2006.0.

 

7)    Although there is a Data Availability Statement on page 14, it would be nice to add some form of thanks to the mentioned institutions for the freely available data used in the work.

Although I am not a proofreader, I believe the English used is very good. A little check is not out of the question.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

[1]      English writing should be further polished.

[2]      Lines 21-22: Here GRS80 and WGS84 are only two ellipsoids, which should be fixed with the real earth by ellipsoid positioning. The reference ellipsoid is different from the ellipsoid. The geoid is fixed with the real earth.

[3]      Line 43: What is geodetic levelling?

[4]      Line 46: What is oceanic levelling?

[5]      Eq. (1): Here should be dot product.

[6]      Eq. (6): In general, h=H+N with no considering effect of deflection of the vertical.

[7]      Eq. (12): In general, h=H*+ζ without considering the effect of deflection of the vertical.

[8]      Table 1: The table cannot consider the effect of ellipsoid positioning.

[9]      Line 185: The coastal data should be removed while DTU MSS model is used because of low precision for coastal MSS data.

[10]   Table 2: What are tide free, zero tide and mean-tide system? Here is the tide include solid earth tide, ocean tide and polar tide?

[11]   Line 238: What earth model is referenced for the Love number?

[12]   Figure 4: How about the effect of uneven distribution of GNSS sites on the final results?

[13]   Figure 6: The resolution of DTM2006 is obviously greater than that of SRTM in the figure.

[14]   Section 4: How to assess the relative offset? How about the precision of the relative offset? How to define uniquely the global height datum?

[15]   References: The literatures should be written in the normal format.

The English writing should be futher polished.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript entitled “Analysis of a Relative Offset between the North American and the Global Vertical Datum in gravity potential space” is interesting. This manuscript focuses on determining the vertical offset between the local and global vertical datums. It discusses the issues affecting vertical offset accuracy, including geodetic reference ellipsoid, zero-order term contribution, tide system, tilt error and omission error on the determination of the vertical datum offset. The authors' approach is commendable, and the manuscript is suitable for publication in Remote Sensing.

The major and minor issues are as follows.

MAIN CONCERNS

(1) Page 8, line 287: The author mentions the "remove-compute-restore (RCR) approach" but does not provide a clear explanation or description of this method. Further elaboration is required regarding the application of the RCR approach and its specific implementation details.

(2) Page 9: The color of the dots in Figure 4 is not specified, leaving the representation unclear.

(3) Page 10, line 326-334: The author employed the EGM2008, EIGEN-6C4, and XGM2019e_2159, and presented details regarding the spheroidal harmonic degree of these models. It is necessary to assess the spectral accuracy of these GGMs, including evaluating their degree errors.

MINOR COMMENTS

(1) Page 1, line 36: 'global vertical datum (GVD)' should be used instead of 'global vertical datum,' and the abbreviation 'GVD' should be employed in the subsequent text.

(2) Page2, line77: ‘the offset between the North American vertical datum (NAVD)’ should be used instead of ‘the offset between the NAVD’.

(3) Page 9, line 310: "-23.4.24 mGal" seems to contain a typographical error with an extra period. It should be corrected to provide a valid value.

The manuscript is well written. 

Author Response

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Author Response File: Author Response.docx