Development, Status and Future Perspectives of Croatian Gravimetric Reference System

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper under review presents a comprehensive review of the development, current status, and future modernization perspectives of the Croatian Gravimetric Reference System (HGRS03). The paper traces the evolution of international gravity reference systems, from the Vienna and Potsdam Gravity Systems to IGSN71, and places the Croatian national gravity infrastructure within this historical and international context. The establishment of the Zero-Order, First-Order, and Second-Order Gravimetric Networks of Croatia is described along with a discussion of the emerging International Gravity Reference System (IGRS) and the current limitations of Croatia's capacity to integrate into it. The paper is primarily a review and documentation work, which is a legitimate and necessary contribution for a national geodetic community and for the broader international audience, given the limited availability of English-language documentation of Croatian gravimetric infrastructure.

Nevertheless, in its current form, the paper has no scientific contribution as it just gives an outline of the current status of gravity networks in Croatia, without any qualitative or quantitative analysis of its current status. Moreover, the aspects on its exapnsion and/or modernization rely on a short citation of two related works, without an attempt by the authors to give their own contribution. The authors could add much more insight in the documention of the zero- first and second-order gravity network acquisition, as well as discuss leasons learned, while proposing the way forward for the future. But, the authors just append some general information, in something short of review study and/or project report, about the current status of the gravity networks in Croatia. The only prospect for the future discussed is the new international gravity referenece system and frame (which is ITGRS/ITGRF and not IGRS/IGRF). Even in that respect, there is no effort to convey some aspects of how this new system can be integrated in the Croation gravity network, how it will affect (based on the new conventions and notions introduced in the ITGRF) the current gravimetric infrastructure, etc.. Nothing qualitative or quantative is given for the case of Croatia, both for the existing and forthcoming gravity networks.

Below, I have some more detailed comments on the paper that form the basis for the outright rejection of the paper, at least in its current form. Morover, the comments below can constitue the basis for the authors to provide a heavily revised vervsion, which will constitute a scientific publication that can convey usefull insights to the geodetic community. A positive aspect of the paper is that it is clearly written, reasonably well structured, and the information compiled will be of use to both national practitioners and to colleagues working on regional gravity field modeling in Central and Southeastern Europe, and especially the Balkan countries. The list below provides some more detailed comments and concerns that must be addressed.

1. The paper is classified by the journal as a Review article, yet the scope of the review is almost entirely limited to Croatian national literature and a relatively narrow set of international references. A review article in this context is expected to provide a critical synthesis of the state of the art at the international level and how that conveys to the Croatian gravity network. If the intended contribution is primarily documentation of Croatian national infrastructure, the paper should be reclassified as a Technical Note accordingly, and the scope of the abstract and introduction should be adjusted. This is something that should be pointed out, as its current framing creates a mismatch between the stated paper type and the actual content delivered.

2. The historical sections on the Vienna Gravity System, the Potsdam Gravity System, and IGSN71 (Sections 2.1–2.3) are largely a retelling of material found in Torge (1989) [Ref. 4] and Morelli et al. (1972) [Ref. 12]. While this context is useful, the treatment adds little that cannot be found in standard textbooks and is obsolete, and least in the extent that is included in the paper, to the theme of the work. These sections should be condensed significantly (by at least half) and the space redirected toward a more substantive technical discussion of the Croatian system itself, its adjustment methodology, accuracy characterization, and connection to neighboring networks.

3. Section 2.4 on the IGRS/IGRF is welcome and topical, but it reads more as a paraphrase of Wziontek et al. (2021) [Ref. 42] than as a critical assessment of what the transition from IGSN71 to IGRS means specifically for Croatia and the broader Balkan/Adriatic region. The authors should discuss the implications more concretely: what accuracy level is required for IGRS participation, what is the current accuracy of the Croatian zero-order stations, and what is the magnitude of the gap? This is not done anywhere in the paper, and without it the discussion remains at a level of generality that reduces its scientific value.

4. In Section 3.2 (Zero-Order Gravity Network), the absolute gravity measurements from the UNIGRACE project (1996–2000) are described, but no gravity values, uncertainty estimates, or standard deviations are reported for the six AGT points. A table listing the station coordinates, observed gravity values, and their formal uncertainties, as published in Čolić et al. [Ref. 6] or Markovinović (2009) [Ref. 38], is essential and should be included. The absence of any numerical content in the description of the primary datum of Croatia is a significant omission for a paper claiming to document the national gravimetric reference system.

5. In Section 3.3 (First-Order Gravity Network), the paper mentions that the network was adjusted in phases using different models, and that a unified adjustment was performed for the first time by Repanić (2017) [Ref. 57]. However, no results of this adjustment are reported, no standard deviations, no gravity values, no comparison with the earlier phase-by-phase solutions. The authors fail to comment on whether the unified adjustment introduced statistically significant changes relative to the individual phase adjustments. A table with at least summary statistics of the adjustment (e.g., residuals, standard deviations of unit weight, range of formal errors) is strongly required.

6. Similarly, in Section 3.4 (Second-Order Gravity Network), the paper notes that the gravity values for the 194 second-order points were never officially adopted by CSGA, and that the adjustment referenced in Repanić (2017) was stated to be "ongoing." This is an important governance and scientific gap that the paper identifies but does not analyze. The authors should discuss the practical implications of this situation for users of Croatian gravity data as for instance, for geoid modeling, height system maintenance and modernization, or GNSS height transformation. It would add to the paper if the authors provided some estimate of the impact on downstream applications. This is especially evident as the paper is a review one hence it could provide some useful information on the imapct that this non-maintained geodetic infrastrucure can have.

7. The Discussion and Conclusions section (Section 4) is the weakest part of the paper. It is brief and largely descriptive, repeating facts already stated in the body of the paper rather than synthesizing them into new insights. Specifically:

• The claim that Croatia maintains "satisfactory point density" is accepted uncritically. The recommended densities cited [Ref. 65] are taken from a Slovenian source, without discussion of whether these recommendations are appropriate for Croatian topography and geodetic applications, particularly in the Adriatic coastal and island regions where the network geometry is irregular.
• The discussion of the urgent need to re-observe the zero-order network would be strengthened considerably by quantitative estimates: how large might the temporal gravity change at the AGT sites be over 30 years, given the known geodynamic setting (Dinaric Alps, Adriatic microplate, seismically active region)? References to relevant geodynamic or absolute gravity monitoring studies in the region would be appropriate here.

8. The paper makes no reference to the connection between the Croatian gravimetric reference system and the Croatian height system. Given that HGRS03 underpins Croatian geoid modeling and therefore the transformation between GNSS ellipsoidal heights and the national normal-orthometric height system (HVRS71), this connection is fundamental and its omission is a serious gap in a paper claiming to document the national gravimetric infrastructure. The authors should at least briefly address the role of HGRS03 in Croatian geoid modeling efforts and the national height datum.

9. To the reviewer's opinion, the paper fails to cite the foundational reference for the IGSN71 network at the level of detail warranted for a review article. The original report by Morelli et al. (1972) [Ref. 12] is cited but not discussed in any critical depth. Furthermore, the important early work on IGSN71 accuracy assessment by Mäkinen and Ihde is absent. The following reference should be added and discussed:

Mäkinen J and Ihde J (2009) The Permanent Tide in Height Systems. In: Sideris M (ed.) Observing our Changing Earth, International Association of Geodesy Symposia Vol. 133, Springer, pp. 81–87.

10. In Section 2.3, the description of the IGSN71 network states that its standard errors are "generally better than ±0.1 mGal." This statement is not supported by any discussion of the heterogeneity of this accuracy across the network as the accuracy of IGSN71 is known to be spatially variable, particularly in regions with sparse absolute gravity coverage such as the Balkans and Eastern Europe. This nuance is important for a proper assessment of HGRS03's accuracy limits.

11. In Section 3.2, the paper refers to the proposal to expand the Zero-Order Network with five new points (Čakovec, Gospić, Rovinj, Split, Zabok), but notes that the expansion was never implemented. The authors should state clearly whether these proposals are still scientifically valid or whether the proposed modernization framework (IGRS integration, use of modern absolute gravimeters) supersedes them. The current text leaves this unresolved. What do the authors propose in this context?

12. Figure 1 is credited to Morelli (1999) [Ref. 8] and Torge (1989) [Ref. 4]. This is a reproduced figure from published sources and its use may raise copyright concerns. The authors should either redraw this figure as an original illustration or obtain explicit permission and add the appropriate acknowledgment. For a journal that operates under MDPI open-access terms, reuse of figures from non-open-access sources requires particular attention.

13. The Figures overall are informative, but Figure 3 (Zero-Order Network) and Figure 4 (First-Order Network) lack coordinate grid labels and scalebars. The AGT point labels in Figure 3 are partially obscured by the topography layer. These figures should be improved to publication quality. Also, Figures 3, 4 and 5 could be combined somehow.

14. The abstract states that "perspectives for future modernization ... are briefly addressed." The word "briefly" accurately reflects the content, but in a review paper this brevity is a weakness rather than a virtue. The abstract should be revised to more accurately reflect the paper's contribution and not understate its own findings.

 

With the above in mind, I would recommend the outright rejection of the paper in its current form. With some substantial effort by the authors, keeping in mind the comments above, the paper can be rewritten and resubmitted for review. 

Author Response

Dear Reviewer,

Thank you for taking the time and reviewing our article.

Please find attached the document with our responses to your comments.

With kind regards,

Authors

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This paper entitled “Development, Status and Future Perspectives of Croatian Gravimetric Reference System” presents a comprehensive review of the development, current status, and future perspectives of the Croatian Gravimetric Reference System (HGRS03) within the broader context of international gravity reference systems. The paper is well written and provides a valuable compilation of historical and technical information regarding national gravimetric infrastructure. However, the paper lacks some key information, e.g., how accuracy of the current geoid, the relevant methodology to build geoid, missing consideration of temporal gravity variations, and limited integration with modern gravity data sources from satellite and so on.  Revisions are required before the paper can be considered for publication.

Author Response

Dear Reviewer,

Thank you for taking your time and reviewing our article.

Please find attached our responses to your comments.

With kind regards,

Authors

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This article is a review. This article should have provided a more comprehensive analysis of the research. It would also have been worth providing recommendations for future research. Could you revise the Data Availability Statement section?

The manuscript provides a useful historical and technical overview of the development of the Croatian gravimetric reference system and its relationship to international gravity reference systems. The topic is relevant to modern geodesy because national gravity infrastructures remain essential for geoid determination, height systems, and integration into emerging international gravity reference frameworks.
The main research question addressed by the paper is how the Croatian gravimetric reference system developed historically, The manuscript clearly identifies this objective in the introduction and revisits it in the discussion and conclusions.
In particular, the discussion connecting Croatian developments with neighboring countries and international gravimetric modernization efforts is valuable.  However, the manuscript is primarily descriptive and historical. Its originality would be strengthened if the authors included a more analytical assessment of the current performance and limitations of HGRS03, rather than mostly summarizing historical developments.
The synthesis of Croatian gravimetric development with broader international gravimetric history is particularly useful for readers unfamiliar with the regional context.
Regarding methodology, several improvements should be considered:
• Since the paper is presented as a review article, the methodology for literature selection and evaluation should be explained more explicitly.
• The authors should provide clearer criteria for evaluating the adequacy and modernization needs of the Croatian gravimetric network.  
• The discussion of future integration into the IGRS would benefit from more technical detail.
• The paper would also benefit from a concise summary table presenting the characteristics of each Croatian gravimetric network (number of stations, epochs, instrumentation, accuracy levels, and current status). This would improve readability and allow easier comparison between network orders.
• Some sections rely heavily on historical narrative. The manuscript could be improved by adding more critical analysis of the current operational challenges, including maintenance, accessibility, temporal stability, and data availability.
The conclusions are generally consistent with the evidence presented in the manuscript. The argument that Croatia urgently needs re-observation of the Zero-Order Network is well supported by the discussion regarding the age of the measurements and regulatory requirements. Similarly, the conclusion that the lack of a national absolute gravimeter limits Croatia’s direct participation in IGRS realization is logically connected to the preceding discussion. However, some conclusions could be strengthened with more quantitative evidence. For example, it would be useful to provide estimates of the expected degradation or uncertainty growth associated with not remeasuring the Zero-Order stations for nearly thirty years.
The references appear generally appropriate and relevant for the subject area.
The tables and figures are generally helpful and relevant. The maps showing the evolution of Croatian gravimetric networks are particularly useful for understanding the spatial distribution and historical development of the networks. However, figure quality and consistency could be improved. Some figures appear visually dense, and labels are difficult to read at normal scale. It would also be beneficial to standardize figure styles, symbols, and color schemes throughout the manuscript. Additionally, the manuscript currently lacks summary tables, which would significantly improve clarity and accessibility for readers.
Overall, the manuscript addresses an important topic and provides a valuable overview of the Croatian gravimetric reference system. With stronger methodological framing, additional quantitative analysis, and improved synthesis of the modernization challenges, the paper could make a stronger contribution to the field.

Author Response

Dear Reviewer,

Thank you for taking your time and reviewing our article.

Please find attached our responses to your comments.

With kind regards,

Authors

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised version of the paper addressed all points indicated during the first review cycle. Hence, I reccommend the paper for publication in its present form.

Reviewer 3 Report

Comments and Suggestions for Authors

I thank the authors for addressing the comments and for their work on improving the manuscript.