Digital Twin Smart City: Integrating IFC and CityGML with Semantic Graph for Advanced 3D City Model Visualization
Round 1
Reviewer 1 Report
Comments and Suggestions for Authors
This paper provides an interesting perspective on the effective use of Digital Twins in Smart Cities through the integration of IFC-CityGML-Semantic Graph 3D City Model Visualization.
The paper is well-written, particularly in its clear presentation of the research methodology and its application.
The conclusion could be improved by replacing the remainder of the paper’s objectives with the paper’s main results and feedback from the case study.
Author Response
We have revised our manuscript according to the proposed reporting format. The reviewer's responses are included in the attached Word file. Thank you for the detailed review.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for Authors
For the overall improvement of your paper may I share the following suggestions with you and request for your revision accordingly to review again as part of a major revision. All reviewing comments are provided in the pdf comment fields.
Comments for author File: Comments.pdf
Comments on the Quality of English Language
Author Response
We have revised our manuscript according to the proposed reporting format. The reviewer's responses are included in the attached Word file. Thank you for the detailed review.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors
In this paper, authors present a semantic mapping framework between IFC and CityGML domain. The modelling and case study visualization are expressed in detail. The main problem for the present form lies in the scientific value and presentation. Some suggestions for further improvement are as follows.
1. Abstract is a bit general, with more process-concentrated info. Specific findings with quantitative analysis are of high significance.
2. Re-consider the paper submission type. The present form is more like a case study report, rather than an academic article. To my personal mind, it seems to fall into case report, instead of research article.
3. What is the main gap between the present platform and proposed visualization model. Benchmark references or comparison with similar available model might be helpful for clarifying the progress.
4. Clarify the key scientific value. The present content paid most attention to depict the staged processes for 3D graph visualization.
5. What are the key new findings? Off-shelf handy models or available softwires integration?
6. Compare the visualization results or 3D graphs with other existing similar studies in urban planning or city related design regards. How about the key advance or progress over available research.
7. Extend the results and discussion substantially, with more emphasis on the application potentials of the proposed integrated visualization framework. Show the design or planning illustrative examples of the model, with comparison with basic BIM or OWL approaches. What are the main advantages for usage after combination or integration.
8. Re-structure the conclusion part. It is highly suggested to list main findings into several short bullets, and to avoid repeating detailed modeling processes.
Comments on the Quality of English Language
Moderate editing of English language required.
Author Response
We have finished editing the manuscript based on your comments. Our responses to your comments are included in the attached Word file. Thank you for your detailed review.
Author Response File: Author Response.pdf
Reviewer 4 Report
Comments and Suggestions for Authors
I would like to thank for the opportunity to review such an interesting manuscript. As the authors state in the abstract, the manuscript mostly deals with a data transformation between the Industry Foundation Classes (IFC) and City Geometry Markup Language (CityGML).
I would recommend clarifying the manuscript in some parts and considering some minor modifications – mostly regarding the terms from the civil engineering branch. The comments for these purposes are the following:
1. The authors state (l. 21 – 25): "The emergence of the "Digital Twin" within the Fourth Industrial Revolution revolutionized urban planning by simulating and analyzing real-world behaviors. This technology, operating at various levels of detail, enables the prediction of performance, process optimization, and cost reduction across sectors such as manufacturing, transportation, and energy." The "Digital Twin" can’t be considered as a one particular technology. Moreover, the consensus on definition of this term is still not clear – see [1]. I would recommend reformulating the second mentioned sentence and instead of "This technology …" use "Technologies for Digital Twins …".
2. The authors state (l. 28 – 30): "Leveraging the data provided by Building Information Models (BIMs), professionals from diverse industries can collaborate seamlessly, facilitating efficient project life-cycle management and cost reduction." Since there are commonly known issues on collaboration using BIM technologies both on software application level (data formats compatibility) and level of management of building information modeling processes, the word "seamlessly" seems to be quite strong. Moreover there is no reference for a such statement. I would recommend to omit the word „seamlessly“ in the sentence.
3. Please check the sentence (l. 57 – 59): "Through this integration, the transformation of Building Information Modeling (BIM) into an ontology using the Semantic Web has been proposed to enhance inference accuracy". The authors mentioned integration of "Semantic Web" into "Industry Foundation Classes (IFC) " in the previous sentence. The meaning of "BIM" abbreviation in the sentence (l. 57-59) seems to be "Building Information Models", not "Modeling" as mentioned. You can check the meanings of BIM abbreviation in senses of Building Information Model and Building Information Modeling in first two paragraphs of [2].
4. Please provide a reference for the following statement (l. 60 – 61): "The CityGML 2.0 concept is widely utilized in academia and industry due to its ease of application and data transformation."
5. The authors state (l. 61 – 63): "However, as the range of applications expands, its current classification assigns all internal features to a single LOD4, necessitating complex representations both externally and internally." I would recommend briefly explaining LOD concept in CityGML 2.0 before this sentence. In civil engineering (especially concerning building information models), the LOD abbreviation is used in meanings of "level of details" or "level of development". LOD is usually followed by a three-digit number specifying the level. More information on this topic can be found in [3, 4].
6. I don’t understand the term "Digital Twin methodology" in the following sentence (l. 69 – 71): "In this study, a method is proposed to incorporate the Digital Twin methodology into urban management practices within the context of a smart village located in Busan, South Korea." In the context of built environments, the Digital Twin is usually understood as a concept supported by various technologies. If the authors know about specification of any methodology regarding Digital Twins in built environments they should provide a reference.
7. Table 1 requires a lot of explanations:
a) Terms provided in "Division" column should be explained. At least meaning of "Sensor", "Indoor", "LoD", "Semantic", and "Attribute" terms are not clear to me. E.g. all data in IFC files are stored as values of attributes of class instances. What "Attribute" mean in the context of the given table?
b) I can’t agree that Geo-referencing is not supported in IFC – please see [5].
c) Please explain the levels of support mentioned in the given table (0 : Basic + : Supported, ++ : Extended support). How they differs?
Regarding the Table 1, the authors state (l. 99 – 102): "We conducted a comparative analysis of 3D data model standards, including IFC, LandXML, 3DF-GML, KML, CityGML, and glTF. Analysis was conducted for each model based on specifications outlined in standard documents and research reports as shown in Table 1." I understood that giving details about such an analysis is out of the scope of this manuscript. But by my opinion this analysis would deserve a separate publication providing details about methodology, data used, results, discussion and conclusion. Providing only short description in a few lines of this manuscript raises more questions than provide answers. E.g. wich versions of IFC standards the authors have analyzed?
8. In section 3.2.1 the authors provide "Semantic mapping from IFC to CityGML". The semantic mapping rule between IFC and CityGML entities is provided in Figure 3. Figure 3 shows that some classes of IFC (mostly subclasses of IfcBuildingElement class) were mapped to CityGML entities. But how they were chosen? BuildingSMART specification of IFC standard version 2x3 [6] defines IfcBuildingElement as "The building element comprises all elements that are primarily part of the construction of a building, i.e., its structural and space separating system." Why you have not mapped all subclasses of IfcBuildingElement class? Why for example IfcColumn class was not included? Please provide the explanation in 3.2.1 section. Moreover particularly, IfcColumn class is mentioned in l. 312 and Figure 6. Why this class is not in the Figure 3?
9. Please check the text in l. 289 – 290. It seems that the sentence "The second IFC reader parsing phase focuses on data transformation." appears twice in the text.
10. Please correct the following typos: "IfcCurtainWanll" (l. 476), "CiytGML" (l.488, 489).
11. The authors state: (l. 492 – 493): "For this experiment, we focused on sensor data that records ambient temperature over a three-day period, as shown in Figure 20(b)." Similarly, part of the caption of Figure 20 is "(b) Visualization of time series data". I can’t see any data about temperature in Figure 20(b). Please make this clear.
[1] Shahzad, M.; Shafiq, M.T.; Douglas, D.; Kassem, M. Digital Twins in Built Environments: An Investigation of the Characteristics, Applications, and Challenges. Buildings 2022, 12, 120. https://doi.org/10.3390/buildings12020120
[2] Wholesale Building Design Guide, Building Information Modeling (BIM), [accessed on 24-05-08], available from: https://www.wbdg.org/bim
[3] What is LOD, LOI, LOG, LOIN?, [accessed on 24-05-08], available from: https://buildext.com/en/what-is-lod-loi-log-loin/
[4] Level of Development (LOD) Specification, [accessed on 24-05-08], available from: https://bimforum.org/resource/lod-level-of-development-lod-specification/
[5] buildingSMART: User Guide for Geo-referencing in IFC, [accessed on 24-05-08], available from: https://www.buildingsmart.org/wp-content/uploads/2020/02/User-Guide-for-Geo-referencing-in-IFC-v2.0.pdf
[6] buildingSMART International Ltd., "Industry Foundation Classes IFC2x Edition 3 Technical Corrigendum 1", [24-05-08], Available from: https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ .
Author Response
We have revised our manuscript according to the proposed reporting format. The reviewer's responses are included in the attached Word file. Thank you for the detailed review.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for Authors
Please refer to the attached pdf with comments.
Overall: consistent structure, major revision well performed, good language for readers outside of the profession. Timely topic!
Comments for author File: Comments.pdf
Comments on the Quality of English Language
Author Response
Thank you for your detailed review. We have revised the manuscript and provided our responses in the attached Word file.
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors
The authors have addressed the last review comments and provided point-wise responses.
Comments on the Quality of English Language
Minor editing of English language required
Author Response
Thank you for your comment. We have checked and corrected minor errors in the English language of the manuscript.
