A Comparison of Surface Deformation Measurement Methods for Slopes

Round 1

Reviewer 1 Report

The article deals with the measurement of land surface deformation using various methods. The topic is interesting, but the article still needs a lot of work and in my opinion it should not be published in its current form. Detailed notes below:
1. Abstract - abstract is poorly constructed. Most of it concerns the description of the results obtained by individual methods. This should be condensed into a general conclusion. There is no indicated research goal or research area.
2. Section 1. Introduction - a very poor overview of the state of research, not leading to an indication of a research gap. How is the novelty of the research? What is the goal? The authors conduct research on three methods (TS, DSS, UAS), and in this section they only briefly cover two.
3. Figures captions should be consistent. See Fig. 1 and Fig. 5. What are the reasons for the differences in wording?
4. Line 130 - "However, CPs are only for validation." To validate what? A geometric model of displacements? GCP constancy? How are GCPs found to be constant? Additional measurements and calculations performed? How was the criterion of constancy identified?
5. Line 164 - "... and the location of the base station was obtained from the 1OPUS (Online Position User Service) used together to correct the surveying coordinates." What do the authors mean by correcting coordinates? Why do they do these things? Have calculations been carried out using exact methods, e.g. least squares method? On what basis are adjustments made?
6. Line 166 - "Since adding GCPs would not improve the accuracy, CPs were used to confirm the accuracy of the surveying." Where did this request come from? The authors conducted the analyses? Usually, in geodetic measurements, increasing the redundancy of observations and the reference base has a positive effect on the obtained results of displacements and their accuracy.
7. Lines 167 - "The TS based surveying, however, was not conducted for this site due to time constraints." If they were not carried out, it means that the authors made incorrect assumptions to conduct the research. If the authors want to study the three methods mentioned above, they cannot abandon one of them due to the lack of time.
8. Line 337 - "The detailed reason is not known at this time, although it is presumed that the elevation of the reference points could have been moved (or elevated) slightly." If the Authors do not know the cause of the errors, it should be checked whether the points maintain mutual stability. If it takes the points as fixed (and they are not), the determination of the displacements is unreliable! This is a basic error in the engineering interpretation of the measurement results and displacement monitoring!
9. Section 5. Concussions - since the authors did not define the purpose of the research, it is impossible to say whether the conclusions are appropriate. In addition, this section does not contain conclusions, rather a brief technical description of the measurement work carried out.
10. The article at the very beginning is written in a way that indicates that geodetic methods are the least favorable to use. This is a very wrong assumption. Especially considering that they are the most precise, it allows you to refer changes to the external reference system, and the obtained displacements are characterized by high accuracy. The explanation that qualified surveying engineers could not be found is unprofessional. Such an approach completely negates the assumption regarding the authors of the study of three measurement methods.
11. References to Wikipedia in a scientific article is very unprofessional. The topic raised by the authors is known and there are many studies that may be the basis for their reference in the article.

Author Response

Authors reply to reviewers’ comments. (Reviewer 1)

Authors show their sincere appreciation to reviewers for taking time to read the manuscript and providing precious comments.

1. Abstract - abstract is poorly constructed. Most of it concerns the description of the results obtained by individual methods. This should be condensed into a general conclusion. There is no indicated research goal or research area.

 

Abstract was rewritten as follows (line 11 to 24);

Abstract:  This study compared the features and field applicability of Total Station (TS) based surveying, Distributed Strain Sensing (DSS) based, and Uncrewed Aerial System (UAS) based deformation measurement methods that were applied for different slopes, aiming to collect reliable deformation data efficiently. The TS-based method was a two-person task, and it provided acceptable results. It is the one with a long history of a "tried and true" reputation. However, it included a major portion of manual work in the field, potentially consuming extended time to obtain high-resolution data. The DSS-based method was a fiber optic cable-based one-person work, and it showed substantially faster and easier measurement. This method needed one operator and possessed the capability of collecting unattended measurements. The method also required anchor posts to measure deformation in segmented sections, some anchor posts became loose from shrinkage cracks and resulted in invalid measurements, particularly for soils of high plasticity. The UAS-based method was an aerial photo-based method, and it provided an extremely high-resolution deformation profile, and the method needed manual surveying for an elevation check at reference points – though the surveying took a short amount of time by utilizing a Global Navigational Satellite Survey (GNSS) technique. This method required one operator and an assistant. From the comparison of the characteristics of three different methods, it was found that each technique has its own pros and cons, and the combination of different methods may greatly enhance the accuracy and convenience of the measurement rather than competing with each other.

 

2. Section 1. Introduction - a very poor overview of the state of research, not leading to an indication of a research gap. How is the novelty of the research? What is the goal? The authors conduct research on three methods (TS, DSS, UAS), and in this section they only briefly cover two.

 

Authors mentioned TS from lines 55 to 57 as follows.

 

In addition, this study included the traditional Total Station (TS) based measurement result in the comparison, even though its usage was limited to only one site (I-180).

 

3. Figures captions should be consistent. See Fig. 1 and Fig. 5. What are the reasons for the differences in wording?

 

All figure captions are revised to be consistent.

 

 

4. Line 130 - "However, CPs are only for validation." To validate what? A geometric model of displacements? GCP constancy? How are GCPs found to be constant? Additional measurements and calculations performed? How was the criterion of constancy identified?

 

Explanation of CPs are revised as follows from line 228 to 235.

 

After surveying, imagery data and coordinates of the marker points were imported into Pix4DMapper. In the next step, the surveyed points were divided into CPs (CheckPoints) and GCPs (Ground Control Points). GCPs are selected in a few images to modify image coordinates and match them with the real-world coordinates at the centimeter level. CPs are used as additional points with real-world coordinate information to confirm the model's accuracy at the centimeter level. Using GCPs significantly increase the accuracy of the surveying, as the GPCs enable restraints on the model to limit the errors.

 

5. Line 164 - "... and the location of the base station was obtained from the 1OPUS (Online Position User Service) used together to correct the surveying coordinates." What do the authors mean by correcting coordinates? Why do they do these things? Have calculations been carried out using exact methods, e.g. least squares method? On what basis are adjustments made?

 

This comment was addressed from line 269-277 as follows.

 

The base station logged continuously throughout the UAS flight, and the location of the base station was obtained using OPUS (Online Position User Service). Here OPUS provided an accurate estimate of the base station, specifically for International Terrestrial Reference Frame coordinates of 0.6 cm horizontally and 2.5 cm vertically. Due to the known accuracy of the PPK platform and the wet slope in the field, CPs were not deployed nor deemed necessary. For likewise reasons of wet and slippery slope, the TS-based surveying technique was also not deployed for safety. The accuracy of the tagged pictures was 2 cm horizontally and 4 cm vertically, with the constructed point clouds having an average ground sampling distance of 1.04 cm.

6. Line 166 - "Since adding GCPs would not improve the accuracy, CPs were used to confirm

the   accuracy of the surveying." Where did this request come from? The authors conducted the analysis? Usually, in geodetic measurements, increasing the redundancy of observations and the reference base has a positive effect on the obtained results of displacements and their accuracy.

 

The authors believe that the reply to comment 5 addressed this comment 6 as well.

7. 
   Lines 167 - "The TS based surveying, however, was not conducted for this site due to time constraints." If they were not carried out, it means that the authors made incorrect assumptions to conduct the research. If the authors want to study the three methods mentioned above, they cannot abandon one of them due to the lack of time.

 

Authors did conduct surveying for I-180 site, but for Highway 84 site. Authors believe that the reviewer understands that some research work might be planned but the work scope might be modified. The authors also believe that the result from I-180 may represent a typical TS result. Also please understand that comparison of different technique was not a part of the research work. Authors used the data collected from two independent research with different time spans.

 

8. 
   Line 337 - "The detailed reason is not known at this time, although it is presumed that the elevation of the reference points could have been moved (or elevated) slightly." If the Authors do not know the cause of the errors, it should be checked whether the points maintain mutual stability. If it takes the points as fixed (and they are not), the determination of the displacements is unreliable! This is a basic error in the engineering interpretation of the measurement results and displacement monitoring!

 

The authors reinspected the data for TS survey and UAS survey and found the measurement dates were different. Therefore, it was addressed as follows from line 376-379.

 

The overall deformation trend is similar to the survey results, but the magnitude of deformation is lower in UAS based method than that shown in the TS survey and DSS results. This discrepancy could be due to the different measurement dates for TS-survey and DSS.

 

9. 
   Section 5. Concussions - since the authors did not define the purpose of the research, it is impossible to say whether the conclusions are appropriate. In addition, this section does not contain conclusions, rather a brief technical description of the measurement work carried out.

 

Conclusions are in line with the purpose now and shown as follows from line 511-558 as follows.

 

This study evaluated the features and field applicability of different deformation measurement methods - Distributed Strain Sensing (DSS), Uncrewed Aerial System (UAS), and Total Station (TS) - based on multiple years of experience in monitoring deformation for two roadside slopes. The two sites have varying conditions such as soil type, geographical locations, and weather conditions. From the comparative study following comparisons are obtained.

The DSS-based method was the quickest and easy method to measure the deformation of the ground surface. The installation of measurement poles was easy. The measurement time took typically less than an hour for one site and needed only one operator. The method was little impaired by vegetation or snow covering once the system was installed. Training an operator was easy. The remote control and remote measurement were possible in some models.

However, the fiber optic cable of DSS might suffer from the activity of wild animals and low-temperature related voltage drops from the batteries. In addition, the PVC posts driven into the ground were observed to be unstable due to shrinkage cracks and impaired the measurement, in highly plastic soils. Galvanized zinc pipes with a depth longer than the frost depth and dry cracks might be more adequate for these soils. DSS used one long fiber optic cable, and a damaged section anywhere in the system could disable the measurement beyond the damage point. To prevent this trouble, installing redundant lines of DSS might be needed.

The UAS-based method offered fast surveying even for a large job site. It was flexible to mother nature's weather condition except for snow coverings encountered in this study and some wind. The method could measure the 3-D surface deformation for the entire area of interest (not just at discrete points) with unmatched high resolution providing deformation images extensively and clearly. The result of the UAS-based method generally agreed well with those of other methods. The method provided rational deformation data even for relatively thick (0.3 m) vegetation areas.

The system for UAS-based method might require dedicated equipment such as high-quality UAS, UAS license, and dedicated software, which might be expensive to acquire and maintain. In addition, a one drone controller and one highway safety crew member are recommended.

Traditional TS surveying was a popular technique; however, this can be challenging to reliably deploy on certain slopes given moisture and safety concerns. The method was a two-person work, however, the output and measurement effectiveness were not better than DSS or UAS-based methods. It was, however, noted that the TS method could the most economic one for certain slopes, particularly when the number of measurement points were not very high.

The error from different methods was not compared due to the drastically different measurement principles of the different method. However, given the ability to measure deformation using remotely sensed data, DSS and UAS techniques, these techniques are preferred for safety and operational needs.

In addition to the capability of individual technique, the authors would like to address a new potential of deploying these techniques in a combined way for successful monitoring. For example, UAS can be quick and useful for monitoring large and remote sites. However, adverse weather conditions such as snow and strong wind may pose limitations. DSS can cover this weakness of UAS by not only enabling remote/real-time measurements that can be conducted in small intervals (e.g., weekly measurement) but also capturing some direct quasi-subsurface (shallow subsurface) movement through anchor poles - which can indicate the movement of soils not affected by weathering element.

10. 
    The article at the very beginning is written in a way that indicates that geodetic methods are the least favorable to use. This is a very wrong assumption. Especially considering that they are the most precise, it allows you to refer changes to the external reference system, and the obtained displacements are characterized by high accuracy. The explanation that qualified surveying engineers could not be found is unprofessional. Such an approach completely negates the assumption regarding the authors of the study of three measurement methods.

 

The authors were not partial to any specific technique. Authors tried to be completely fair in comparing different techniques, particularly in the revised version. And authors made it clear that each technique has pros and cons, and combination of different techniques would compensate each other’s weakness. Particularly refer line 551 to 558.

11. 
    References to Wikipedia in a scientific article is very unprofessional. The topic raised by the authors is known and there are many studies that may be the basis for their reference in the article.

 

References from Wikipedia was revisited. Their original source was checked and properly quoted in the revised version.

Reviewer 2 Report

1.       Articles are not well organized;

2.       Abstracts describe a lot of background, and reviewers have no idea what the authors intend to study in the manuscript;

3.       Is it contact measurement or non-contact visual measurement? The author did not explain clearly;

4.       Image quality could be improved. The language of the article needs deep polishing;

5.       Insufficient discussion of measurement error analysis.

Author Response

Authors reply to reviewers’ comments. (Reviewer 2)

Authors show their sincere appreciation to reviewers for taking time to read the manuscript and providing precious comments.

1. Articles are not well organized;

 

Authors feel sorry for comment. The revised version is reorganized for easy to read.

 

2. Abstracts describe a lot of background, and reviewers have no idea what the authors intend to study in the manuscript;

 

Abstracts are revised as follows to make it as clear as possible.

 

Abstract:  This study compared the features and field applicability of Total Station (TS) based surveying, Distributed Strain Sensing (DSS) based, and Uncrewed Aerial System (UAS) based deformation measurement methods that were applied for different slopes, aiming to collect reliable deformation data efficiently. The TS-based method was a two-person task, and it provided acceptable results. It is the one with a long history of a "tried and true" reputation. However, it included a major portion of manual work in the field, potentially consuming extended time to obtain high-resolution data. The DSS-based method was a fiber optic cable-based one-person work, and it showed substantially faster and easier measurement. This method needed one operator and possessed the capability of collecting unattended measurements. The method also required anchor posts to measure deformation in segmented sections, some anchor posts became loose from shrinkage cracks and resulted in invalid measurements, particularly for soils of high plasticity. The UAS-based method was an aerial photo-based method, and it provided an extremely high-resolution deformation profile, and the method needed manual surveying for an elevation check at reference points – though the surveying took a short amount of time by utilizing a Global Navigational Satellite Survey (GNSS) technique. This method required one operator and an assistant. From the comparison of the characteristics of three different methods, it was found that each technique has its own pros and cons, and the combination of different methods may greatly enhance the accuracy and convenience of the measurement rather than competing with each other.

 

3. Is it contact measurement or non-contact visual measurement? The author did not explain clearly;

 

DSS and TS are contact measurement technique. But UAS is a non-contact measurement technique. Revised manuscript is clearer in addressing different techniques.

 

4. Image quality could be improved. The language of the article needs deep polishing;

 

Number of pictures were replotted or replaced for the readability. And the language was thoroughly revised.

5. Insufficient discussion of measurement error analysis.

Errors were addressed whenever possible (e.g. line 271- 277).

Here OPUS provided an accurate estimate of the base station, specifically for International Terrestrial Reference Frame coordinates of 0.6 cm horizontally and 2.5 cm vertically. Due to the known accuracy of the PPK platform and the wet slope in the field, CPs were not deployed nor deemed necessary. For likewise reasons of wet and slippery slope, the TS-based surveying technique was also not deployed for safety. The accuracy of the tagged pictures was 2 cm horizontally and 4 cm vertically, with the constructed point clouds having an average ground sampling distance of 1.04 cm.

However, it is noted that accuracy of the used technique was not significant for all technique in detecting the movement of the slope. The manuscript described broad aspects of different deformation techniques including conventional techniques and new techniques. Its grand conclusion is,

“In addition to the capability of individual technique, the authors would like to address a new potential of deploying these techniques in a combined way for successful monitoring. For example, UAS can be quick and useful for monitoring large and remote sites. However, adverse weather conditions such as snow and strong wind may pose limitations. DSS can cover this weakness of UAS by not only enabling remote/real-time measurements that can be conducted in small intervals (e.g., weekly measurement) but also capturing some direct quasi-subsurface (shallow subsurface) movement through anchor poles - which can indicate the movement of soils not affected by weathering element.” (line 551 to 558).

Reviewer 3 Report

This manuscript explained and compared different methods for measuring surface deformation including Total Station, Distributed Strain Sensing, and Uncrewed Aerial System. But the contents are basically common sense, I did not see any novelty. Therefore, a rejection is suggested:

1.      Where is the literature review? What did the other research do? What is the novelty here? I did not see the original contribution of the manuscript

2.      Please refine the abstract, a background of surface deformation measurement and a comprehensive conclusion are needed in this section.

3.      Please add some figures to show the geological condition and the location of the project sites.

4.      The principles of different methods should be introduced before the section namely Essential information on project sites.

5.      The format of this manuscript is kind of a mess, please refine.

6.      It is better of visualize the workflow of different methods.

7.      The qualities of figures are supposed to be upgraded. (for example: Fig. 5 and Fig.7)

8.      Where is the discussion section? Some limits, challenges and future suggestions should be provided in discussion.

9.      Please include more recent papers into references, or the concept is kind of out of date. You can refer to “Probabilistic stability analysis of Bazimen landslide with monitored rainfall data and water level fluctuations in Three Gorges Reservoir, China.” And “Displacement prediction of Jiuxianping landslide using gated recurrent unit (GRU) networks”

10.   Check the format of references, especially for the last two!

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors made the necessary corrections.

Author Response

Thanks to reviewer 1.

No specific review comments raised by reviewer 1.

However, the manuscript was thoroughly revised to enhance the clarity of technical contents and readability.

Sincerely,

Authors

 

Author Response File: Author Response.docx

Reviewer 2 Report

The readability and typesetting of the revised manuscript are too poor. Especially pages 20-22, totally incomprehensible

Author Response

Dear Reviewer 2

Thank for your comments.

The manuscript was thoroughly revised from the first page to the last page.

Thanks for your time to review the manuscript.

Author Response File: Author Response.docx