Optimization of Photogrammetric Flights with UAVs for the Metric Virtualization of Archaeological Sites. Application to Juliobriga (Cantabria, Spain)

Round 1

Reviewer 1 Report

In this manuscript, the Taguchi DOE Method has been proposed by the authors to form a methodology for optimizing the accuracy of the models applied for the virtualization of archaeological sites. They follow a statistical procedure to optimize selecting four crucial flight parameters to obtain the best accuracy, thus optimizing the UAS data acquisition process.

The manuscript is well structured and with sufficient bibliography.

In the methodology section 2.2. it is not clear the range of values considered for height, overlaps, and inclination. Furthermore, the authors selected specific values with significant differences between height 15-45-75 (difference 30 meters), camera tilt 30 degrees. They should clearly define their selection as it is evident that the higher with less overlap the UAS data acquisition is realized, the lower the virtualization results.

The authors state that according to their study based on the Taguchi DOE Method, the suitable combination of factors to optimize the photogrammetric flight is the one that fixes the height of flight at 15 m, the longitudinal and transversal overlaps at 80% both, and the angle of inclination set to nadiral. This result is evident as it is known that when flying higher with less overlap and taking oblique aerial images, the photogrammetric results are not optimal as GSD, slantrange, and scale varies to the images acquired.

Furthermore, following the Taguchi method, the authors didn’t realize the optimal flight (15m, 80% overlaps, 0^o inclination); thus, they don’t have results to compare as for the virtualization concern. They should present the virtualization differences between the best and other flights with different parameters, as this is the theme of their manuscript according to the title.

In this context, the authors should consider to realize and to add material from the proposed optimal flight comparing with others. As they state that their approach will optimize the photogrammetric flight to generate the information required for metric virtualizations, they should illustrate and discuss the virtualization differences between the scenarios presented.

Lines 426-427:… the measurement of distance of the eight stadias was not possible for all the designed flights due to several reasons. The authors should define the reasons and if the lack of measurements affects and how their results.

Additionally, this statement is contradictory to line 391: …..so as to guarantee the absence of hidden areas in the model.

 

Author Response

Dear Reviewer,

First of all we would like to thank your comments, they have helped us to improve the comprehensibility of our work. Please, find below our answer to your observations, along with the list of modifications that we have made. To ease its reading, a PDF file with format has been included, but the text is the same. 

“In this manuscript, the Taguchi DOE Method has been proposed by the authors to form a methodology for optimizing the accuracy of the models applied for the virtualization of archaeological sites. They follow a statistical procedure to optimize selecting four crucial flight parameters to obtain the best accuracy, thus optimizing the UAS data acquisition process.

The manuscript is well structured and with sufficient bibliography.

In the methodology section 2.2. it is not clear the range of values considered for height, overlaps, and inclination. Furthermore, the authors selected specific values with significant differences between height 15-45-75 (difference 30 meters), camera tilt 30 degrees. They should clearly define their selection as it is evident that the higher with less overlap the UAS data acquisition is realized, the lower the virtualization results.”

Taguchi DOE Method is described in a general way in section 2.2. In section 2.3., the methodological proposal adapted for the optimization of flights is described. Table 3 comprises all the values that were selected for this test. The aim is the optimization of the height of flight, longitudinal and transversal overlaps, and angle of inclination in order to obtain the best product possible, with reasonable effort in terms of time, costs and work. This is the reason why three different levels were proposed for each parameter. The consideration of a higher number of levels would have increased the number of experiments in a significant way. However, when considering the levels for each factor, better, intermediate and worse levels were set, with a noticeable leap among them, so as to obtain a significant response in the results. These levels were set taking into account other authors’ works (References 25 to 28 have been included, and reference 1 could be included among these sources). For example, In the specific case of the height, flights over 120 meters are not allowed in our country, so a minimum height of 15 meters was chosen, along with two more levels at 45 and 75 meters. Flights over the latter were considered as not appropriate beforehand due to the poor results that would provide.  

“The authors state that according to their study based on the Taguchi DOE Method, the suitable combination of factors to optimize the photogrammetric flight is the one that fixes the height of flight at 15 m, the longitudinal and transversal overlaps at 80% both, and the angle of inclination set to nadiral. This result is evident as it is known that when flying higher with less overlap and taking oblique aerial images, the photogrammetric results are not optimal as GSD, slantrange, and scale varies to the images acquired.”

Although the answer might seem obvious, this work, and more specifically Table 3 in the previous draft, Table 2 in the current one, highlight the scarce variability of the longitudinal overlap between 70% and 80%, which implies that the adoption of the lower value would be feasible. This can reduce the effort while maintaining quality. It has been emphasized in the discussion section (page 17, lines 513-514 of the current manuscript).  In addition to this, although the inclination of the camera is unadvisable on a first approach, most of the pilots take inclined shots, especially when dealing with vertical facades.  References to works that have been developed with different angles (perpendicular to the surface of the element to be captured, oblique shots and the combination of both alternatives) are included in the current draft (References [15-17], Page 5, lines 200-201).

“Furthermore, following the Taguchi method, the authors didn’t realize the optimal flight (15m, 80% overlaps, 0^o inclination); thus, they don’t have results to compare as for the virtualization concern. They should present the virtualization differences between the best and other flights with different parameters, as this is the theme of their manuscript according to the title.”

This is a due to the fact that managing four parameters and three different levels, Taguchi DOE Method reduces the required number of experiments from 81 test to just 9. This implies a reduction in terms of time, costs, the amount of work, etc. However, given the small number of experiments that are developed, the best one could not be included in this initial set, which is a common situation. Nevertheless, this circumstance does not impede Taguchi DOE Method, which is a well-established method, from providing the best possible flight. Our aim was not the contrast of the best possible flight, but to determine the methodology to define it.

“In this context, the authors should consider to realize and to add material from the proposed optimal flight comparing with others. As they state that their approach will optimize the photogrammetric flight to generate the information required for metric virtualizations, they should illustrate and discuss the virtualization differences between the scenarios presented.”

Despite the unavailability of the best possible fight according to Taguchi DOE Method, materials shown in section 3.4. try to provide information about the potential actions aimed at the virtualization of the area that can be developed from the best flight that was included in the initial set. In order to illustrate the variations in the quality of the product, Figure 9 has been modified. The new figure includes two images which correspond to the best and worst flights included in the initial set.

Lines 426-427:… the measurement of distance of the eight stadias was not possible for all the designed flights due to several reasons. The authors should define the reasons and if the lack of measurements affects and how their results.

The reason that impeded the measurement of three stadias was not related to shadowed areas, but to the luxuriance of the grass itself (it has been included in the manuscript (page 13, line 427 of the current  version). The incidence in the analysis is minimal, as it is shown in former Table 6, current Table 7, only three of the stadias were measured in the models obtained from seven flights instead of eight, and this fact was taken into account when calculating the Signal-to-Noise ratios (Equation 3).

Additionally, this statement is contradictory to line 391: …..so as to guarantee the absence of hidden areas in the model.

The statement has been modified: “guarantee the absence ” has been substituted with “reduce” (page 11, line 376 of the current version).

Please, find at the end of this letter the list of changes that have been made. We hope that these changes meet your criteria. Once again, we would like thank your comments.

Best regards,

 

Dr. Rubén Pérez Álvarez.

Corresponding Author.

 

RELATION OF MODIFICATIONS:

 

Please note that these references are defined according to the numbers of page and line according to the document with the track changes function enabled.

 

[Page 1, line 30]: “In this regard” has been removed to simplify the redaction of this section.  

[Page 1, line 39]: “If required” has been removed to simplify the redaction of this section.  

[Page 2, line 44]: “stands out greatly in the field of archaeology. This technique” has been removed to simplify the redaction of this section.

[Page 2, line 45]: “, as it has been mentioned” has been removed to simplify the redaction of this section.  

[Page 2, line 46]: “itself” has been removed to simplify the redaction of this section.  

[Page 2, line 52]: “, with all the advantages that it implies, as it has been mentioned.” has been removed to simplify the redaction of this section.  

[Page 2, line 57]: “are the prime matter for the development of the previously mentioned outputs. Due to this, the images” has been removed to simplify the redaction of this section.  

[Page 2, line 68]: “establishes” instead of “established”.

[Page 2, line 71]: “, such as the height of flight, longitudinal and transversal overlap, speed of light, sensor features, climatic conditions, etc. There is a set of parameters or technical factors that must be considered when dimensioning the flight project” has been removed to reduce the introduction.

[Page 2, line 74]: “The definition of these technical factors” instead of “This definition”.

[Page 2, line 75]: “Themselves” has been removed.

[Page 2, line 77]: “and” has been removed.

[Page 2, line 77]: “On this purpose” has been removed.

[Page 79, line 79]: The acronym “DOE” has been included.

[Page 5, line 196]: “angle” instead of “angled”.

[Page 5, line 196]: “principal axis” instead of “focal length”.

[Page 5, line 204]: The following sentence and references have been included “In this regard, and considering the relative position of the surface the element to be captured and the principal axis of the camera [15], examples of perpendicular[16], oblique and a combination of both types of shots can be found in literature [17].”

[Page 5, lines 215, 222, 223, 224, 226]: The numbers of the references has been adjusted.

[Page 5, line 228]: The sentence has been simplified, avoiding the explanation of control and noise factors.

[Pages 5-6, line 244-253]: The stages of Taguchi DOE Method have been removed to reduce this section.  

[Page 6, line 261]: This paragraph referred to Table 1. Both of them have been removed to simplify this paragraph.

[Page 6, line 266-268; Table 1]. The number of the table has been adjusted.

[Page 7, line 283]: The reference to the aim of this research has been avoided to reduce redundancies through the text.

[Page 7, line 286]: The reference order has been updated.

[Page 7, line 299]: The table that is referenced has been modified (Table 2 instead of Table 3).

[Page 7-8, lines 300-301]: The following sentence and references have been introduced to justify the values that have been adopted for the different parameters and levels: “The levels for each parameter range according to the recommendations and values applied in several works of research related to the application of aerial photogrammetry with UAV to the analysis of archaeological sites and structures [1, 28-31]”.

[Page 8, line 303]: The number of the table has been corrected.

[Page 8, lines 310, 312]: The number of the table has been corrected.

 [Page 8, lines 312-312]: The reference to the aim of this research has been avoided to reduce redundancies through the text.

[Page 8, line 327]: References [32-33] have been included to justify the idoneity of stadias as standardize elements.

[Page 9, line 347]: “North” instead of “Nort”.

[Page 9, lines 347,356; Page 10, line 368]: Reference number have been updated.

[Page 11, lines 385 and 392]: The number of the references to the tables have been updated.

[Page 11, lines 392-394]. Figure 3, Table 5 and the following statement have been introduced to provide an idea about the influence of the combination of parameters in the complexity of the flights: “. The combination of factors affects the complexity of the project in a substantial way (Figure 4, Table 5).”

[Page 12, line 402]: The number of the reference to the table has been edited.

[Page 12, line 406]: “reduce” instead of “guarantee the absence of”.

[Page 12, lines 410 and 419]: Reference to the figure has been updated.

[Page 12, line 421]: The number of the figure has been edited.

[Page 12, line 422]: The reference to the figure has been updated.

 [Page 13, line 423]: The number of the figure has been modified.

[Page 13, line 430]: “Agisoft Metashape” instead of “Agisoft”.

[Page 13, lines 430-437]: The importance of the autocalibration feature and the self-calibration parameters (Table 6) have been introduced.

[Page 14, line 447]: The reference to the table has been modified.

[Page 14, line 451]: The reason why some stadias were impossible to measure, and its minimal incidence in the final calculations, have been noted.

 [Page 14, lines 458 and 464; Page 15, lines 469, 489, 493 and 496]: The references to tables and figures have been updated.

[Page 16, line 499]: The figure number has been modified.

[Page 16, lines 503, 506, 510 and 516]: The references to figures 7, 8 and 9 have been modified.

[Page 16, line 517; Page 17, lines 519-520]: Figure 9 has been modified to offer a comparison between the best and worst flight in terms of the model quality.

[Page 17, line 538]: The possibility to optimize the overlap due to the low difference in the variabilities between 70 and 80% is emphasized. “which allows adopting the first value to reduce costs while maintaining quality”.

[Pages 19-20, lines 611, 618, 621, 624, 628, 631, 634, 638, 643, 646]. The references have been edited to adequate the format of the volumes and issues.

The following references have been included:

Gómez-López, J.M.; Pérez-García, J.L.; Mozas-Calvache, A.T.; Delgado-García, J. Mission flight planning of RPAS for photogrammetric studies in complex scenes. ISPRS Int. J. Geo-Inf. 2020, 9, 392. doi:10.3390/ijgi9060392

Taddia, Y.; González-García, L.; Zambello, E.; Pellegrinelli, A. Quality Assessment of Photogrammetric Models for Facade and Building Reconstruction Using DJI Phantom 4 RTK. Remote Sens. 2020, 12, 3144. doi: 10.3390/rs12193144.

Piech, I.; Ruzyczka, A. Generating of building facades orthophotoplans with UAV and terrestrial photos. IOP Conf. Ser.: Earth Environ. Sci. 2019, 221 , 012074. doi:10.1088/1755-1315/221/1/012074.

Morgenthal, G.; Hallermann, N. Quality assessment of unmanned aerial vehicle (UAV) based visual inspection of structures. Adv. Struct. Eng. 2014, 17 (3), 289–302. doi: 10.1260/1369-4332.17.3.289

Stek, T. D. Drones over Mediterranean landscapes. The potential of small UAV’s (drones) for site detection and heritage management in archaeological survey projects: A case study from Le Pianelle in the Tappino Valley, Molise (Italy). J Cult. Heritage 2016, 22, 1066–1071. doi: 10.1016/j.culher.2016.06.006

Laguela, S.; Diaz-Vilarino, L.; Roca, D.; Lorenzo, H. Aerial thermography from low-cost UAV for the generation of thermographic digital terrain models. Opto-Electron. Rev. 2015, 23 (1), 76–82. doi: 10.1515/oere-2015-0006.

Rakha, T.; Gorodetsky, A. Review of Unmanned Aerial System (UAS) applications in the built environment: Towards automated building inspection procedures using drones. Autom. Constr. 2018, 93, 252–264. doi: 10.1016/j.autcon.2018.05.002

Elsadig Ali, A. Accuracy of stadia tacheometry with optical theodolites and levels. J. King Saud Univ.-Eng.Sci. 1995, 7(2), 175–184.  doi: 10.1016/S1018-3639(18)30625-1.

Ibraheem, A.Th.; Hasim Mehdi, A.; Adil Najeeb, Z. The utilization of stadia measurements for different constructions works. Int. J. Tech. Res. Appl. 2015, 3(6), 37–44.

The following reference was removed to simplify Section 2.

151. Sharma, P.; Verma, A.; Sidhu, R.K.; Pandey, O.P. Process parameter selection for strontium ferrite sintered magnets using Taguchi L₉ orthogonal design. J. Mater. Process. Technol. 2005, 168(1), 147–151. doi: 10.1016/j.jmatprotec.2004.12.003.

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The research aims at the optimization of a subset of significant variables influencing the accurate metric performance of a photogrammetric UAV flight, namely: height off light, longitudinal and traversal overlap, and inclination of the principal optical axis to the nadir. The 3D virtualization of an archaeological site is adopted as a relevant case study. A DOE is accomplished (Taguchi) for specifying the set of experiments under different measuring conditions, and metric performance is evaluated in each one compared to convenient metric references (as length measuring errors to reference stadias). Despite the soundness and novelty of the research topic (i.e.: evaluating the set of variables affecting the metric performance of a photogrammetric flight), a major review is requested according to the following:

• Adopting a maximal overlap between images, along with a constant nadiral inclination during a measuring flight are concluded as the optimal set of measuring variables under study. However, concerning image overlap, this result seems to be straightforward without the need of accomplishing any DOE for determining an optimum. The greater the overlap between images is, the greater information is available in the total set of images, given that additional images are taken in a flight with more overlapping percentage for covering a constant measuring field (indeed, authors do not refer to the total images taken at each flight). Authors should better report the number of images and corresponding flight and computing times required for each overlapping level and balance that effort to the increase of metric performance (which could be expected to be asymptotical, that is to say, with no practical increase of metric performance after an optimal traversal overlap).
• Concerning angle inclination, a fixed angle inclination is adopted at each flight, along with a planar flight grid. However, relevance of inclination for getting accurate results better relates to getting images of an object from different points of view, so that, given an adequate arrange of images at different inclination angles, error propagation at triangulation between images and corresponding metric performance are optimized, as reported in close range photogrammetry studies applied to accurate industrial metrology [1]. As a result, compared to just keeping a constant nadiral flight, a more significative influence could be observed in the metric performance if the inclination angle between images was varied during each flight (that is to say, for example, getting a hemispherical UAV flight keeping the principal/optical axis of the camera radially tilted to the centre of the archaeological site, instead of designing just a planar grid for the flight). Moreover, relevance of inclination reported by the authors can be also explained through an indirect reduction of the effective overlap of information in between images, more than through a practical observation of the relevance of the inclination angle itself in between images.

As a result, authors demonstrate a valuable first step but major changes are required for getting conclusive and reproducible results, including updated post-processing and analysis. Authors are encouraged to further review and analyse the asymptotic nature of the metric performance to the overlapping between images in a flight, balancing the effort required to the increase of accuracy. These analyses could be accomplished using the measuring flights so far. A valuable discussion and relevant guideline results could be obtained. On the contrary, results so far concerning inclination angle can´t be considered conclusive and should require further testing (i.e.: hemispherical flights with varying inclination during flight vs planar flights with constant nadiral flight).

Additionally, minor changes could increase the quality of the paper:

• Intro and section 2 seem to be too long, synthetizing them might be welcomed by the reader. Mentions to the paper scope at the intro and further sections result too repetitive. In a similar way, section 2 should be shortened so that the reader does not get lost (e.g.: Taguchi method is a widely known DOE methodology that must be just referred to as adequately, no need for such a large description).
• A mention should be also included to the relevance camera self-calibration functionality of the Agisoft Metashape software. Most of the photogrammetric software (but not all of them), enables the self-calibration of the internal parameters (e.g.: focal length, radial and tangential distortion coefficients, etc.) so that low cost camera thermo-mechanical instabilities are compensated for using the redundant information available in the images [2]. Self-calibrated internal parameters should be reported for the UAV flights, so that readers aiming at reproducing the results could keep in mind the relevance of considering this self-compensating functionality (otherwise, for instance, ambient temperature changes could be also considered as a major factor affecting camera condition, and as a result, the metric performance of a flight).
• A reference should be included to the acknowledged use of length stadias.
• Depicting the flights in 3D (set of images) for a representative flight could help the reader visualizing the flights themselves, as well as getting insight into the flight grids themselves, which are the practical depiction and result of the variables under study (e.g.: two flights with significative difference in their traversal overlapping between images, describing the difference in the total number of images of each flight). In the same way, having such pictures presented at a very early stage of the paper will help the reader (first figure appears already at page 9).
• Terminology: refer better to “principal axis” instead of “focal length” when presenting the inclination angle variable at section 2 (line 196).

Bibliographical references:

[1] Luhmann, T., Robson, S., Kyle, S., & Harley, I. (2006). Close range photogrammetry. Whittles, Caithness.

[2] Mendikute, A., Yagüe-Fabra, J. A., Zatarain, M., Bertelsen, Á., & Leizea, I. (2017). Self-calibrated in-process photogrammetry for large raw part measurement and alignment before machining. Sensors, 17(9), 2066.

Author Response

Dear Reviewer,

First of all we would like to thank your comments, they have helped us to improve the understability of our work. Please, find below our answer to your observations and the list of modifications that we have made. To ease its reading, a PDF file has been uploaded, but the text is the same.

“The research aims at the optimization of a subset of significant variables influencing the accurate metric performance of a photogrammetric UAV flight, namely: height off light, longitudinal and traversal overlap, and inclination of the principal optical axis to the nadir. The 3D virtualization of an archaeological site is adopted as a relevant case study. A DOE is accomplished (Taguchi) for specifying the set of experiments under different measuring conditions, and metric performance is evaluated in each one compared to convenient metric references (as length measuring errors to reference stadias). Despite the soundness and novelty of the research topic (i.e.: evaluating the set of variables affecting the metric performance of a photogrammetric flight), a major review is requested according to the following:

• Adopting a maximal overlap between images, along with a constant nadiral inclination during a measuring flight are concluded as the optimal set of measuring variables under study. However, concerning image overlap, this result seems to be straightforward without the need of accomplishing any DOE for determining an optimum. The greater the overlap between images is, the greater information is available in the total set of images, given that additional images are taken in a flight with more overlapping percentage for covering a constant measuring field (indeed, authors do not refer to the total images taken at each flight). Authors should better report the number of images and corresponding flight and computing times required for each overlapping level and balance that effort to the increase of metric performance (which could be expected to be asymptotical, that is to say, with no practical increase of metric performance after an optimal traversal overlap).”

 

Although we agree in the fact that a maximum overlap between images could produce the best photogrammetric flight, the application of Taguchi DOE Method is not aimed at obtaining the best flight, but the best possible flight with the optimized combination of the initial set of parameters and levels. Regarding the overlap, it is worth mentioning that this work highlights that the longitudinal overlap offers very little differences in terms of variability between 70% and 80%, which could contribute to maintain quality and reduce costs. Table 5 (Page 11) has been included to report the number of images and times of flight and data management.

 

• “Concerning angle inclination, a fixed angle inclination is adopted at each flight, along with a planar flight grid. However, relevance of inclination for getting accurate results better relates to getting images of an object from different points of view, so that, given an adequate arrange of images at different inclination angles, error propagation at triangulation between images and corresponding metric performance are optimized, as reported in close range photogrammetry studies applied to accurate industrial metrology [1]. As a result, compared to just keeping a constant nadiral flight, a more significative influence could be observed in the metric performance if the inclination angle between images was varied during each flight (that is to say, for example, getting a hemispherical UAV flight keeping the principal/optical axis of the camera radially tilted to the centre of the archaeological site, instead of designing just a planar grid for the flight). Moreover, relevance of inclination reported by the authors can be also explained through an indirect reduction of the effective overlap of information in between images, more than through a practical observation of the relevance of the inclination angle itself in between images.

As a result, authors demonstrate a valuable first step but major changes are required for getting conclusive and reproducible results, including updated post-processing and analysis. Authors are encouraged to further review and analyse the asymptotic nature of the metric performance to the overlapping between images in a flight, balancing the effort required to the increase of accuracy. These analyses could be accomplished using the measuring flights so far. A valuable discussion and relevant guideline results could be obtained. On the contrary, results so far concerning inclination angle can´t be considered conclusive and should require further testing (i.e.: hemispherical flights with varying inclination during flight vs planar flights with constant nadiral flight).”

Indeed, the inclination of sensor has a special relevance for the generation of images with different points of view, but especially when dealing with elements with pronounced vertical component (facades) o flights with close objects. References to works that have been developed with different angles (perpendicular to the surface of the element to be captured, oblique shots and the combination of both alternatives) are included in the current draft (References [15-17], Page 5, lines 200-201). These circumstances are not present in this particular site, which is practically horizontal. Hence, the final conclusion is that the nadiral flight is the most interesting alternative given the characteristics of the site.

Although the metric has been considered as useful variable to select the best possible flight, this research is not aimed at analyzing the model of propagation of the error, but to obtain a methodology to define the flight parameters that allow generating the best possible flight. Otherwise, working with metric cameras would be advisable. The relevance of the inclination angle could be attributed to the indirect reduction of the overlap, but the application of Taguchi DOE Method, and the consideration of the contributions of the different parameters justify the relation between the reduction of the angle and the increase of the signal-to-noise ratio, which indicates the improvement in the quality.

Additionally, minor changes could increase the quality of the paper:

• “Intro and section 2 seem to be too long, synthetizing them might be welcomed by the reader. Mentions to the paper scope at the intro and further sections result too repetitive. In a similar way, section 2 should be shortened so that the reader does not get lost (e.g.: Taguchi method is a widely known DOE methodology that must be just referred to as adequately, no need for such a large description).”

The introduction and section 2.2. have been reduced. Further mentions to the aim through the text have been avoided (Page 6, line 251; Page 7, line 299, of the current draft). Introduction has been edited in lines 31, 39.44,45,46,52-53,57-58,68,71-75, 77,78, 81 and 82 of the original draft, so as to reduce the total length from 54 lines to 49. Regarding section 2, the explanation about Taguchi DOE Method has been simplified (from 91 lines to 68): Noise and control factors are mentioned instead of defined (lines 224-226 of the original draft). The main stages to be followed when applying the method are not related (lines 241-250) and Table 1 (relation between arranges and number of factors has been deleted. (line 258 of the original draft).

• “A mention should be also included to the relevance camera self-calibration functionality of the Agisoft Metashape software. Most of the photogrammetric software (but not all of them), enables the self-calibration of the internal parameters (e.g.: focal length, radial and tangential distortion coefficients, etc.) so that low cost camera thermo-mechanical instabilities are compensated for using the redundant information available in the images [2]. Self-calibrated internal parameters should be reported for the UAV flights, so that readers aiming at reproducing the results could keep in mind the relevance of considering this self-compensating functionality (otherwise, for instance, ambient temperature changes could be also considered as a major factor affecting camera condition, and as a result, the metric performance of a flight).”

Mention to the advantages of the autocalibration feature and, in order to allow the repetition of the experiments, the particular parameters for this camera have been provided. (Page 12, lines 406-410 and Table 6).

• “A reference should be included to the acknowledged use of length stadias.”

References 32 and 33, which apply the stadia as standardized element for surveying purposes, have been included (Page 12, line 391. Page 20, lines 665-668).

• “Depicting the flights in 3D (set of images) for a representative flight could help the reader visualizing the flights themselves, as well as getting insight into the flight grids themselves, which are the practical depiction and result of the variables under study (e.g.: two flights with significative difference in their traversal overlapping between images, describing the difference in the total number of images of each flight). In the same way, having such pictures presented at a very early stage of the paper will help the reader (first figure appears already at page 9).”

Figure 6 (page 11, it has not been possible to insert it before) has been included to provide and enhanced approach to the issue. This figure comprises the flight plans of three different itineraries that are characterized by different conditions of height and overlap.  

• “Terminology: refer better to “principal axis” instead of “focal length” when presenting the inclination angle variable at section 2 (line 196).”

It has been modified (Page 4, Line 191 of the current draft).

Please, find annexed at the end of this document the whole set of changes that have been developed in the manuscript.

We hope that these changes meet your criteria. Once again, we would like to show our gratitude for your comments.

Kind regards,

 

Dr. Rubén Pérez Álvarez.

Corresponding Author.

 

RELATION OF MODIFICATIONS:

 

Please note that these references are defined according to the numbers of page and line according to the document with the track changes function enabled.

 

[Page 1, line 30]: “In this regard” has been removed to simplify the redaction of this section.  

[Page 1, line 39]: “If required” has been removed to simplify the redaction of this section.  

[Page 2, line 44]: “stands out greatly in the field of archaeology. This technique” has been removed to simplify the redaction of this section.

[Page 2, line 45]: “, as it has been mentioned” has been removed to simplify the redaction of this section.  

[Page 2, line 46]: “itself” has been removed to simplify the redaction of this section.  

[Page 2, line 52]: “, with all the advantages that it implies, as it has been mentioned.” has been removed to simplify the redaction of this section.  

[Page 2, line 57]: “are the prime matter for the development of the previously mentioned outputs. Due to this, the images” has been removed to simplify the redaction of this section.  

[Page 2, line 68]: “establishes” instead of “established”.

[Page 2, line 71]: “, such as the height of flight, longitudinal and transversal overlap, speed of light, sensor features, climatic conditions, etc. There is a set of parameters or technical factors that must be considered when dimensioning the flight project” has been removed to reduce the introduction.

[Page 2, line 74]: “The definition of these technical factors” instead of “This definition”.

[Page 2, line 75]: “Themselves” has been removed.

[Page 2, line 77]: “and” has been removed.

[Page 2, line 77]: “On this purpose” has been removed.

[Page 79, line 79]: The acronym “DOE” has been included.

[Page 5, line 196]: “angle” instead of “angled”.

[Page 5, line 196]: “principal axis” instead of “focal length”.

[Page 5, line 204]: The following sentence and references have been included “In this regard, and considering the relative position of the surface the element to be captured and the principal axis of the camera [15], examples of perpendicular[16], oblique and a combination of both types of shots can be found in literature [17].”

[Page 5, lines 215, 222, 223, 224, 226]: The numbers of the references has been adjusted.

[Page 5, line 228]: The sentence has been simplified, avoiding the explanation of control and noise factors.

[Pages 5-6, line 244-253]: The stages of Taguchi DOE Method have been removed to reduce this section.  

[Page 6, line 261]: This paragraph referred to Table 1. Both of them have been removed to simplify this paragraph.

[Page 6, line 266-268; Table 1]. The number of the table has been adjusted.

[Page 7, line 283]: The reference to the aim of this research has been avoided to reduce redundancies through the text.

[Page 7, line 286]: The reference order has been updated.

[Page 7, line 299]: The table that is referenced has been modified (Table 2 instead of Table 3).

[Page 7-8, lines 300-301]: The following sentence and references have been introduced to justify the values that have been adopted for the different parameters and levels: “The levels for each parameter range according to the recommendations and values applied in several works of research related to the application of aerial photogrammetry with UAV to the analysis of archaeological sites and structures [1, 28-31]”.

[Page 8, line 303]: The number of the table has been corrected.

[Page 8, lines 310, 312]: The number of the table has been corrected.

 [Page 8, lines 312-312]: The reference to the aim of this research has been avoided to reduce redundancies through the text.

[Page 8, line 327]: References [32-33] have been included to justify the idoneity of stadias as standardize elements.

[Page 9, line 347]: “North” instead of “Nort”.

[Page 9, lines 347,356; Page 10, line 368]: Reference number have been updated.

[Page 11, lines 385 and 392]: The number of the references to the tables have been updated.

[Page 11, lines 392-394]. Figure 3, Table 5 and the following statement have been introduced to provide an idea about the influence of the combination of parameters in the complexity of the flights: “. The combination of factors affects the complexity of the project in a substantial way (Figure 4, Table 5).”

[Page 12, line 402]: The number of the reference to the table has been edited.

[Page 12, line 406]: “reduce” instead of “guarantee the absence of”.

[Page 12, lines 410 and 419]: Reference to the figure has been updated.

[Page 12, line 421]: The number of the figure has been edited.

[Page 12, line 422]: The reference to the figure has been updated.

 [Page 13, line 423]: The number of the figure has been modified.

[Page 13, line 430]: “Agisoft Metashape” instead of “Agisoft”.

[Page 13, lines 430-437]: The importance of the autocalibration feature and the self-calibration parameters (Table 6) have been introduced.

[Page 14, line 447]: The reference to the table has been modified.

[Page 14, line 451]: The reason why some stadias were impossible to measure, and its minimal incidence in the final calculations, have been noted.

 [Page 14, lines 458 and 464; Page 15, lines 469, 489, 493 and 496]: The references to tables and figures have been updated.

[Page 16, line 499]: The figure number has been modified.

[Page 16, lines 503, 506, 510 and 516]: The references to figures 7, 8 and 9 have been modified.

[Page 16, line 517; Page 17, lines 519-520]: Figure 9 has been modified to offer a comparison between the best and worst flight in terms of the model quality.

[Page 17, line 538]: The possibility to optimize the overlap due to the low difference in the variabilities between 70 and 80% is emphasized. “which allows adopting the first value to reduce costs while maintaining quality”.

[Pages 19-20, lines 611, 618, 621, 624, 628, 631, 634, 638, 643, 646]. The references have been edited to adequate the format of the volumes and issues.

The following references have been included:

Gómez-López, J.M.; Pérez-García, J.L.; Mozas-Calvache, A.T.; Delgado-García, J. Mission flight planning of RPAS for photogrammetric studies in complex scenes. ISPRS Int. J. Geo-Inf. 2020, 9, 392. doi:10.3390/ijgi9060392

Taddia, Y.; González-García, L.; Zambello, E.; Pellegrinelli, A. Quality Assessment of Photogrammetric Models for Facade and Building Reconstruction Using DJI Phantom 4 RTK. Remote Sens. 2020, 12, 3144. doi: 10.3390/rs12193144.

Piech, I.; Ruzyczka, A. Generating of building facades orthophotoplans with UAV and terrestrial photos. IOP Conf. Ser.: Earth Environ. Sci. 2019, 221 , 012074. doi:10.1088/1755-1315/221/1/012074.

Morgenthal, G.; Hallermann, N. Quality assessment of unmanned aerial vehicle (UAV) based visual inspection of structures. Adv. Struct. Eng. 2014, 17 (3), 289–302. doi: 10.1260/1369-4332.17.3.289

Stek, T. D. Drones over Mediterranean landscapes. The potential of small UAV’s (drones) for site detection and heritage management in archaeological survey projects: A case study from Le Pianelle in the Tappino Valley, Molise (Italy). J Cult. Heritage 2016, 22, 1066–1071. doi: 10.1016/j.culher.2016.06.006

Laguela, S.; Diaz-Vilarino, L.; Roca, D.; Lorenzo, H. Aerial thermography from low-cost UAV for the generation of thermographic digital terrain models. Opto-Electron. Rev. 2015, 23 (1), 76–82. doi: 10.1515/oere-2015-0006.

Rakha, T.; Gorodetsky, A. Review of Unmanned Aerial System (UAS) applications in the built environment: Towards automated building inspection procedures using drones. Autom. Constr. 2018, 93, 252–264. doi: 10.1016/j.autcon.2018.05.002

Elsadig Ali, A. Accuracy of stadia tacheometry with optical theodolites and levels. J. King Saud Univ.-Eng.Sci. 1995, 7(2), 175–184.  doi: 10.1016/S1018-3639(18)30625-1.

Ibraheem, A.Th.; Hasim Mehdi, A.; Adil Najeeb, Z. The utilization of stadia measurements for different constructions works. Int. J. Tech. Res. Appl. 2015, 3(6), 37–44.

 

The following reference was removed to simplify Section 2.

 

151. Sharma, P.; Verma, A.; Sidhu, R.K.; Pandey, O.P. Process parameter selection for strontium ferrite sintered magnets using Taguchi L₉ orthogonal design. J. Mater. Process. Technol. 2005, 168(1), 147–151. doi: 10.1016/j.jmatprotec.2004.12.003.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for the replies and changes. I think it is an interesting approach the use the Taguchi Design of Experiments Method to optimize Photogrammetric Flights. 

I would recommend the authors, if possible, in their following research to realize the proposed optimal flight. I believe that this flight should have been realized and the virtualization results presented in this manuscript.

Author Response

Dear Reviewer,

First of all, we would like to show or gratitude for your kind comments, time and contribution to the enhancement of our manuscript. We will keep in mind your recommendation and will develop the optimal flight in subsequent works. Due to time and climatic constraints, we have not been able to expand the initial campaign as desired.

Considering the changes with respect to the previous draft, the following lines have been included to highlight the importance of the autocalibration function of the software, and to discuss the requirements in terms of the effort needed to generate the model derived from best flight among those developed:

[Pages 12-13, lines 408-417]: “One of the advantages of this software is related to the autocalibration function. By applying the Exchangeable Image File Format (EXIF) data of the images, it allows a quasi-automatic self-calibration. This permits solving an important problem in the field of photogrammetry by means of analytic processes [37], as it improves the internal model and physical parameters of the camera, and minimizes the influence of the variables that affect its metric performance [38]. This is particularly indicated for low-cost cameras. The autocalibration developed with Agisoft Metashape determines the focal length, the principal point, and the coefficients that are obtained from a polynomial adjustment that is aimed at solving the radial and tangential distortions. The focal length and the coordinates of the principal point can be expressed in mm or pixels, while the coefficients are dimensionless parameters.”

[Page 17, lines 541-554]: “Time requirements and their relation with the design of the flight should be noticed. The comparison among the flights that have been developed shows that the differences between those with the best and the worst metric qualities (flights 4 and 5, Table 5) are just 23 images, one minute of flight, and 23 minutes of data processing. The best flight in terms of metric performance required 58 images, 10 minutes of acquisition and 95 minutes of processing. All the foregoing implies that the difference in the total amount of time needed for the generation of the worst model is only 22,85% lower than that needed to get the best one within the initial set. In addition to the foregoing, the difference in terms of effort between the flights with the highest and lowest requirements (flights 8 and 3, Table 5) is wide. If the efforts needed to generate the models from the different flights are compared, the requirements related to the best one in terms of metric performance is well positioned and perfectly assumable. Hence, it can be stated that it is well balanced in terms of effort. A deeper analysis of the effort could be developed by setting another experiment based on the application of Taguchi DOE Method, which should be the aim of another research.”

[Page 20, lines 701-704]. References [37] and [38] have been included.

We hope that these changes meet your criteria. The attached PDF file comprises the same information that is included here.

Once again, thank you for your valuable contribution to the improvement of this work.

Kind regards,

The Corresponding Author.

Author Response File: Author Response.pdf

Reviewer 2 Report

Thanks to the authors for adequatelly considering the comments and the significant improvement on the paper, specially concerning the more precise definition of the paper scope and aims. New pictures are really wellcome (specially Figure 4).

Minor changes are recommended to the reviewed version:

1) Table 5. Measuring effort figures are now presented (number of images, processing time). A discussion should be included (e.g.: in section 4) concerning which might be the optimal flight configuration that better balances metric performance to the required measuring effort. Setting a feasible or desirable limit to the measuring effort for the case under study could be helpful. 

2) Table 6. Self-calibrated internal camera parameters are now reported (Brown´s model [1]). Units are pending for radial and tangential coefficients. A more complete description is pending in section 3.3 related to the relevance of the camera internal parameter self-calibration capability. This funcitonality [2], along with the adoption of an adequate model for camera internal model characterization (e.g.: Brown for Agisoft), enables minimizing the influence of variables affecting the camera condition (e.g.: temperature changes between flights), and thus, the resulting metric performance. That is to say, this funcionality enables indirect control and in-process compensation of a relevant variable (i.e.: changes in camera condition between flights). Due to its relevance, authors are encouraged to further describe this fact.

[1] Brown, D.C. Decentering distortion of lenses. Photogramm. Eng. Remote Sens. 1996, 32, 444–462.

[2] Mendikute, A., Yagüe-Fabra, J. A., Zatarain, M., Bertelsen, Á., & Leizea, I. (2017). Self-calibrated in-process photogrammetry for large raw part measurement and alignment before machining. Sensors, 17(9), 2066.

Author Response

Dear Reviewer,

First of all, the Authors would like to express their gratitude for your time and helpful contribution to the enhancement of this manuscript. Please, find the response to your comments in the following lines:

“Thanks to the authors for adequatelly considering the comments and the significant improvement on the paper, specially concerning the more precise definition of the paper scope and aims. New pictures are really wellcome (specially Figure 4).

Minor changes are recommended to the reviewed version:

1)Table 5. Measuring effort figures are now presented (number of images, processing time). A discussion should be included (e.g.: in section 4) concerning which might be the optimal flight configuration that better balances metric performance to the required measuring effort. Setting a feasible or desirable limit to the measuring effort for the case under study could be helpful.” 

The following bullet has been included to discuss the requirements in terms of effort in section 4:

[Page 17, lines 541-554]: “Time requirements and their relation with the design of the flight should be noticed. The comparison among the flights that have been developed shows that the differences between those with the best and the worst metric qualities (flights 4 and 5, Table 5) are just 23 images, one minute of flight, and 23 minutes of data processing. The best flight in terms of metric performance required 58 images, 10 minutes of acquisition and 95 minutes of processing. All the foregoing implies that the difference in the total amount of time needed for the generation of the worst model is only 22,85% lower than that needed to get the best one within the initial set. In addition to the foregoing, the difference in terms of effort between the flights with the highest and lowest requirements (flights 8 and 3, Table 5) is wide. If the efforts needed to generate the models from the different flights are compared, the requirements related to the best one in terms of metric performance is well positioned and perfectly assumable. Hence, it can be stated that it is well balanced in terms of effort. A deeper analysis of the effort could be developed by setting another experiment based on the application of Taguchi DOE Method, which should be the aim of another research.”

“2) Table 6. Self-calibrated internal camera parameters are now reported (Brown´s model [1]). Units are pending for radial and tangential coefficients. A more complete description is pending in section 3.3 related to the relevance of the camera internal parameter self-calibration capability. This funcitonality [2], along with the adoption of an adequate model for camera internal model characterization (e.g.: Brown for Agisoft), enables minimizing the influence of variables affecting the camera condition (e.g.: temperature changes between flights), and thus, the resulting metric performance. That is to say, this funcionality enables indirect control and in-process compensation of a relevant variable (i.e.: changes in camera condition between flights). Due to its relevance, authors are encouraged to further describe this fact.

[1] Brown, D.C. Decentering distortion of lenses. Photogramm. Eng. Remote Sens. 1996, 32, 444–462.

[2] Mendikute, A., Yagüe-Fabra, J. A., Zatarain, M., Bertelsen, Á., & Leizea, I. (2017). Self-calibrated in-process photogrammetry for large raw part measurement and alignment before machining. Sensors, 17(9), 2066.”

Section 3.3 has been expanded with the following passage, which considers those aspects:

[Page 17, lines 541-554]: “Time requirements and their relation with the design of the flight should be noticed. The comparison among the flights that have been developed shows that the differences between those with the best and the worst metric qualities (flights 4 and 5, Table 5) are just 23 images, one minute of flight, and 23 minutes of data processing. The best flight in terms of metric performance required 58 images, 10 minutes of acquisition and 95 minutes of processing. All the foregoing implies that the difference in the total amount of time needed for the generation of the worst model is only 22,85% lower than that needed to get the best one within the initial set. In addition to the foregoing, the difference in terms of effort between the flights with the highest and lowest requirements (flights 8 and 3, Table 5) is wide. If the efforts needed to generate the models from the different flights are compared, the requirements related to the best one in terms of metric performance is well positioned and perfectly assumable. Hence, it can be stated that it is well balanced in terms of effort. A deeper analysis of the effort could be developed by setting another experiment based on the application of Taguchi DOE Method, which should be the aim of another research.”

The references that you kindly suggested us enhance this section have been included in this paragraph and the list of references. [Page 20, lines 701-704. References 37 and 38].

We hope that these changes meet your criteria. The attached PDF includes the same information that is related here. Once again, thank you for your time and contribution to the improvement of this work.

Kind regards,

The Corresponding Author.

Author Response File: Author Response.pdf