Accurate and Cost-Efficient 3D Modelling using ...

Accurate and Cost-Efficient 3D Modelling using Motorized Hexacopter, Helium Balloons and Photo Modelling: a Case Study 1

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Britt Lonneville , Berdien De Roo1, Cornelis Stal1, Bart De Wit , Alain De Wulf1, 1 Philippe De Maeyer 1

Department of Geography, Ghent University, Krijgslaan 281 (building S8), 9000 Ghent, Belgium

{Britt.Lonneville, Berdien.DeRoo, Cornelis.Stal, Bart.DeWit, Alain.DeWulf, Philippe.DeMaeyer}@ugent.be

Abstract. The destructive nature of archaeological excavations and the spatial character of archaeological finds make 3D models valuable contributions to the documentation of archaeological information. Laser scanning allows highly accurate 3D reconstructions, but involves considerable costs and expert knowledge. Therefore, photo modelling could be considered as a useful alternative. In this paper, we will demonstrate on the one hand the cost-efficiency and the consequent time-efficiency of the technique and on the other hand its (sub-decimeter) accuracy. Furthermore, the possibilities and advantages of motorized unmanned aerial vehicles (UAV) and helium balloons as airborne platform for image acquisition are shown. For this purpose, a case study is performed at the Mayan archaeological site of Edzna (Mexico). Using the Structure from Motion (SfM) and Multi-View Stereo (MVS) algorithm, terrestrial and aerial photographic recordings are processed into the final 3D models. For the quality assessment the photographic recordings are supplemented with topographic measurements. Keywords: Data Acquisition, 3D, SfM-MVS, Hexacopter, Helium Balloon.

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Introduction

Recently, 3D representations of real-world objects have gained a significant importance. This results from both increasingly accurate acquisition methods, such as laser scanning and photo modelling, improved computer performance and availability of processing software. Multiple disciplines such as geology, civil engineering and archaeology benefit from these 3D models and their applications. Considering the destructive nature of archaeological excavations and the spatial component of archaeological finds, such 3D models significantly contribute to the conservation of archaeological information. They allow archaeologists to revisit the site in a virtual space after the excavation has been concluded [1]. This way, all elements can be studied in their original configuration and context. Photo modelling is considered as a

useful technique to generate cost-efficient and photo-realistic 3D visualizations of archaeological objects [2]. The resulting 3D models could for the basis of digital heritage systems [3]. The images used for photo modelling can be processed using Structure from Motion (SfM) and Multi-View Stereo (MVS) [4]. The SfM algorithm determines the displacement of the camera (motion) and hereby reconstructs a sparse point cloud (structure). MVS calculates a dense point cloud and creates a 3D mesh. Afterwards, some software allow the construction of texture maps. Both free (e.g. Bundler) and commercial (e.g. Agisoft PhotoScan) software are available to process these 3D models [5, 6]. However, the application of SfM and MVS imposes some constraints concerning the acquisition procedure. Images should contain sufficient overlap, although panoramic imagery cannot be used for this purpose. Furthermore, weather conditions could limit the possibilities of this method, as with other methods. Barazzetti et al. describe the problems due to sunny weather, high temperature and refraction, as these affect the quality of the images [7]. Consequently, the data acquisition should preferably been done on a cloudy day. However, laser scanning is faced with the same limitations. Next to these environmental effects, physical barriers can also obstruct the data acquisition, e.g. the inaccessibility of an archaeological site, the presence of obscuring features like trees and buildings in the environment of the object or the geometrical complexity of the structures. In these cases, the use drones have shown great promise in several contexts [8, 9]. These platforms are equipped with a digital camera and are controlled either manually, semi-automatically or automatically [10]. Drones, however, might not be applicable in any given situation due to local legislation or difficulties concerning the control over the platform. In such cases, a system using helium balloons could provide a useful alternative. For this research, the use of this 3D modelling methods, was applied during a 3D measuring campaign of the archaeological Mayan site of Edzna (Mexico). In order to create accurate and qualitative 3D models, two types of Unmanned Aerial Vehicles (UAV) were deployed: a motorized hexacopter and helium balloons. These platforms allowed the researchers to take aerial pictures of the different structures, which provided a considerable added value to the project. This project paper focuses on the level of accuracy and cost-efficiency gained by using photo modelling and the added value of airborne platforms in this regard. In paragraph 2, the study area is delimited. Paragraph 3 provides a description of the data acquisition procedure and the subsequent processing of the resulting 3D models. The quality assessment of these models is given in paragraph 4. Finally, paragraph 5 discusses these results in light of the research questions and reaches a conclusion.

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Archaeological Site of Edzna

The study area consists of the Mayan site of Edzna, which is located on the Mexican peninsula of Yucatan at approximately 50 km from the state capital Campeche (Fig. 1). Edzna – which signifies either ‘House of the Itzas’, ‘House of the grimace’ or ‘House of the echo’ – was inhabited from 600 BC until 1450 and had a population of

25 000 citizens at its climax [11]. The structures were influenced by the Puuc construction style, which was also detected at the temples of Uxmal and Chichen Itza.

Fig. 1. Location of the Mayan site of Edzna.

Fig. 2. Overview of the Mayan site of Edzna.

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Data Acquisition and Processing

The measuring campaign in Mexico took place in November 2013 and comprised both topographic measurements and photographic recording. Preliminary processing was executed on site to check the coverage and the data quality. The final processing was conducted at Ghent University (Belgium) during the following months. 3.1

Terrestrial Topographic Measurements

The aim of the terrestrial topographic measurements was georeferencing the 3D models and providing data for the quality assessment. The measurements were conducted using a Trimble M3 total station and Garmin Etrex handheld GPS. The total station has a distance accuracy of ± (2mm +2 ppm) (using a prism) and ± (3mm +2 ppm) (reflectorless), and an angular accuracy of 2”. The GPS has a two-meter accuracy in SBAS (WAAS) mode. A network of first and second order points was established in order to measure characteristic points on the buildings using the total station. As the focus of the project lay on the Great Acropolis, its constituting buildings(the Five-story building, Moon temple and North temple) were measured in great detail. Redundant points were used for the quality assessment. The remaining buildings were measured in order to create a complete, locally referenced overview of the site. The total station measurements were processed using Octopus. GPS measurements were used to obtain WGS 84 coordinates for the first and second order points. As the accuracy of these measurements was inferior to the accuracy of the total station measurements, these coordinates were used to a lesser extent. 3.2

Photographic Recording

Photographic recording was executed both through terrestrial and aerial pictures in order to obtain maximal coverage and create integral and qualitative 3D models. Using exclusively either terrestrial pictures or airborne pictures would result in incomplete models containing holes.

Fig. 3. Helium balloons (left) and motorized hexacopter (right).

Terrestrial photographic recording was conducted using two types of camera: Sony NEX-5R (16.1 MP, 16-50 mm lens, EXMOR APS-C 23.4 x 15.6 mm sensor) and Canon EOS 450D (12.2 MP, 10-22 mm lens, CMOS APS-C 22.2 x 14.8 mm sensor). Aerial photographic recording was done using two platforms: helium balloons and a motorized hexacopter (Fig. 3). The helium balloons were mainly provided in case something went wrong with the drone, as there might be unforeseen technical problems. The helium balloons were equipped with an aluminum frame holding the camera (Sony NEX-5R) and controlled by two operators. Photographic recording was executed using a ‘Timelapse’ app, which allows taking pictures automatically defining a certain interval. The use of this app was obstructed by changing weather conditions, as these required the modification of the initial settings which was impossible once the system was running. Table 1 holds the advantages and disadvantages of this system within the context of this particular research. It shows that this method is mainly useful for modelling uncomplicated buildings on sites which are accessible for the operators and when it has not to be applied frequently. Table 1.Advantages and disadvantages helium balloons. Advantages Autonomy (no batteries involved) Cost (when used only once) No need for experienced operator

Disadvantages Limited flexibility/aiming possibilities No live view Accessible site required Limitations ‘Timelapse’ app Difficulties in windy conditions

The hexacopter thus served as the main platform for aerial photographic recording. This project employed a TSH GAUI 540H hexagonal drone with a live video transmission system. The UAV was controlled by one operator and pictures were taken through the use of the ‘Timelapse’ app or a trigger function on the UAV’s remote control. The camera (Sony NEX-5R) was mounted on the drone with 0°, 30° and 60° nadir angels. This platform was particularly useful during this project. It allowed to model complex structures and its flexibility enabled to fly in densely vegetated areas. The advantages and disadvantages of this system can be deducted from Table 1, as the advantages of the helium balloons are the disadvantages of the drone and vice versa. However, both systems are influenced by windy conditions. 3.3

Modelling the Maya Site in 3D

Agisoft PhotoScan Professional was used to process both the terrestrial and aerial imagery. This software applies the SfM-MVS algorithm through a four-step workflow, as shown in Fig. 4. During this process, ground control points (GCP) known by terrestrial topographic measurements are indicated on all models in order to align them in the local coordinate system that was established on the site. To connect the terrestrial images to the aerial images, chunks are integrated in the workflow. These chunks are processed separately and joined through the use of either GCPs or manually indicated markers. By reducing the angle of the camera with respect to the

ground, the connection between both types of imagery might be accomplished automatically in the future.

Align photos

Build dense point cloud

Build mesh

Build texture

Fig. 4. Workflow Agisoft PhotoScan.

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3D Models and Quality Assessment

The quality assessment is based on the processing of the Five-story building, Moon Temple and North Temple (Fig. 2), as these buildings were topographically measured in great detail within the local network. On each building, a series of GCPs (8 to 12) is indicated. These GCPs are spread out evenly across the building and the models are adjusted based on these points. Subsequently, the coordinates of the remaining points (12 to 63) are estimated and compared to the measurements, which results in an overview of the errors in 3D.Based on this analysis, mean absolute errors (MEA) of 0.015 to 0.020 m were found with a root mean square error (RMSE) of the same magnitude. Consequently, models with a high local accuracy are available for analysis within the a priori established sub-decimeter accuracy, as shown in Fig. 5. These models are presented as well on the website ‘Edzna 3D’, in order to communicate the results of the case study to a broader audience [12]. In the future, these models will also be implemented in a web GIS in order to improve the user’s experience and provide analytical possibilities. A similar quality assessment is conducted based on the absolute coordinates of the Five-story building. The poor results of this assessment (MAE ranging from 0.498 m to 1.588 m and RMSE ranging from 0.700 m to 2.326 m) can be explained by the limited accuracy of the GPS device.

Fig. 5. 3D model Five-story building (left) and Platform of Knives (right)

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Discussion and Conclusion

Several authors establish photo modelling as a cost-efficient and accurate method in order to create 3D models of real-world objects. Furthermore, the use of airborne platforms such as drones or helium balloons could create a considerable added value. This premise was confirmed during the presented research. Photo modelling has proven to be a very cost-efficient method, as the main costs involve the purchase of a camera, a powerful computer and software licenses. In comparison to laser scanning, where a powerful computer and software licenses are required as well, the initial cost for the scanner is significantly lower. Moreover, photo modelling enables a fast acquisition and processing. Based on the configuration of the research team – one person for photographic recording and two persons for topographic measurements – one day should suffice in order to acquire and process buildings with similar dimensions. By this time-efficient character of the method the cost-efficiency is even increased. Secondly, during this case study it is proven that this method generates highly accurate 3D models. The quality assessment indicates a 3D mean absolute error of 1-2 cm for the locally referenced models, which lies within the a priori established subdecimeter accuracy limit. Consequently, photo modelling is clearly a valuable method that can be introduced into the archaeological workflow and offers archaeological researchers several advantages, among which its cost- and time-efficiency and accuracy. Furthermore, the deployment of airborne platforms forms a substantial advantage during this research. Both platforms – motorized hexacopter and helium balloons – create the possibility of recording aerial imagery and thus generating qualitative 3D models. This was essential, as the site contains several complex structures (e.g. stairs). The drone proves itself to be the most promising platform, given its flexibility and reliability. During low-cost, short term projects, however, the use of helium balloons might be considered. National legislation might also limit the import and usage of a UAV. Moreover, both platforms are susceptible to weather conditions, whereas a laser scanner is less subject to these circumstances. Nevertheless, both platforms were indispensable during this particular project and have proven to be important additions to archaeological research. In conclusion, it has become apparent that acquisition through photo modelling and the use of airborne platforms such as drones and helium balloons are promising techniques which might aid and enrich archaeological research. Considering the often limited time and budget archaeologists are granted, they enable a cost-efficient and accurate acquisition of the site and allow archaeologists to revisit any archaeological site in its original configuration. Furthermore, these visually attractive and realistic models can be used to inform and involve the general public and tourists. The developed project website takes a first step in this direction [12]. Considering the current developments in the establishment of an archaeological 3D GIS, the resulting 3D models might even gain more importance during future research and facilitate researcher’s understanding of archaeological phenomena through thorough computeraided analysis [13].

Acknowledgments. The authors would like to thank the division of the Mexican Instituto Nacional de Antropología e Historia (INAH) in Campeche for their cooperation. Furthermore, financial support from the Special Research Fund (BOF) of Ghent University and the Research Foundation Flanders (FWO) are gratefully acknowledged.

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