**1. Introduction**

During the last decade, UAV-imaging has developed rapidly, enabling advancements in three-dimensional (3D) modeling for preservation, documentation, and management of cultural heritage sites [1,2]. Terrestrial surveying using terrestrial laser scanners is accepted for producing high-quality cultural heritage images, while the advent of robotic total stations has enabled researchers to gather large quantities of data more easily than ever before [3–6]. Unfortunately, the use of such geomatics-based approaches requires considerable expertise and a large surveying budget [2]. Alternatively, unmanned aerial vehicles (UAVs) can be used to easily and feasibly acquire 3D models. Furthermore, imaging using UAVs can be conducted economically and offers an ideal means for surveying complex archaeological sites [7–9]. There are different forms of UAVs, including fixed-wing [10], multi-rotor [11], and gyro. Furthermore, current platforms such as quadrotor [12] are more robust than their previous forms, due to their vertical take-off and landing capability, insensitivity to varying environments, high mobility and stability, and ease of operation [13,14]. The combination of computer vision and photogrammetry present in [15] has the following advantages: (1) Remote control systems, permitting the UAV to be perfectly positioned to collect images at varying heights and angles; (2) different sensors, ensuring that different image types can be used, including infrared,

visible-spectrum, and thermal images captured from both calibrated and non-calibrated cameras, and (3) high-quality outcomes, enabling a researcher to control the reliability and accuracy of the results.

This study focuses on the low cost, portability, and completeness provided by the unmanned aerial vehicle structure-from-motion (UAV-SfM) method for cultural heritage site management. Two pagodas in Wat Maha That, Thailand (Chedi and Prang) are considered as a case study. The results demonstrate the accuracy of 3D models obtained using ground control points (GCPs), checkpoints (CPs), and terrestrial laser scanning (TLS). The study aims to apply the UAV-SfM method, which is cost-effective, convenient, and provides acceptable accuracy for building the 3D models compared with traditional techniques, such as TLS. The constructed 3D model could be employed in different applications in the future, such as tourism and generating the rapid mapping of cultural heritage in a short period, while ensuring acceptable quality.

## **2. Previous Work**

Several previous studies have sought to use UAV technology for archaeological projects. Among these, Brutto [16] used the microdrone MD4-200 and the Sensefly Swinglet CAM UAV to photogrammetrically record the Temple of Isis. The equipment used in this case consisted of two UAV systems with different performance and characteristics fitted using a global station. Nadhirah and Khairul [17] proposed the use of UAV photogrammetry as a tool to capture and generate a 3D model in Negeri Sembilan, Malaysia.

Several other authors have used UAVs with certain geotechnological approaches; for example, Ebolese [18] used UAVs to produce a high-resolution 3D model of Lilybaeum, the ancient city of Marsala in Southern Italy. Chiabrando [19] studied an archaeological site in Hierapolis, Phrygia, Turkey, using UAV photogrammetry to collect aerial images of the site. Stek [20] used an inexpensive method involving drones to collect particularly high-quality spatial geoinformation, which could be employed to study the landscape archaeology at Le Pianelle in the Tappino Valley of Molise, Italy. Bolognesi [21] used a remotely piloted aircraft system for aerial photogrammetry at the Delizia del Verginese Castle of Italy. This was combined with a terrestrial laser scanner to generate a 3D archaeological model.

Research indicates that UAVs can be used for low-altitude imaging and remote sensing of spatial data [7,22–24]. UAVs are used for exploring cultural heritage sites because they are reliable and easy to use [25,26]. The latest developments in photogrammetry technology provide a simple, cost-effective manner to create a relatively accurate 3D model from 2D images [27,28]. These techniques provide a new set of tools for cultural heritage professionals to capture, store, process, share, and display images and annotate 3D models in the field. A review by Colomina and Molina [7] showed that the use of UAVs in the exploration of cultural heritage is increasing, owing to the ease of use and quality of processed measurements.

## **3. Materials and Methods**
