# The Integration of Earthwork Design Review and Planning Using UAV-Based Point Cloud and BIM
## 3 Study Area and Methods
### Earthwork Design and Data Acquisition  
The initial design was created by architectural designers based on the digital elevation map (DEM) from the National Geographic Information Institute (NGII) [24] which is officially managing DEMs for public use. As shown in Figure 1a, the draft of the site design was created (see red line). The DEM was created by aerial survey in 2005, and it has not been updated since, therefore, design review is essential prior to be constructed. In the initial stage of housing design, architects concede the DEM was conducted as a primary source unless on-site mapping (because the DEM from NGII is the only available source). The difference between the actual terrain and the DEM is typically corrected during construction. As we mentioned previously, there are several ways to verify the design. This study employed a UAV to create topographic digital terrain model. A quadcopter UAV, DJI's Inspire 2 was used to acquire aerial pictures of the site [25] that attached with a gimbal and camera, Zenmuse X4S [26], and Pix4 DmapperUAV based photogrammetry was used [27]. Ground Sample Distance (GSD) was 1.77 cm/px (flight height was 60 m). Ten ground control points (GCPs) were installed to increase the accuracy of the point cloud. The installation of the GCPs was carried out according to a guideline [28] and were easily accessible to personnel and located evenly at the target site as shown in Figure 1b. The latitude and longitude coordinates of each ground reference point were measured using the real-time kinematic-Global Positioning System (RTK-GPS) and the altitude was directly measured since even RTK GPS has low reliability on the altitude estimation. Leica Geosystem's GS18T was used for surveying. According to the specification, measurement accuracy during network RTK is Hz 8 mm + 0.5 ppm/V 15 mm + 0.5 ppm. The coordinate of the GCPs is shown in Table 1, and the coordinate was based on the Bessel ellipsoid and Transverse Mercator (TM) Korean Central Belt projections, and altitude was the result of direct level measurements using RTS. Figure 2 shows the flowchart of GCP marking. First, every UAV flight must obey the Act on Promotion of Utilization of Drones and Creation of Infrastructure Therefor [29]. In this regard, a UAV pilot must check the no-fly zone (e.g., military-related areas, near the airport, etc.) before take-off to make sure that the flight path stays out of the restricted areas. Second, GCPs (ground control points) were acquired via RTK (real-time kinematic) GPS as well as leveling using a total station. The location of the GCPs was shown in Table 1. Lastly, the GCPs were properly marked on the surface [28]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**GCPs** & **Nothing (m)** & **Easting (m)** & **Elevation (m)** \\ \hline
1 & 354,794.355 & 211,658.751 & 72.34 \\ \hline
2 & 354,788.336 & 211,653.043 & 71.959 \\ \hline
3 & 354,736.735 & 211,687.548 & 75.175 \\ \hline
4 & 354,723.373 & 211,703.816 & 74.206 \\ \hline
5 & 354,768.343 & 211,679.64 To acquire the point cloud, aerial photography using UAVs was carefully planned during the planning phase. An aerial photography plan was established to prevent the collisions of UAVs, include all sites, and ensure the quality of data by considering the presence of obstacles, flight altitude, long corridor, flight time, optimal route, and light during UAV take-off and landing. Figure 3 shows the results of the flight plan considering the presence of obstacles, flight altitude, long corridor, flight time, etc. It was able to cover the entire site with one flight and flew at an altitude of 60 m above the mountainous area. At the time of aerial photography, the weather was clear, and it flew in relatively stable flight conditions with a wind speed of 2.6 m/s and a temperature of 24 \({}^{\circ}\)C. The flight time zone used the minimum shadow time across the site around 3 p.m., and the overlap of the picture was set at 90% longitudinal and 90% lateral. The original earthwork design was established based on the DEM from the NGII.