# Optimization of Number of GCPs and Placement Strategy for UAV-Based Orthophoto Production
## 1 Introduction
If orthophotos or DEMs are produced in units of cities of districts without GCPs or other control points, significant geolocation errors may occur. In order to minimize such errors, an analysis from a previous study [11] reported that GCPs could be installed to guarantee a certain degree of precision and reduce the error rate. In summary, GCPs play an important role in ensuring the accuracy, precision, and reliability of orthophotos produced with UAV-based platforms, providing the necessary references for georeferencing and contributing to the overall quality of mapping or surveying results. With a growing demand for accurate and up-to-date geospatial information/data, there is a clear need to establish a standardized methodology for UAV-based orthophoto production. An important challenge is to identify the ideal density, spatial distribution, and optimal number of GCPs considering changes in topography, land cover, and characteristics of the UAV used and the target site. Georeferencing involves assigning real-world coordinates to digital data (usually images or maps). A GCP functions as a key player in this process by providing a known reference location. The geolocation of GCPs is measured using surveying techniques that provide accurate and known coordinates for a specific point on the Earth's surface. Commonly used methods include real-time kinematic (RTK) GPS, high-precision global navigation satellite system (GNSS) receivers, and terrestrial light detection and ranging (LiDAR) [12]. Among these methods, RTK GPS is often considered one of the most reliable methods for achieving high positioning accuracy. RTK GPS uses satellite signals for the real-time determination of the exact location of a GPS receiver. By using a base station with known coordinates to transmit corrected data to the rover (GPS receiver of a GCP), centimeter-level resolution/accuracy can be achieved. The absence of clear guidelines in general regulations/specifications about measuring geolocations of GCPs for georeferencing may result in suboptimal georeferencing, affecting the overall quality of the resulting orthophoto. According to the current version of the Guideline for Public Survey Using Unmanned Aerial Vehicles in Korea, the requirements for the installation of the target signboard for aerial surveying and the number of GCPs are set as nine points per km\({}^{2}\) in principle. However, it can be verified that a larger than necessary number of GCPs against the size of the target site or inefficient placement of GCPs would result in wasted time and manpower. Therefore, in this study, we aim to achieve improved accuracy by adjusting the number and placement of GCPs to increase their process efficiency and speed. However, considering the previous studies conducted to date, although there has been a range of comparative evaluations or analyses on the impact of having no GCPs or GCPs on the X and Y coordinates, few studies have investigated the impact of the number and distribution of GCPs on the accuracy of the resulting orthophoto. Therefore, considering the potential for utilization in practical applications, more in-depth research is needed on the impact of the number and distribution of GCPs. Against this backdrop, in this study, we aim to analyze the impact of the number and distribution of GCPs on the accuracy of orthophoto production based on images acquired by UAVs, to understand the impact on the quality of the survey results. In addition, based on the results, we propose the optimal number per unit area and distribution of GCPs. If the findings of this study can contribute to an improved accuracy in UAV-based photogrammetry, it is expected that UAV-based platforms can be more extensively utilized in a range of applications such as process control at construction sites or facility inspection, as well as surveying, building status surveys, and investigations of unauthorized buildings.