# Optimization of Number of GCPs and Placement Strategy for UAV-Based Orthophoto Production
## 1 Introduction  
In recent years, the development of various types of unmanned aerial vehicles (UAVs) has led to the active use of UAV-based platforms or systems across various industrial sectors. In particular, there has been a significantly increased interest in the use of UAVs to acquire low-altitude images [1, 2]. UAV-based aerial photogrammetry has emerged as a relatively low-cost alternative to the traditional manned aerial photogrammetry, enabling fast and efficient data acquisition and processing for surveying, even for locations that are difficult for humans to access or over large areas. The versatility and advantages of UAV platforms have attracted continuous research interest owing to their potential in a wide range of applications [3]. The use of UAVs provides a rapid, cost-effective, and efficient method for the generation of high-resolution datasets, including digital elevation models (DEMs) and orthophotos [4, 5]. When creating DEMs or producing orthophotos based on UAVs, images and altitude data should be properly aligned with the Earth's surface to obtain accurate positioning information. This is essential for ensuring accuracy in mapping, geospatial analysis, and integration of UAV data with data from other geomatic systems [6]. When used properly, UAV-based systems enable the production of orthophotos and DEMs with a sufficient level of accuracy and quality; however, the quality of these resulting models or photos largely depends on how the mission and surveys are optimally performed to achieve the highest possible accuracy under given settings [7]. For applications including surveying or mapping, the acquisition of geospatial information with minimal error is essential toobtaining accurate measurements. A high positional accuracy ensures the reliability of information about distances, areas, and volumes calculated from the data acquired with UAVs, which is useful in a variety of applications including construction, agriculture, and environmental monitoring. To achieve a practical degree of positional accuracy for orthophotos produced based on the structure from motion (SFM) algorithm by acquiring aerial photos, an appropriate georeferencing strategy needs to be adopted [8]. This step is critical to ensuring the accuracy of these images for various purposes including mapping, surveying, and environmental monitoring. Accurate georeferencing allows the reliable comparison of heterogeneous datasets acquired at different times. It also plays an important role in monitoring changes over time, including the tracking of vegetation growth, evaluation of land use changes, and detection of changes in surface topography. In the UAV-based image mapping stage, ground control points (GCPs) are usually used for georeferencing to ensure orthophoto geolocation/positioning accuracy [9]. A GCP is an artificial or natural point on the Earth's surface with defined coordinates. This refers to the point of observation by installing a target signboard for aerial surveying on the ground for geometric correction of the acquired photo. In some cases, other objects such as crosswalk signs, manholes, or braille blocks are used instead of installing target signboards. To use something other than the target signboards as a GCP, requirements that must be considered consist of the following: first, the location of the pixels, enabling the clear identification of a point; second, locations with a clear difference in brightness; and third, points invariant to the time at which photos are taken and changes in time. After determining the GCPs, devices used for stereo matching, such as a global positioning system (GPS), are used to obtain values of the stereo corresponding with the applicable points [10]. Without GCPs, orthophotos are at risk of experiencing distortions and errors from factors such as GPS inaccuracy, sensor drift, and topographic changes. GCPs help to minimize these errors and improve the overall quality of orthophotos.