Effect of ground control point spacing on unmanned aerial vehicle orthomosaic and elevation model accuracy

The aim of this study was to analyze how the distance between ground control points (GCPs) affects the accuracy of orthophotomaps and elevation data generated from unmanned aerial vehicle (UAV) imagery.Unlike most previous research focused on small areas or GCP density per unit area, this study examines a large area of 3027 ha and introduces a spacing-based approach to define GCP distribution.A total of 48,000 images and 216 GCPs were used, with seven data processing variants differing in minimum GCP spacing (0-2500 m) and control-check point configurations.The results show that increasing GCP spacing does not significantly affect horizontal accuracy (X, Y) but significantly reduces elevation accuracy (Z) and overall spatial accuracy (XYZ).Moreover, once a spacing threshold of about 1000 m is exceeded, further increases have little effect on data quality.The findings highlight the possibility of optimizing the number and placement of GCPs without substantial loss of accuracy, which can greatly reduce the cost and duration of UAV mapping over large areas.