Influence of quantity, quality, horizontal and vertical distribution of ground control points on the positional accuracy of UAV survey

Quantity and distribution of Ground Control Points (GCPs) play a significant role in determining the positional accuracy of UAV photogrammetry. A dense GCP network helps in achieving good accuracy. However, the cost, time, and feasibility of setting up a dense network are challenging. Therefore, it is crucial to assess whether high accuracy can be achieved using minimal GCPs and its optimal distribution. This study investigated the effects of quantity, quality, horizontal, and vertical distribution using 0, 3–11 GCPs to identify a suitable configuration for a sparse GCP network. Thirty-eight configurations were experimented by distributing GCPs in the corners, edges, centre and vertically. Also, another sixteen configurations were used to understand the influence of incorrectly surveyed GCPs on positional accuracy. Horizontal and vertical Root Mean Square Error (RMSE) values were calculated from 79 Check Points for accuracy assessment. Initially, on assessing the effect of quantity, a higher count of GCPs produced high accuracy, but specific configurations using 4–5 GCPs rendered accuracy levels similar to 9–11 GCPs. On further investigation, configurations with few GCPs at the corners showed better accuracy than GCPs distributed only in the edge or centre. A significant reduction in RMSE_z of ± 1.5 cm was witnessed by adding vertically distributed GCPs. Based on the results, configurations using 4–5 GCPs distributed vertically and at corners equalled the RMSE values of configurations using 8–11 GCPs, proving it to be an ideal distribution while using fewer GCPs. The poor quality of GCP resulted in low positional accuracy when a sparse number of GCPs were used.