Accuracy assessment of orthophoto using ground control point derived from various global positioning system techniques

Abstract

Unmanned Aerial System (UAS) is a rapid mapping method that is capable to capture many details in short span of time from various altitudes. UAS comprises of two components: (i) Unmanned Aerial Vehicle (UAV) and (ii) Ground Control System (GCS). The functions of GCS are to monitor and control the UAV from the ground during the process of data collection. UAS has the capability to produce accurate set of data for mapping, nevertheless, the accuracy of the data need to be assessed. The aim of this study is to assess the accuracy of mapping using UAS data based on Ground Control Points (GCP) derived from various Global Positioning System (GPS) techniques. The GCPs are used as control points for production of orthophoto. In this study, a fixed-wing UAV attached with a digital camera was flown over Universiti Teknologi Malaysia campus at an altitude of 300 m for digital aerial images acquisition. The study area is divided into: (i) small study area of Lingkaran Ilmu with approximate area of 35000 m2 and (ii) large study area with approximate area of UTM of 7000000 m2. These different sizes of the study area were used to investigate the effect of different GPS techniques. In this study, the GCPs location is well distributed in both study areas and the GCPs were established using three different GPS techniques: i) relative static, ii) absolute static and ii) Network Real Time Kinematic (NRTK). These different techniques were used to investigate its effect on orthophoto production. The accuracy assessment is performed by comparing the orthophoto measurements with reference values based on Check Points (CPs) established using GPS technique. Results show that the Root Mean Square Error (RMSE) of NRTK technique is consistently small with 0.39 m and 0.55 m for the small and large study areas respectively. While for the relative static technique, the RMSE show inconsistent results with 1.61 m and 0.50 m for the small and large study areas respectively. Meanwhile, absolute static technique gives the biggest RMSE with 0.84 m and 2.27 m for the small and large study areas respectively. In conclusion, NRTK technique is proved to be the best technique of GCP establishment in terms of accuracy and the UAS can be employed for mapping purposes.