# A Simple Method to Estimate Weed Control Threshold by Using RGB Images from Drones
## 2 Materials and Methods
### Trial Setup and Data Collection  
The experimentation was carried out in June-September 2019 at the Agro-Environmental Research Centre \"[PERSON]\" of the University of Pisa, located in Central Italy (43.68 lat. N, 10.34 long. E), on two contiguous fields of approximately 4000 m\({}^{2}\) of surface, and cultivated with silage maize. The farming practices used were those usually adopted by local farmers [35]. The soil (Typic Xerofluvent, USDA classification) was clay-loam (clay 29%, silt 38%, sand 33%, USDA method) and the main chemical characteristics in the 0.00-0.30 m layer were the following: pH 8.1, organic matter 2.2% (\(w/w\)) ([PERSON] method), total nitrogen 1.39 (mg kg\({}^{-1}\)) ([PERSON] method), assimilable phosphorus 4.0 ppm ([PERSON] method), and cation exchange capacity 18 meq 100 g\({}^{-1}\) ([PERSON] method). Climatic conditions are representative of Mediterranean coastal areas, with about 900 mm of annual rainfall and a 15 \({}^{\circ}\)C mean temperature.  
Eight plots of 4 m\({}^{2}\) (2.0 m \(\times\) 2.0 m) were set up, each containing four rows of maize plants (Figure 1).  
Figure 1: Experimental plots (control plot to the (**left**) and pulling weed plots, to the (**right**)).  
At the 4-5 leaf stage, the number of plants of maize (NM) and weeds (NW) was counted on each plot. Afterwards, half of the plots were weeded (weeded plots = WP) by using 0.5 l ha\({}^{-1}\) of Aric 480 L.S. (41% Dicamba) and 1.5 l ha\({}^{-1}\) of Nifuron (4.18% Nicosolfuron), whereas no intervention was carried out on the remaining four plots (control plots = CP). At harvest time, the aboveground biomass of all maize plants was collected from each plot (WP and CP) and weighed after oven drying at 60 \({}^{\circ}\)C until constant weight. On the other four plots of 4 m\({}^{2}\) (2.0 m \(\times\) 2.0 m), the number of maize plants was counted and all weeds were removed manually before the UAV flight (pulling weed plots = PWP) in order to estimate more easily the average cover value of a single maize plant.  
The UAV flight was made on the 25 th of June over the entire surface of both fields, just before the weed and crop scouting. We used the drone \"Agri-Efesto\", made available by Sigma Ingegenria and the Precision Agriculture group of the CNR-IBIMET of Florence. The drone was a modified multi-rotor Mikrokopter (HiSystems GmbH, Moomeland, Germany) with six MK3538 and APC propellers 12 \(\times\) 3.8 inches. Autonomous fight through a predefined route was set using the onboard navigation system based on a GPS receiver (U-blox LEA6S) connected to a navigation board (Navy-Ctrl 2.0) and a small Microelectromechanical System (MEMS)-based IMU (Inertial Measurement Unit) (Mikrokopter Flight Controller ME V2.1). A universal camera mount equipped with three servomotors permitted accurate image acquisition through the compensation for tilt and rolling effects. The Agri-Efesto was equipped with a Sony Cyber-shot DSC-QX100 RGB camera (Sony Corporation, Tokyo, Japan), which mounts a 20.2-megapixel CMOS Exmor R sensor and a Carl Zeiss Vario-Sonnar T lens.  
The UAV flight was made at 10 and 20 m in height, with a ground-size pixel equal to 3.5 mm and 7.1 mm, respectively, maintaining an overlap of 70% between the individual frames.