Creating a field-wide forage canopy model using UAVs and photogrammetry processing

The canopy of alfalfa reveals useful information for managing this important forage crop, but manual measurements of alfalfa canopies are time consuming and impractical at field-scale. Photogrammetry processing with images taken from Unmanned Aerial Vehicles (UAVs) can create a three-dimensional model of the crop canopy across the entire field. The goal of this study was to determine the appropriate flight parameters for the UAV that would enable consistent and reliable generation of canopy models at all stages of alfalfa growth and with at least the 2.54 cm resolution used in manual field measurements. Flights were conducted over two separate fields on four different dates using three different flight parameters to control a Phantom 4 Pro. This provided 24 flights. The flight parameters considered were: 30 m altitude with 90° camera gimbal angle, 50 m altitude with 90° camera gimbal angle, and 50 m altitude with 75° camera gimbal angle. A total of 32 three-dimensional canopy models were created in Pix4D. This included one model from each separate flight and combination models. They were analyzed based on Model Ground Sampling Distance (GSD), Model Root Mean Square Error (RMSE), and Camera Calibration Difference. Of the 32 attempted models, 30, or 94%, were judged acceptable. The flights at 30 m altitude had the lowest Model GSD, though there was no significant difference in Model RMSE between the various flight parameters, but the flights at 50 m altitude with the 75° camera gimbal angle did have lower variance. There was no benefit to combining the images from multiple flights with different flight parameters. It is suggested to use flights at a 50 m altitude with a 75° camera gimbal angle to reduce flight durations and processing time.