Comparison of four off-the-shelf unmanned aerial vehicles (UAVs) and two photogrammetry programmes for monitoring pasture and cropping field trials

Rapid advancements in UAVs, computing power and photogrammetry techniques now permit low cost biological-monitoring applications using off-the-shelf hardware and software. The utility of four UAV models costing $1,200 - $11, 000 and two photogrammetry programmes were assessed in separate experiments to evaluate their ability to detect standardised plant targets and to generate useable orthomoasic images. The colour and contrast of standardised targets influenced detection by UAVs more than their size as height increased. A large green rosette (50.8 cm2) could be detected by all UAVs from 28–90 m, while a yellow target 13 times smaller could be detected at 36–100 m, with the more expensive UAVs being effective at the higher altitudes. Monitoring vegetation cover or flowering plants is possible at the minimum allowable height altitude of 20 m by all four UAVs. However, identification of species in their vegetative state would require the UAVs with the better camera optics. The two photogrammetry programmes produced suitable orthomosaic images under the pasture, maize and hill country scenarios tested.