Abstract
Multispectral observations from unmanned aerial vehicles (UAVs) are currently used for precision agriculture and crop phenotyping applications to monitor a series of traits allowing the characterization of the vegetation status. However, the limited autonomy of UAVs makes the completion of flights difficult when sampling large areas. Increasing the throughput of data acquisition while not degrading the ground sample distance (GSD) is, therefore, a critical issue to be solved. We propose here a new image acquisition configuration based on the combination of two focal length (f) optics: an optics with f = 4.2 mm is added to the standard f = 8 mm (SS: single swath) of the multispectral camera (DS: double swath, double of the standard one). Two flights were completed consecutively in 2018 over a maize field using the AIRPHEN multispectral camera at 52 m altitude. The DS flight plan was designed to get 80% overlap with the 4.2 mm optics, while the SS one was designed to get 80% overlap with the 8 mm optics. As a result, the time required to cover the same area is halved for the DS as compared to the SS. The georeferencing accuracy was improved for the DS configuration, particularly for the Z dimension due to the larger view angles available with the small focal length optics. Application to plant height estimates demonstrates that the DS configuration provides similar results as the SS one. However, for both the DS and SS configurations, degrading the quality level used to generate the 3D point cloud significantly decreases the plant height estimates.
Highlights
Since recent years, unmanned aerial vehicles (UAVs) have become very popular to estimate several crop traits for application to precision agriculture [1, 2] and high-throughput plant phenotyping [3]
The camera with f = 4:2 mm used for the double swath (DS) configuration (Figure 3(c)) shows as expected a higher number of images per pixel because of the 80% overlap between images designed for this camera
We proposed to use an imaging system where two different focal length lenses were used concurrently aboard a UAV
Summary
Since recent years, unmanned aerial vehicles (UAVs) have become very popular to estimate several crop traits for application to precision agriculture [1, 2] and high-throughput plant phenotyping [3]. Significant progress has been achieved to extract traits such as crop height, the cover fraction, green area index (GAI), or chlorophyll and nitrogen contents from different optical sensors [5,6,7,8,9]. Among these optical sensors, multispectral cameras working in the visible (400-700 nm) and near-infrared (700-1000 nm) spectral domain are well suited for vegetation monitoring [10, 11]. Despite remarkable research and applications, there are still some bottlenecks that limit the efficiency and accuracy of multispectral cameras onboard UAVs
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