Tower remote-sensing system for monitoring energy crops; image acquisition and geometric corrections

A stand-alone tower remote-sensing platform was developed to monitor energy crops using multispectral imagery. The sensing system was capable of collecting red, green and blue (RGB) and colour infrared (CIR) images for miscanthus, switchgrass and prairie, which were transferred through a wireless network over the growing season. A motorised, variable lens (18–200 mm) provided high spatial resolution. The system was rotated between 0° and 355° in the horizontal plane and tilted ±90° in the vertical plane by a pan/tilt device, which was installed along with a multispectral camera at the top of a tower, 38 m above the ground. A digital compass was incorporated into this system to record the yaw and pitch of the camera position. An algorithm was developed to control automatic image collection in real time for a set of experimental plots. The intrinsic parameters of lens distortion were calibrated for focal lengths of 18, 50, 100, 150 and 200 mm. Images were compiled from each of the experimental plots and were transformed into a site-specific map. A 50 mm focal length was selected, which corresponded to a spatial resolution that depends on the distance and camera tilt along the horizontal and vertical planes. Ortho-rectification was performed by post-processing of the images, and geo-referencing of the images were performed using geographical information system (GIS). The spatial resolution achieved was 120 mm per pixel after mosaic generation and geo-referencing. An normalised difference vegetation index (NDVI) map was prepared to identify the crop response for understanding of vegetation status.