Standardization of shooting parameters of digital image technique to estimate nitrogen concentration of winter rapeseed

ObjectivesDigital image technique is useful for rapid and non-destructive prediction of nitrogen concentration of winter rapeseed. The establishment of the standardization of the shooting parameters can provide the basis for the interaction of different crops and camera models.

MethodsWinter rapeseed was used in field experiment with five nitrogen application rates (N:0, 90, 180, 270 and 360 kg/hm²). The canopy pictures of winter rapeseed were obtained in different light intensities, times, heights, angles, pixels, modes and image formats by using digital camera (Nikon-D7000, 1620 million pixels) at the seedling stage (79-83 days after transplanting). Significant analyses of canopy digital normalized redness intensity (NRI) were also made in different imagery modes. Other conventional diagnosis parameters, such as aboveground biomass, leaf nitrogen and leaf chlorophyll concentration were determined to reveal their correlations to NRI.

ResultsCanopy NRI acquired either at sunny or cloudy days could interpret digital information of canopy well, however, it was slightly better in sunny days than cloudy days. The correlation coefficient between NRI and leaf nitrogen concentration was -0.802** at noon, better than those in the morning and afternoon. No significant difference was observed among three image height of 1.5, 2.0 and 2.5 m. It showed very prominent relationships between NRI obtained in three image angles (30°, 60ånd 90°) and aboveground biomass, leaf nitrogen concentration and chlorophyll concentration, however, it was more convenient to get canopy NRI at 30°-60°. The correlation coefficient between NRI and leaf nitrogen concentration was -0.802** at auto exposure mode, higher than other exposal models. No significant difference was observed among three photo pixels (4928×3264, 3696×2448 and 2464×1632). NRI in JPEG fine format was superior to other image formats.

ConclusionsOverall, the optimal operating range of digital image technique to estimate nitrogen nutrition index of winter rapeseed was followed:the best shooting time is sunny noon because of its relatively greater solar elevation angle and stable light, the image can be taken at any height near the ground, it is easy to operate at the image angle of 30°-60°, the shooting mode should be automatic exposure mode, the photo pixel is better in relatively clear and high resolution, and the images saved in compressed JPEG accurate format with less space occupying is recommended. Our study provides a theory and technique foundation of digital image technique on nondestructive nitrogen diagnostic using low-altitude unmanned aerial vehicles remote sensing.