Optimisation of oblique-view remote measurement of crop N-uptake under changing irradiance conditions

A method was developed to optimise ground-based oblique-view remote measurement of N-uptake under changing irradiance conditions. The method consists of two elements: a special viewing geometry and an optimised vegetation index, both chosen in order to minimise disturbing irradiance effects. Regarding the viewing geometry, it is shown that multiple simultaneous measurements into two or more directions can largely reduce errors induced by changing solar and view azimuth angles. Vegetation indices were then calculated and ranked according to both their capability to predict N-uptake and their insensitivity towards changes in daily irradiance. Results show that ratio vegetation indices with appropriately chosen wavelengths are much better predictors of crop N-uptake than more commonly used indices such as the Infrared-to-Red-Ratio, the NDVI or the Red-Edge-Inflection Point.