By Junwei Zhang
Abstract
The position of the inflexion point in the red edge region (680-780nm) of the spectral reflectance signature, termed the red edge position (REP), is affected by biochemical and biophysical parameters and has been used as a means to estimate foliar nitrogen content and it provides a very sensitive indicator of, among other things, vegetation stress. The high spectral resolution of airborne imaging spectrometers now offers the potential for determining the REP of vegetation canopies at regional scales. However, the accurate estimation of the REP is dependent upon sensor band positions, widths and sensor type. Various techniques have been developed to minimize the error in estimating the REP, such as inverted Gaussian fitting and linear four-point interpolation in the region of the red edge. In this report, aim is to assess different Remote Sensing techniques for estimation of nitrogen content of vegetation in the Millingerwaard nature reserve using hyperspectral data and to test which one is best.
The feasibility of using information from the red edge position region of the spectrum was tested by estimating nitrogen content for one test site with different years (2004 and 2005). The site is a heterogeneous natural area in the floodplain Millingerwaard along the river Waal in the Netherlands. The spectral information in both two years was obtained with an ASD FieldSpec spectrometer. In addition, in 2004 HyMap airborne imaging spectrometer data were acquired and in 2005 AHS airborne imaging spectrometer data were obtained.
From the REP extracted using four different techniques, in case of 2004 data, they have similar R2 just like in 2005. During estimation of nitrogen concentration, the lowest R2 is for the maximum band depth (MBD) in interval 550 nm to 751 nm and the highest R2 for the maximum band depth normalized to the area (BNA) in interval 550 nm to 751 nm. There are big R2 differences between estimated nitrogen content in 2004 and 2005. The continuum removal technique showed a high correlation with foliar nitrogen concentration and nitrogen content for FieldSpec measurements in 2005. Two coarser resolution spectra (Hymap and ASD) offered little help during the research and the maximum first derivative technique has very bad results in comparison to the other three techniques.