Retrieval of spruce leaf chlorophyll content from airborne image data using continuum removal and radiative transfer

We investigate combined continuum removal and radiative transfer (RT) modeling to retrieve leaf chlorophyll a & b content (C-ab) from the AISA Eagle airborne imaging spectrometer data of sub-meter (0.4 m) spatial resolution. Based on coupled PROSPECT-DART RT simulations of a Norway spruce (Picea abies (L.) Karst.) stand, we propose a new C-ab sensitive index located between 650 and 720 nm and termed ANCB(650-720). The performance of ANCB(650-720) was validated against ground-measured C-ab of ten spruce crowns and compared with C-ab estimated by a conventional artificial neural network (ANN) trained with continuum removed RT simulations and also by three previously published chlorophyll optical indices: normalized difference between reflectance at 925 and 710 nm (ND925&710), simple reflectance ratio between 750 and 710 nm (SR750/710) and the ratio of TCARI/OSAVI indices. Although all retrieval methods produced visually comparable C-ab spatial patterns, the ground validation revealed that the ANCB(650-720) and ANN retrievals are more accurate than the other three chlorophyll indices (R-2=0.72 for both methods). ANCB(650-720) estimated C-ab with an RMSE=2.27 mu g cm(-2) (relative RRMSE=4.35%) and ANN with an RMSE= 2.18 mu g cm(-2) (RRMSE=4.18%), while SR750/710 with an RMSE=4.16 mu g cm(-2) (RRMSE=7.97%), ND925&710 with an RMSE= 9.07 mu g cm(-2) (RRMSE=17.38%) and TCARI/OSAVI with an RMSE=12.30 mu g cm(-2) (RRMSE=23.56%). Also the systematic RMSEs was lower than the unsystematic one only for the ANCB(650-720) and ANN retrievals. Our results indicate that the newly proposed index can provide the same accuracy as ANN except for C-ab values below 30 mu g cm(-2), which are slightly overestimated (RMSE=2.42 mu g cm(-2)). The computationally efficient ANCB(650-720) retrieval provides accurate high spatial resolution airborne C-ab maps, considerable as a suitable reference data for validating satellite-based C-ab products. (c) 2012 Elsevier Inc. All rights reserved.