AVA – short description
The agricultural vegetation analyzer (AVA) is based on the PROSPECT-5 [1] and 4SAIL models [2], coupled to “PROSAIL”. The 4SAIL model simulates the bi-directional reflectance of homogeneous canopies as a function of soil reflectance, illumination and viewing geometries, as well as several structural and biophysical variables, such as LAI, average leaf angle (ALA) and a hot spot parameter. Leaf optical properties (reflectance and transmittance) are simulated by the PROSPECT-5 model as a function of a structure parameter N, leaf chlorophyll content (Cab), dry matter content (Cm), carotenoids (Car) and leaf water content
(Cw). For the estimation of the variables from the PROSAIL model, a flexible and fast look-up table (LUT) inversion procedure is implemented (e.g. [3,4]). The LUT contains is composed of 50’000 variable combinations, randomly drawn within (truncated) Gaussian distributions in defined bounds [5]. Bi-directional reflectance is interactively calculated for all variable combinations according to the geometric characteristics of the given imagery. Additionally, fractional vegetation coverage (fCover) and fraction of photosynthetically active radiation
(fAPAR) are calculated by the model and included in the LUT. A simple cost function composed of the root mean square error (RMSE) is then employed for the inversion. For the solution, the spectra of the closest (radiometric) match with the measured signal are selected. The final estimated variables are calculated as average of all variables from those spectra situated within less than 20% of the lowest RMSE value. Moreover, for each pixel a
“model uncertainty” value – being the coefficient of variation of all spectra within the 20% criteria - is provided.
References:
[1]
[2]
[3]
[4]
[5]
J.-B. Feret, C. Francois, G. P. Asner, A. A. Gitelson, R. E. Martin, L. P. R. Bidel, S. L. Ustin, G. le Maire, and S. Jacquemoud, "PROSPECT-4 and 5: Advances in the leaf optical properties model separating photosynthetic pigments," Remote Sens. Environ., vol. 112, pp. 3030-3043,
Jun 16 2008.
S. Jacquemoud, W. Verhoef, F. Baret, C. Bacour, P. J. Zarco-Tejada, G. P. Asner, C.
Francois, and S. L. Ustin, "PROSPECT plus SAIL models: A review of use for vegetation characterization," Remote Sens. Environ., vol. 113, pp. S56-S66, Sep 2009.
M. Weiss, F. Baret, R. B. Myneni, A. Pragnere, and Y. Knyazikhin, "Investigation of a model inversion technique to estimate canopy biophysical variables from spectral and directional reflectance data," Agronomie, vol. 20, pp. 3-22, Jan-Feb 2000.
K. Richter, C. Atzberger, F. Vuolo, P. Weihs, and G. D'Urso, "Experimental assessment of the
Sentinel-2 band setting for RTM-based LAI retrieval of sugar beet and maize," Canadian
Journal of Remote Sensing, vol. 35, pp. 230-247, Jun 2009.
F. Baret, O. Hagolle, B. Geiger, P. Bicheron, B. Miras, M. Huc, B. Berthelot, F. Nino, M.
Weiss, O. Samain, J. L. Roujean, and M. Leroy, "LAI, fAPAR and fCover CYCLOPES global products derived from VEGETATION - Part 1: Principles of the algorithm," Remote Sens.
Environ., vol. 110, pp. 275-286, Oct 15 2007.