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Lake Maggiore - Italy, 3-6 April 2000

Estimation of land surface albedo and vegetation biophysical properties using SPOT-4 VGT and semi-empirical BRDF models.

M. J. Barnsley, T. L. Quaife, P. D. Hobson and J. Shaw,
Earth Observation & Environmental Monitoring Group,
Department of Geography,
University of Wales Swansea, U.K.

P. Lewis and M. I. Disney,
Remote Sensing Unit,
Department of Geography,
University College London, U.K.

Paper (pdf file, 856 k)

The VGT instrument facilitates analysis of the land-surface angular signature by virtue of its wide swath width ¾ multiple over-passes of a location provide image data at varying sun-sensor geometries.

In recent years considerable effort has been directed towards devising methods to assess the information content of such data, most notably in the formulation of models of the Bidirectional Reflectance Distribution Function (BRDF). The three commonly cited potential products of BRDF model inversion are 1) the radiometric normalisation of off-nadir pixels, 2) the improved estimation of narrow- and broadband albedo and 3) the estimation of land-surface biophysical properties such as Leaf Area Index (LAI). Semi-empirical linear models of the BRDF — such as the AMBRALS suite proposed for use with the MODIS sensor — are rapidly invertible and have the potential to provide the aforementioned outputs.

Data have been collected from the VGT sensor for the period 17th of May to1st of October 1999, covering Western Europe. This constitutes a total of 478 scenes (all P level products). The data were atmospherically corrected in-house using the Simplified Method for Atmospheric Correction (SMAC) and cloud masked using the reflectance information in the blue channel.

Three areas have been selected for detailed study:

1) East Anglia, UK — a MODLAND validation site with which some of the authors are involved, and for which field data is available. The area is flat with large arable farming areas.

2) Bordeaux, France — Focusing on the Les Landes area, and

3) SE Spain — a semi-arid area with low cloud-contamination.

The AMBRALS BRDF model was inverted using VGT data for the above scenes. The potential for angular normalisation and estimation of spectral albedo is clearly demonstrated. Investigation of the potential for biophysical parameter and broad band albedo estimation is ongoing -- the main focus of which is fieldwork in East Anglia, including investigations with plant growth models and the temporal trajectories of the BRDF model parameters.