VEGETATION - 2000

Lake Maggiore - Italy, 3-6 April 2000

BRDF correction in SPOT 4/VEGETATION ten-days composite imagery for mapping of boreal forest

D. Erchov ⁽¹⁾, S. Bartalev ⁽²⁾, M. Deshayes ⁽³⁾, J. R. Dymond ^(4), International Forest Institute, Moscow, Russia
+7-095-332 68 77, fax +7-095-332 29 17,
⁽²⁾ Joint Research Center of the European Commission, Ispra, Italy
+39-332-786396, fax +39-332-789073,
⁽³⁾ Systemes et Structures Spatiaux Cemagref-ENGREF, Montpellier, France
+33-(0)4 67 54 87 51, fax +33-(0)4 67 54 87 00,
⁽⁴⁾ Landcare Research, Palmerston North, New Zealand
+64 6 356 7154, fax +64 6 355 9230,

The appearance of the SPOT 4/VEGETATION instrument opens new capabilities of forest mapping at continental and sub-continental scales. The examination of ten-day synthesis imagery, acquired over Russia’s boreal forests, has revealed the strong influence of Sun-Earth-sensor geometry. This non-Lambertian vegetation reflectance can be described and corrected with the use of BRDF models.

In the present research, we used a physically based and simple BRDF model (the WAK model) to correct ten-day synthesis imagery for non-Lambertian reflectance. Observation conditions were standardised to nadir view and 45^o solar zenith angle. Model parameters were derivable directly from the VEGETATION imagery, for predominant vegetation groups of the condition-zoning map of Russia.

Model parameter determination is based on the solution of a system of equations describing the reflectance of reference sites viewed from different directions. Not less than three pairs of reference sites for each vegetation type, or land cover class, are required. Each reference site was located across the boundary between two series passes to provide different view directions.

The efficiency of the BRDF correction with reference to boreal forest mapping is quantified. The limitations of the given approach and direction of further improvement of BRDF correction in the SPOT 4/VEGETATION imagery are discussed.