VEGETATION - 2000

Lake Maggiore - Italy, 3-6 April 2000

VALIDATION OF BIOPHYSICAL PRODUCTS DERIVED FROM LARGE SWATH SENSORS FOR GLOBAL BIOSPHERE MONITORING

F. Baret⁽¹⁾ D. Allard⁽²⁾, A. Begue⁽³⁾, G. Dewispelaere⁽³⁾, E. Dufrene⁽⁴⁾, J.P. Gastellu⁽⁵⁾, D. Guyon⁽⁶⁾, A. Kuusk⁽⁷⁾, J.P. Lagouarde⁽⁶⁾, T. Leonidas⁽⁸⁾, M.Leroy⁽⁵⁾, P. Lewis⁽⁹⁾, D. Lo Seen⁽³⁾, W. Mauser⁽¹⁰⁾, J. Moreno⁽¹¹⁾, E. Mougin⁽⁵⁾, T. Nilson⁽⁷⁾, S. Rambal⁽¹²⁾, J.L. Roujean⁽¹³⁾, M. Weiss⁽¹⁾
INRA Bioclimatologie, Domaine Saint-Paul, 84 914 Avignon Cedex 9, France
INRA Biométrie, Domaine Saint Paul, 84 914 Avignon Cedex 9 France
CIRAD, Maison de la télédétection, 34 093 Montpellier cedex 05, France
Laboratoire d’écologie Végétale, Université Paris sud, 91 400, Orsay, France
CESBIO, 18 avenue E.Belin, BP 2801, 31041 Toulouse Cédex 4, France
INRA Bioclimatologie, Domaine de la grande Ferrade, BP 81, 33 883 Villenave-d’Ornon cedex, France
Tartu Observatory, 61 602, Toravere, Estonia
NAGREF, Institute for soil classification and mapping, 1, Theophrastos street, 413 35 Larissa, Greece
University College London, Department of geography, 26 Bedford way, WC1H OAP, London, UK
Institut für Geography, Luisentr. 37, 80 333 Muenchen, Germany
Dept. Termodinamica, Facultad de Fisicas, C/Dr. Moliner 50, 46 100 Burjassot (Valencia), Spain
CEFE, Route de Mende, BP 5051 34 033 Montpellier, cedex 1, France
CNRM, 42, Av. Coriolis, 31 057 Toulouse Cedex, France

Several large swath sensors (VEGETATION, AVHRR, POLDER, SEAWIFS, MSG, MERIS, AATSR, MODIS, GLI) will provide concurrently global monitoring of the Earth's surface. The radiometric data collected by these sensors are then transformed in biophysical products (albedo, LAI, fAPAR, and fCover) used both in ecosystem models and in GCMs to define the surface boundary conditions. However, very little work is dedicated to the evaluation of the actual accuracy of these products as well as to the inter-comparison of products between possible algorithms and sensors that could be exploited to combine their data and get enhanced products (spatial resolution, revisit frequency, accuracy on biophysical products).

The objective of the VALERI project is to develop a network of sites and a methodology designed to evaluate the accuracy of the biophysical products derived from large swath satellites and propose ways to combine them and improve their performances.

The VALERI project is based on a network of 10 to 15 sites covering the Earth's surface and representing a wide range of conditions. Each site is about 100km², a dimension consistent with large swath satellite sensors such as VEGETATION, AVHRR, POLDER, SEAWIFS, MSG, MERIS, AATSR, MODIS, GLI. The biophysical products envisioned are either instantaneous quantities or result from a temporal synthesis spanning over a maximum one month period. A methodology is developed to allow accurate measurements of ground level measurements of the biophysical variables of interest: LAI, fAPAR, fCover, albedo representative both in time and space. It is based on the following steps:

1- Selection of a set of elementary places (around 30 to 50) based on a previous high resolution satellite image (SPOT, TM) thanks to geostatistical methods.

2- Ground measurement of the biophysical quantities using the LAI2000 instrument.

3- Extrapolation of the set of local biophysical quantities measurements to the whole site thanks to a high spatial resolution image acquired during the period of interest and geostatistical methods.

4- Estimation of the biophysical quantities at the resolution of the large swath sensors by agregation, and evaluation of the associated uncertainty.

5- Comparison of the values computed from the large swath sensor data thanks to a given algorithm and the values measured from ground level over the ensemble of sites and the several period of measurements.

The project will therefore provide the basic information to evaluate the absolute accuracy of the proposed sensor/algorithms. It will also provide inter-comparison between products derived from several sensors and thus allow to propose possible ways to combine the data collected concurrently by several large swath sensors.

The VALERI project is complementary to the validation effort conducted in the USA (MODLAND), and exchanges of satellite and ground level data will insure efficient synergy between both projects.