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Mesures physiques et signatures en télédétection

itself result in relative errors larger than 10% for the MIR foliar reflectances. Consequently, the validity of the
predictive equations will be much affected. In the case of poor atmospheric corrections, a solution would be to
determine the predictive equations directly from the remotely acquired data. A major drawback would be that
these predictive equations would be both time and site dependent.
V. Concluding remarks
Chemical analyses combined with spectral measurements of dried and ground pine needles allowed us to
determine linear relationships that are predictive of lignin, nitrogen and cellulose concentrations within these
pine needles. The equations were extrapolated to AVIRIS spectrometric information. This extrapolation
yielded poor results whenever predictive equations were applied to AVIRIS-derived canopy reflectances. The
use of atmospheric corrections did not significantly improve results. Therefore, in order to take into account
external factors such as the canopy geometry and the density of pine needles, an indirect approach was
adopted. Through the inversion of two canopy reflectance models, predictive equations were applied to
reflectances of individual needles. Preliminary results are encouraging. Compared to the direct approach,
correlations are definitely better. This shows that similar predictive equations can be applied both on a
laboratory and a remote sensing level. Some extrapolation is possible from laboratory spectrometry to
airborne spectrometry. As an example of application, the predictive equations were used to map local
chemical concentrations in order to provide spaual information suitable for the "forest BGC" ecosystem model
(Running and Hunt, 1992).
The validation of the extrapolation of laboratory-derived predictive equations was based on a limited number
of samples for making any definite statement. Therefore, the preliminary results are encouraging but should be
confirmed. ISM data from a 1993 campaign above the same study area are being analysed for that purpose.
Acknowledgements: this research would not have been possible without the financial support of the Centre
National pour la Recherche Scientifique (CNRS), la Région Midi Pyrénées, ICIV and CEFE. We thank Dr T.
Le Toan, Dr Blasco F. and Dr Fromard F. for their kind help in this work.
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