Stéphane Flasse

Detecting Vegetation Leaf Water Content Using Reflectance in the Optical Domain.

This paper outlines the first part of a series of research studies to investigate the potential and approaches for using optical remote sensing to assess vegetation water content. It first analyzes why most methods used as approximations of vegetation water content (such as vegetation stress indices, estimation of degree of curing and chlorophyll content) are not suitable for retrieving water content at leaf level. It then documents the physical basis supporting the use of remote sensing to directly detect vegetation water content in terms of Equivalent Water Thickness (EWT) at leaf level. Using laboratory measurements, the radiative transfer model PROSPECT and a sensitivity analysis, it shows that shortwave infrared (SWIR) is sensitive to EWT but cannot be used alone to retrieve EWT because two other leaf parameters (internal structure and dry matter) also influence leaf reflectance in the SWIR. A combination of SWIR and NIR (only influenced by these two parameters) is necessary to retrieve EWT at leaf level. These results set the basis towards establishing operational techniques for the retrieval of EWT at top-of-canopy and top-of-atmospheric levels.

Designing a spectral index to estimate vegetation water content from remote sensing data: Part 1: Theoretical approach

This paper describes the methodology used to create a spectral index to retrieve vegetation water content from remotely sensed data in the solar spectrum domain. A global sensitivity analysis (GSA) using radiative transfer models is used to understand and quantify vegetation water content effects on the signal measured at three levels: leaf, canopy, and atmosphere. An index is then created that optimises retrieval of vegetation water content (in terms of water quantity per unit area at canopy level) and minimises perturbing effects of geophysical and atmospheric effects. The new index, optimised for the new SPOT-VEGETATION sensor, is presented as an example. Limitations and robustness of the index are also discussed.

Designing a Spectral Index to Estimate Vegetation Water Content from Remote Sensing Data. Part 2. Validation and Applications.

The Global Vegetation Moisture Index (GVMI) was developed to retrieve vegetation water content from local to global scale rapidly and reliably using SPOT-VEGETATION data. This paper validates the GVMI with field measurements of vegetation water content measured over four different ecosystems in Senegal. Two of the sites show exact concordance between GVMI-derived and field-measured water content. The remaining two sites show differences in value but provide identical evolution over time. Comparison between ecosystems illustrates that GVMI-derived water content is consistent with field measurements of water content expressed as a quantity of water per unit area. Additional study shows that GVMI is not related to the vegetation moisture content expressed as a percentage of water per quantity of biomass. Comparison between the GVMI and NDVI methods also illustrates that the NDVI provides different information (vegetation greenness), which is not directly related to the quantity of water in the vegetation. Potential applications of the new GVMI are also discussed.

A contextual algorithm for AVHRR fire detection

Abstract A contextual algorithm for fire detection with NOAA-AVHRR-LAC data was developed. Unlike ‘traditional’ fire detection algorithms (e.g., multichannel thresholds), the decision to record a fire is made by comparing a fire pixel with the pixels in its immediate neighbourhood. The algorithm is self-adaptive and therefore very consistent over large areas as well as through seasons. The algorithm appears to operate successfully in most areas of the world. This Letter presents the contextual approach and describes the algorithm.

Fire detection and fire growth monitoring using satellite data

The objective of this chapter is to review and discuss the use of near real-time satellite data for fire detection and fire growth monitoring, focusing on NOAA-AVHRR images. Capabilities and limitations of these images, as well as existing fire detection algorithms, are presented. Discussion on the potentials of future remote sensing systems for real-time fire detection concludes the chapter.

An in situ study of the effects of surface anisotropy on the remote sensing of burned savannah

This Letter presents field‐based evidence of the perturbing effects of surface anisotropy on the remote sensing of burned savannah. The analysis is based on bidirectional spectral reflectance data collected at different solar illumination angles and convolved to Moderate‐resolution Imaging Spectroradiometer (MODIS) reflective bands. Results from a grass savannah site show that burning reduces the anisotropy of the surface compared to its pre‐burn state. In contrast, at a shrub savannah site, burning reduces or increases surface anisotropy. Spectral indices defined from 1.240 µm and 2.130 µm reflectance, and 1.640 µm and 2.130 µm reflectance, provided stronger diurnal separation between burned and unburned areas than individual reflectance bands but do not eliminate anisotropic effects. The Normalized Difference Vegetation Index (NDVI) provides weak diurnal separation relative to these near‐ and mid‐infrared based indices. Implications of the findings are discussed for burned area mapping.

Remote sensing of vegetation fires and its contribution to a fire management information system

In the last decade, research has proven that remote sensing can provide very useful support to fire managers. This chapter provides an overview of the types of information remote sensing can provide to the fire community. First, it considers fire management information needs in the context of a fire management information system. An introduction to remote sensing then precedes a description of fire information obtainable from remote sensing data (such as vegetation status, active fire detection and burned areas assessment). Finally, operational examples in five African countries illustrate the practical use of remotely sensed fire information.

Modeling Spectralon's bidirectional reflectance for in-flight calibration of Earth-orbiting sensors

The in-flight calibration of the EOS Multi-angle Imaging SpectroRadiometer (MISR) will be achieved, in part, by observing deployable Spectralon panels. This material reflects light diffusely, and allows all cameras to view a near constant radiance field. This is particularly true when a panel is illuminated near the surface normal. To meet the challenging MISR calibration requirements, however, very accurate knowledge of the panel reflectance must be known for all utilized angles of illumination, and for all camera and monitoring photodiode view angles. It is believed that model predictions of the panels bidirectional reflectance distribution function (BRDF) can be used in conjunction with a measurements program to provide the required characterization. This paper describes the results of a model inversion which was conducted using measured Spectralon BRDF data at several illumination angles. Four physical parameters of the material were retrieved, and are available for use with the model to predict reflectance for any arbitrary illumination or view angle. With these data the root mean square difference between the model and the observations is currently of the order of the noise in the data, at about +/- 1%. With this success the model will now be used in a variety of future studies, including the development of a measurements test plan, the validation of these data, and the prediction of a new BRDF profile, should the material degrade in space.

Pareto boundary: a useful tool in the accuracy assessment of low spatial resolution thematic products

Low resolution remotely sensed data, providing consistent coverage of large areas with high temporal frequency, are increasingly used for the production of continental and global thematic maps. Nonetheless, current accuracy assessment methods relate mainly to local scale mapping investigations. In order to obtain a quantitative evaluation of the limitations due to the low spatial resolution of the data, we suggest to use the Pareto boundary. The Pareto boundary allows to determine the maximum user and producers accuracy values that could he attained at the same time and to represent such a lower limit as a boundary in a bidimensional space. We apply the Pareto boundary to a dataset of Landsat 7 images, analysing how the spatial resolution of available medium and low resolution sensors limits the accuracy of the results.