Michel M. Verstraete

Evaluation of the performance of various vegetation indices to retrieve vegetation cover from AVHRR data

Abstract A variety of vegetation indices have been used to assess the state and monitor the evolution of the terrestrial biosphere. Early indices were easy to compute but very sensitive to soil and atmospheric effects. Modified indices with a reduced sensitivity to soil brightness changes were then proposed. More recently, new indices have been designed to be less affected by either atmospheric or soil conditions, or both. In this paper, we propose and demonstrate an objective method to evaluate, through model simulation studies, the performance of a representative sample of three such indices (NDVI, SAVI and GEMI) with respect to their capability to retrieve the fractional vegetation cover and the leaf area index from AVHRR optical data. The proposed performance criterion is based on the concept of signal to noise ratio, where the signal is defined to capture the sensitivity of the index to the desired information, and the noise is designed to measure the sensitivity of this index to undesirable perturba...

Extracting information on surface properties from bidirectional reflectance measurements

The retrieval of surface parameters from remotely sensed data is of prime interest for the estimation of surface properties of various planets in the solar system, including the Earth. Bidirectional reflectance measurements taken over natural surfaces in visible and near-infrared spectral bands represent one data set from which these surface properties could be estimated. To achieve this goal, it is necessary to have both physical models predicting the bidirectional reflectance field as a function of the relevant surface parameters, and numerical procedures allowing the inversion of these models using a limited sampling of the bidirectional reflectance field. Given that theoretical models of the bidirectional reflectance have been published (Hapke, 1981, 1984, 1986; Verstraete et al., 1990) and that numerical procedures are available (Pinty et al., 1990), this paper focuses on the errors and uncertainties in the retrieved parameters which may arise because of (1) the weaknesses in our theoretical understanding and representation of the surface radiation transfer and (2) the errors in the bidirectional reflectance data. For instance, it is shown that the addition a posteriori of an amplitude parameter in the function accounting for the opposition effect can drastically modify the retrieved values of the optical and morphological parameters of the surface. The consequences of uncertainties in the reflectance data are also investigated, and the redistribution, by the inversion procedure, of such uncertainties on the retrieved parameters is discussed. Finally, synthetic reflectance data contaminated by a known random noise are used to examine the numerical stability of the retrieval and the compatibility between the models.

The effect of surface anisotropy and viewing geometry on the estimation of NDVI from AVHRR

Abstract Terrestrial surfaces are observed routinely from space using the AVHRR instrument on the NOAA satellites. Observations in the red and near‐infrared spectral bands are used to compute vegetation indices (such as the NDVI), which are then interpreted in terms of various vegetation properties. Orbital and engineering constraints severely limit the range of illumination and viewing angles under which various ecosystems are observed. Since terrestrial surfaces are anisotropic, all spectral reflectance measurements obtained with a small instantaneous field of view instrument are specific to these angular conditions, and the value of the corresponding NDVI, computed from these bidirectional reflectances, is relative to the particular geometry of illumination and viewing at the time of the measurement. This paper documents the importance of these geometric effects through simulations of the AVHRR data acquisition process, and investigates the systematic biases that result from the combination of ecosyste...

Towards a quantitative interpretation of vegetation indices Part 1: Biophysical canopy properties and classical indices

Abstract The information content of vegetation indices currently derived from satellite observations is discussed using a physically‐based approach. These indices can, in principle, be related to some of the biophysical properties of the canopy through a complex set of equations, provided atmospheric effects, the anisotropy of the coupled surface‐atmosphere system, and the morphology and optical heterogeneity of the canopy are taken into account. Atmospheric effects are discussed on the basis of an approximate analytical expression applicable to vegetation indices built from AVHRR channels 1 and 2. These indices strongly depend on the aerosol and water vapor content, a fact that limits their quantitative interpretation in terms of physical and biophysical properties of the canopy. The interaction between the electromagnetic solar radiation and the surface is discussed using physical models of albedos developed for vegetation canopies. Finally, the biophysical leaf model PROSPECT is used to parameterize re...

The impact of multi‐angular measurements on the accuracy of land‐surface Albedo retrieval: Peliminary results for the proposed ESA LSPIM mission

This paper documents, through a sensitivity study, the retrieval accuracy of the albedo and the bidirectional reflectance factor (BRF) field as a function of the number of observations and their angular distribution. A BRF data base is generated for different surface types, solar positions, and wavelengths. A realistic amount of random noise is added to approximate actual measurement conditions. A parametric BRF model is inverted against the noisy data strings of variable lengths and subsequently used in a forward mode to reconstruct the BRF field and estimate the albedo. The retrieval accuracy is estimated by comparing the original and the reconstructed fields and values. For a single observation, the retrieval accuracy based on the assumption of a Lambertian surface reflectance is compared to the one based on the assumption of a typical BRF field.

The IWMMM-2 Conference: Background, Achievements and Recommendations.

The Second International Workshop on Multiangular Measurements and Models (IWMMM-2) was held in Ispra, Italy, on September 15-17, 1999. This international event was jointly sponsored by the European Network for the development of Advanced Models to interpret Optical Remote Sensing data over terrestrial environments (ENAMORS) supported by the European Commission DG-XII under the Fourth Framework Programme, the Space Applications Institute (SAI) of the European Commission DGJoint Research Centre (JRC), the US National Aeronautics and Space Administration (NASA) and the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia. The meeting was attended by over 100 scientists from Western and Eastern Europe, the United States, Australia, New Zealand, Canada, China and Japan. These participants, listed in Annex 1, were involved or interested in modeling the anisotropy of the radiation field, in multiangular measurements, or in the exploitation of these tools and data in their own applications. This 3-day meeting addressed three main objectives: (1) to establish the state of the art in direct and inverse modeling techniques, including those proposed for the operational exploitation of earlier instruments

ETUDE ET EXTRACTION DES PROPRIETES DE SURFACE PAR TELEDETECTION SPATIALE DANS LES COURTES LONGUEURS D'ONDE

RESUME Lee satellites constituent a priori un excellent moyen experimental pour caractCriser et assurer le suivi, I dee echellee de temp. et d’ecpace adequate., des proprietee dee surfaces continentales. Ii. peuvent donc apporter beaucoup I l’analyse des phCnomeneedilmatiques mettani en cause lee perturbations de ces propriCtes de surface. Que cc soit pour valider des results.ts de modele. dlimatiques, pour specifier leurs conditions initiales I Ia limite inferieure, on pour extraire des information, physique. quantitative. de donnee. de tClCdCtection spatiale, des modIles physique. reprCsentant Ic transfert de rayonnement I Ia surface eont necessaires. Un modIle analytique nouveau exprimant la reflectance bidirectionnelle .ur des surfaces eat pruentC. Lee surfaces eont considerCescomme des miieux poreux constitues d’Clementsoptiquemeni homogInee. Une attention particuliIre eat accoidee an traitement des effets dus I l’orientation des ClCmentset I leur arrangement relatif dan. le milieu. Ce modIle a Cte inverse et validC I partir de mesure, effectuCes au laboratoire, sur Ic terrain et par avion. Ce modIle offrede nouvelle. perspectives quant I l’analyse et l’interprCtation de donnee. satellitaires, et permet de .pkifier des conditions limit.. cur lee continents dane les nsodIlee de climat.