Evaluating the sensitivity of clay content prediction to atmospheric effects and degradation of image spatial resolution using Hyperspectral VNIR/SWIR imagery
Résumé
Visible, near-infrared and short wave infrared (VNIR/SWIR, 0.4-2.5 µm) hyperspectral satellite imaging is one of the most promising tools for topsoil property mapping for the following reasons: i) it is derived from a laboratory technique that has proven to be a good alternative to costly physical and chemical laboratory soil analysis for estimating a large range of soil properties ; ii) it can benefit from the increasing number of methodologies developed for VNIR/SWIR hyperspectral airborne imaging ; and iii) it provides a synoptic view of the area under study. Despite the significant potential of VNIR/SWIR hyperspectral airborne data for topsoil property mapping, the transposition to satellite data must be evaluated. The objective of this study was to test the sensitivity of clay content prediction to atmospheric effects and to degradation of spatial resolution. This study may offer an initial analysis of the potential of future hyperspectral satellite sensors (HYPerspectral X Imagery – HYPXIM, Spaceborne Hyperspectral Applicative Land and Ocean Mission - SHALOM, PRecursore IperSpettrale della Missione Applicativa - PRISMA, Environmental Mapping and Analysis Program - EnMAP and Hyperspectral Infrared Imager - HyspIRI) for soil applications. This study employed VNIR/SWIR AISA-DUAL airborne data acquired in a Mediterranean region over a large area (300 km²), with an initial spatial resolution of 5 m. These hyperspectral airborne data were simulated at the top of the atmosphere and aggregated at 6 spatial resolutions (10, 15, 20, 30, 60 and 90 m) to fit with the future hyperspectral satellite sensors. The predicted clay content maps were obtained using the partial least squares regression (PLSR) method. The large area of the studied region allows analysis of different pedological patterns in terms of soil composition and spatial structures. Our results showed the following: (i) when a correct compensation of atmosphere effects was performed, only slight differences were detected between clay maps retrieved from the airborne imagery and those from spaceborne imagery (both at 5 m of spatial resolution), (ii) the PLSR models built from data with 5 to 30 m spatial resolution had robust performances and allowed clay mapping, although variation in clay content related to short scale succession of parent material was imperfectly captured beyond 15 m of spatial resolution; (iii) the PLSR models built from data with 60 and 90 m spatial resolution were inaccurate and did not enable clay mapping; and (iv) the two latter results could be explained by the combination of a small short-scale clay content variability and small field sizes observed in the study area. Therefore, in the Mediterranean context with short-scale clay content variability and under the spectral specifications of the airborne sensors, most of the future hyperspectral satellite sensors (four among the five sensors which are studied here) would be useful for clay content mapping.
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