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TitleEffect of the calibration of AVHRR data on the Normalized Difference Vegetation Index and compositing
AuthorD'Iorio, M; Cihlar, J; Morasse, C R
SourceCanadian Journal of Remote Sensing vol. 17, no. 3, 1991 p. 251-262
Alt SeriesEarth Sciences Sector, Contribution Series 20041242
Mediapaper; on-line; digital
File formatpdf
AbstractThe calibration of NOAA data from Digital Signal Level (DSL) to apparent radiance is a crucial step in the processing of these data before compositing. Maximum Normalized Difference Vegetation Index (NDVI) composites are used for vegetation monitoring of the Canadian landmass. The composite NDVI values display a smooth transition regionally within a given land cover type; however, the corresponding channel 1 and 2 values often show a bimodal distribution, rendering a speckled appearance to the resulting composite images. In the majority of cases, the brighter pixels originate from high viewing angles. Furthermore, the sun-sensor geometry accentuates observable problems in early to mid-summer images acquired from NOAA mid-afternoon ascending passes because of a "hot spot" being created, where shadows are minimized. Using a digital land cover map of Canada, it was found that, while the speckle problem is apparent over all types of land cover, it is more pronounced in agricultural areas. The radiometric calibration of DSL from channels 1 and 2 increases relatively the NDVI values of near-nadir pixels. As a result, maximum NDVI composites derived from calibrated data are substantially improved when more near-nadir pixels are selected. Nevertheless, residual speckle remains in composite channel 1 and 2 values. The reasons for this effect and the potential strategies for overcoming it are discussed in this paper.

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