EFFECT OF RADIOMETRIC CORRECTIONS ON NDVI-DETERMINED FROM SPOT-HRV AND LANDSAT-TM DATA

The normalized difference vegetation index (NDVI), which is generally considered as an index minimizing the radiometric errors on image data has to be corrected radiometrically when a quantitative analysis is performed. In this article, the main factors affecting NDVI are analyzed: proper characteristics (MTF) and absolute calibration of the satellite sensor, Sun zenith angle, Earth-Sun distance, and atmospheric condition. The effects of these factors are theoretically and practically analyzed on two SPOT-HRV and Landsat-TM images acquired the same day over the same area in southeast France. Some simplified correction methods are proposed. The results show that: i) The conversion of digital counts into apparent reflectance is the most important step for NDVI correction. Without this correction, a relatively constant error affects NDVI depending on the sensor considered (- 0.18 for SPOT-HRV and - 0.10 for Landsat TM). ii) The MTF correction does not affect the average NDVI value; its interest is to restore the radiometric level of individual pixels that have a large contrast with their surroundings. iii) The atmospheric effects are similar in the homologous spectral bands of SPOT-HRV and Landsat TM. Their correction increases the dynamic range of NDVI variation (around 24% in the example presented) and consequently the contrast between different targets. The effect of the noncoincidence of SPOT-HRV and Landsat-TM spectral bands is also studied. This effect can be considered either as a source of error or as a supplementary source of information. An example shows that the combination of the spectral information given by the two satellites can be used to improve the discrimination of some targets such as bare soil and soil with a low vegetation density.