Realistic parameterization of land surface processes must take into account heterogeneities in the land surface. In the case of a sparse canopy, interpretation of remotely sensed measurements is very difficult and somewhat questionable in attempts to relate the vegetation indices (VIs) to fractional vegetation cover information. This paper provides an intercomparison of satellite observations at different scales for the purpose of assessing and monitoring vegetation changes at a regional scale. It is designed (1) to evaluate the level of association that can be expected from a model relating basic tools such as spectrally derived VIs from AVHRR and green biomass data for a set of heterogeneous surfaces in a representative semi-arid region and (2) to determine the best strategy for using satellite imagery in that context. The quantitative relationships between radiation data collected in space and characteristics of land surfaces are investigated in the context of the HAPEX-Sahel study over the Niger. A north-south vegetation gradient was accurately located and documented. Corresponding SPOT data, acquired on the same day for the same test site, at 20m spatial resolution were then resampled to the plate carree projection for comparison with National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA AVHRR) HRPT data (1km spatial resolution). This processing helped in the description and full interpretation of the evolution of various vegetation indices derived from NOAA AVHRR data on these semi-arid regions.