If the dependence of the viewing conditions for a specific surface is known, a standardized NDVI 
corresponding to near madir viewing angle can be inferred. This could be very useful when comparing 
NDVI maps generated from AVHRR data and from LANDSAT which looks in nadir direction. In 
particular, the annual variation of solar height and the development of the vegetation canopy complicate the 
matter considerably. Due to the viewing angle, the surface reflectance depends on the solar zenith angle, 
which varies in the course of the year and even during the above mentioned repetition period. From daily 
data, the instantaneous bidirectional reflectance factor of a limited period can be derived as shown in Fig. 7. 
This can be used for reduction to nadir conditions to get a better intercomparison with LANDSAT TM data. 
Belmonte (Spain) 19. July - 7. Aug. 1991 
Fig. 6 . Example of dependence on viewing conditions. Technical albedo (TOA) of ch 1 (1) and of ch 2 (2), 
corrected for atmospheric effects ch 1 (3), ch 2 (4), angle between sun and satellite (5), solar azimuth ( 6 ) 
FORWARD-SCATTERING 
BACKWARD-SCATTERING 
ZENITH ANGLE OF OBSERVATION 
Fig. 7 Bidirectional reflectance function for Rada de Haro derived from multitemporal AVHRR 
measurements. AVHRR-1 (1) and -2 (5) with atmospheric correction, ch 1 TOA (2), ch 2 TOA (4) and 
measurements from Deering et al., (1990) normalized to same nadir albedos (3). 
3.2 - Image Interpretation And Upscaling 
3.2.1. Multisource Remote-Sensing Data Integration For The Analysis Of Spatial Scaling. Comparison 
between ground (meter scale) and satellite measurements (kilometer scale) implies the consideration of the 
scaling problem, both for particular measurements and for mathematical models which relate them, and this 
is particularly so when using low resolution satellite data (from NOAA AVHRR or METEOSAT). The 
problem becomes really complicated when the comparison is not between direct measurements but 
precisely between values derived from mathematical models (indirect measurements). In this case, the 
mathematical relationships between different magnitudes at ground level are not the same as at a low spatial 
resolution as measured by a satellite. When dealing with mathematical relationships, the spatial average of