722 
5. DISCUSSION AND CONCLUSIONS 
This paper presents observations on the definition and testing of an atmospherically resistant 
vegetation index, the Angular Vegetation Index (AVI) for use with data from the second Along Track 
Scanning Radiometer (ATSR-2). Examination of this index using surface reflectance data, generated 
with a combined PROSPECT and SAIL model, revealed it to be insensitive to changes in soil 
brightness and chlorophyll concentration, at low LAI. Above an LAI of 2, however, the index was 
sensitive to chlorophyll concentration. Comparison with the NDVI revealed that AVI was potentially 
better for detecting the presence of vegetation. The main intention behind development of the index 
was, however, resistance to the atmosphere. Examination of effects of ground visibility and aerosol 
size distribution revealed that AVI was unaffected by a change from a Rural to a Maritime aerosol and 
was relatively insensitive to ground visibility over the range 10-50 km. 
These results are limited to one solar position and the nadir view. The canopy reflectance simulation 
was restricted to vegetation that is represented by the SAIL model with a spherical leaf angle 
distribution and only two soils were used to examine the variation with soil brightness. A more 
exhaustive examination of the potential of AVI is therefore required to determine the sensitivity to 
variation in atmosphere and ground surface. The performance of AVI at other solar and view positions 
and over other surface types is currently being analysed, in particular for forest canopies. The degree to 
which variation in the curvature of the soil reflectance curve in the green-red-near infrared feature 
space affects AVI is also being determined using the Purdue Soil Reflectance Database. 
6. ACKNOWLEDGEMENTS 
The authors would like to thank Larry Biehl and Purdue University for access to the Purdue Soil 
Reflectance Database and Stephane Jacquemoud for the source code for PROSPECT. 
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