Topographic effects in AVHRR NDVI data

An investigation is made of the errors induced in the normalized difference vegetation index (NDVI) by topographic variation. The Advanced RAdiometric RAy Tracer (ARARAT) is used to simulate AVHRR imagery for various viewing and illumination conditions at 1.1-km and 50-m resolution. Topographic error is calculated as the difference between the variable terrain simulation and a flat plane simulation. Correlation between this error and various topographic factors such as shadowing and adjacement hill illumination are examined. The importance of sky occlusion as a topographic factor is highlighted. While topographic errors of 13.5% are found in the 50-m resolution simulations, these errors are reduced markedly in the 1.1-km dataset to approximately 3%.