The normalized difference vegetation index (NDVI) is a measure of greenness. NDVI was calculated as: NDVI = (NIR - R) / (NIR + R), where NIR is the spectral reflectance in the AVHRR near-infrared channel (0.725-1.1 m, channel 2) where light-reflectance from the plant canopy is dominant, and R is the reflectance in the red channel (0.5 to 0.68 m, channel 1), the portion of the spectrum where chlorophyll absorbs maximally (Markon et al. 1995). The grid of NDVI values was produced from the same AVHRR data as the false-color CIR image. NDVI values are saved as scaled integers, where the NDVI value can be calculated as:
NDVI values ranged from -0.112 to 1.000 (grid integer values 113 to 252). The final CAVM NDVI map was plotted as 8 categories that were useful in distinguishing vegetation patterns: <0.03, 0.03-0.14, 0.15-0.26, 0.27-0.38, 0.39-0.50, 0.51-0.56, 0.57-0.62, >0.62. Areas with NDVI < 0.03 are bare rock or ice. Areas with NDVI = 0.03-0.14 are very sparsely vegetated. The remaining categories were evenly divided, except for the highest NDVI categories, which were split up to distinguish very shrubby areas in the southern Arctic.
ESRI. 1993. Digital Chart of the World. September 1993, Ed. 1. Environmental Systems Research Institute Inc. Redlands, CA, USA.
Markon, C.J. M.D. Fleming and E.F. Binnian. 1995. Characteristics of vegetation phenology over the Alaskan landscape using AVHRR time-series data. Polar Record 31:179