The Normalized Difference Vegetation Index (NDVI) is a simple numerical indicator
that can be used to analyze remote sensing measurements. NDVI is related to
vegetation, where healthy vegetation reflects very well in the near infrared part of the
spectrum. Index values can range from -1.0 to 1.0, but vegetation values typically
range between 0.1 and 0.7.
Free standing water (ocean, sea, lake, river, etc.) gives a rather low reflectance in both
spectral bands and thus result in very low positive or even slightly negative NDVI
values.
Soils which generally exhibit a near-infrared spectral reflectance somewhat larger than
the red, and thus tend to also generate rather small positive NDVI values (say 0.1 to
0.2).