How to calculate NDVI

To calculate NDVI (Normalized Difference Vegetation Index) values, you can use the MapCalc function NDVI(a,b).

This function requires 2 satellite bands (one with visible or red values and the other near-infra red values). The function performs the calculation:

(b - a) / (a + b)

When using the NDVI(a,b) function, replace a with the band with visible or red reflectance, and b with the band with near-infrared reflectance.

Example:

If TmBand3 is the band with red values, and TmBand4 is your band with near-infra red values, type on the command line of the Main window:

VegInd = NDVI(TmBand3,TmBand4)

Output map VegInd contains NDVI values.

NDVI values range from -1 to 1:

• Vegetated areas will generally yield high values because of their relatively high near-infrared reflectance and low visible reflectance.
• In contrast, water, clouds, and snow have larger visible reflectance than near-infrared reflectance. Thus, these features yield negative index values.
• Rock and bare soil areas have similar reflectances in the two bands and result in vegetation indices near zero.