ENVI Image Processing (6): NDVI and Vegetation Index

NDVI

Definition: NDVI (Normalized Difference Vegetation Index, Normalized Difference Vegetation Index, Standard Difference Vegetation Index), also known as biomass index change, can separate vegetation from water and soil.

NDVI basic principle:
Plant leaf tissue strongly absorbs blue light (470nm) and red light (650nm), and strongly reflects green and infrared light.The spongy tissue in the center of the leaf and the backside of the leaf are highly reflective to near-infrared radiation (NIR, 700-1000 nm).From red light (Red) to infrared light, the reflectivity of bare ground is high but the increase is small.The higher the vegetation coverage, the smaller the red light reflection and the greater the near-infrared light reflection.Red light absorption saturates quickly, while near-infrared light reflection increases with vegetation.Therefore, any mathematical transformation that enhances the difference between Red and NIR can be used as a vegetation index to describe vegetation conditions.The normalized vegetation index (NDVI) is defined as:

NDVI = [ (NIR/Red-1)/(NIR/Red+1)].

Indicates range: -1<=NDVI<=1

• Negative values ​​indicate that the ground is covered with clouds, water, snow, etc., highly reflective to visible light
• 0 means rock or bare soil, etc. NIR and R are approximately equal
• A positive value indicates that there is vegetation coverage, and it increases with the increase of coverage;

Vegetation Index

There is one more index to introduce here: Vegetation Index

The algorithm that can extract vegetation according to the ratio between the bands is called Vegetation Index (VI)

Vegetation Index:

• Ratio Vegetation Index (RVI:Ratio Vegetation Index): RVI=IR/R
• NDVI:Normalized Vegetation Index: NDVI = (IR-R)/(IR+R)
• DVI:Difference Vegetation Index: DVI=IR-R

ENVI Actions

NDVI

The following is demonstrated in ENVI
Parameter list:

• input file type: input data type (can be viewed in the header file)
• NDVI bands—red: red bands
• NDVI bands—near IR: near infrared bands
  

band math

Four operations of band math, the result is the same

ViewNDVI value, it is easy to know that the upper right NDVI is positive, and the lower left NDVI is generally negative, so the vegetation coverage on the upper right is high, and the vegetation coverage on the lower left is low

quick stat chart

Data analysis:

• Maximum value: -1.000000
• Minimum: 1.000000
• Average: -0.006670
• Standard Deviation: 0.153373

DN is -1.000000, there are 157443 pixels, accounting for 0.261674, indicating that the overall DN is not low and the vegetation coverage is good