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• Acquire definition and equation for Normalized Difference Vegetation Index (NDVI) from an Internet site.
• Download images of Visible and Near-infrared Reflectance.
• Convert image formats from the Internet using Graphic Converter.
• Use NIH Image (or Scion Image) Image Math to create an image of NDVI.
• Use NIH Image (or Scion Image) Image Math to compute NDVI.

Introduction

Vegetative Index is a measure of the amount of vegetation on the ground. It is important for many things, including studying bird habitat and other wildlife studies. Normalized Difference Vegetative Index (NDVI) is computed from data from two channels on an AVHRR satellite. Channel 1 measures how much light is reflected from the earth. Channel 2 measures how much heat in the form of infrared wavelengths is reflected. AVHRR is Advanced Very High Resolution Radiometer in a National Oceanographic and Atmospheric Administration (NOAA) satellite. The information comes to us from the U. S. Geological Survey’s EROS Data Center. This data is used by farmers in the Midwest United States. (today's image)
 

AVHRR Channel 1 Reflected Visible Light from Earth's Surface	AVHRR Channel 2 Infrared Heat Emited from Earth's Surface

1. Open the site Global Land Biosphere Data and Resources, Visible and Near-infrared Reflectance to see both the visible and the near-infrared reflectance images taken on the same clear day of the Nile valley in Egypt on the continent of Africa.

• The image on the left is a Channel 1 image, reflected visible light.
• The image on the right is a Channel 2 image, reflected near-infrared wavelengths.

3. To locate the formula used to calculate the NDVI from the reflected visible light (Channel 1) and reflected near-infrared wavelengths (Channel 2) images, open Global Land Biosphere Data and Resources, Normalized Difference Vegetation Index site.
 

4. Use a Graphic Converter program, such as GIFConverter or Paint Shop Pro, to convert the two images you downloaded from GIF to PICT. This will allow us to use NIH Image to process the images and determine the NDVI.

5. Open both images with NIH Image. Place them side-by-side. Describe their differences. (HINT: Since Chlorophyll absorbs more light it will be darker.)

Each pixel in each of the images has a value. The values in the Channel 1 image represent brightness of light being reflected. The values in the Channel 2 images represent the amount of near-infrared wavelength being reflected.

6. Write down the value of the pixel located at 90,250 on each image.
 

To create an image of the NDVI, we must perform some image math using these two images and the formula for NDVI. First, we will subtract each pixel value in the CH 1 image from each pixel value in the CH 2 image. You can see from Question 7 that that would be a lot of numbers to subtract if we didn’t have some help.

9. Second, we will compute the denominator of the equation you wrote down in Question 3a. Select Image Math from the Process menu. Select the options that permit CH 2 + CH 1. (Other options: X .05, + 0, do not select Real Result, save as Denominator.) In this operation the computer program adds every pixel value in CH 1 to the corresponding value in CH 2 image to create a new image whose pixel values are the sums. Describe the Denominator image you just created.

10. Finally, you will perform the division in the equation. You will use the computer to:

Select Image Math from the Process menu. Next, select the options that permit Numerator/Denominator. (Other options: X 255, + 0, do not select Real Result, save as NDVI.) In this operation the computer program divides each pixel value in the Numerator image by each corresponding value in the Denominator image. The resulting image will have pixel values that are the quotients of all these divisions. It will be the visual representation (image) of the Normalized Difference Vegetative Index (NDVI). Describe the NDVI image you just created. (HINT: Notice the light area at the delta of the NILE!) You may want to use a different LUT to enhance the image.

Online Extension Data Locations

Here are some NDVI images from around the globe:

• NDVI departure from normal
• NDVI Images for Africa
• Climate Data Set
• Australia Vegetation Greenness maps

Assessment Ideas

• Students are given Visible and Near-infrared Reflectance images to compare values of identified pixels with the pixel values of the NDVI image.
• Students create an NDVI image from Visible and Near-infrared Reflectance images.
• Students are able to analyze changes in NDVI over a 1 year period or 10 year period.

Online Resources