Introduction

The NDVI is a calculation of the photosynthetic output in a pixel within a satellite image. It measures, in effect, the amount of green vegetation in the image.  In this exercise, you will use MultiSpec32 to create a new channel in an image to display the NDVI for an image.
 

Background

NDVI calculations are based on the principle that growing plants strongly absorb radiation in the visible region of the spectrum (PAR, or photosynthetically active radiation), while strongly reflecting radiation in the near infrared (NIR) region.

    Given the following abbreviations:

            PAR = value of photosynthetically active radiation from a pixel
           NIR = value of near infrared radiation from a pixel

    The NDVI for a pixel is calculated using the formula:

            NDVI =   NIR - PAR
                            NIR + PAR.

This formula yields a value that ranges from -1 (usually water) to +1 (strongest vegetation).

In this exercise we will use only the red image band (band 3) instead of the whole range of PAR.

As such, our formula will be:

            NDVI =     NIR -  red band
                              NIR + red band
 

Creating the New Channel

• Launch MultiSpec32 and Open your TM image file.
• From the Processor menu, choose Reformat.
• Make certain that the Change Image File Format button is selected, and click OK
• The Set Image File Format Change Specifications window opens.  Make sure that your image size in pixels is correctly indicated by clicking on the button to the left of Line - Column in the Area to Reformat box.
• Check the box next to Transform Data.
• The Set Reformat Transform Parameters opens.  Select New Channel for General Algebraic Transformation.
• Enter the following values in the appropriate boxes:

where C3 = channel 3 (TM band 3 [red]) and C4 =  channel 4 (TM band 4 [near IR]).  The "C" in the channel designations must be upper case.

Note:  The equation above creates a new channel (band) for the image, whose contents, pixel by pixel, are the NDVI values for the image.  However, since NDVI normally has a maximum value of 1, and other pixel values in an 8-bit satellite image range from 0-255, we must multiply the NDVI by 256 so that channel values will be compatible with other channels on the image.

• Once you are sure your formulas are correct, click OK.
• The Set Image File Format Change Specifications screen appears.  Click OK
• Save As dialog appears.  Give your file a descriptive name such as imagename_ch8 (Hint: Use the next highest channel number).
• When the file is saved, you will be returned to the original image window. Do not close this window.
• With the original window opened, select Open Image... from the File menu.
• In the Open dialog window, select Link to Active Image Window, select your NDVI channel file, click Open, then click Cancel.
• To re-save your image with your new NDVI channel, select Processor, Reformat..., select Change Image File Format, click OK
• The Set Image File Format Change Specifications screen appears.  Click OK
• The Save As dialog appears.  Save your file with a different name that before, such as, imagename_withNDVI.lan.

Viewing your NDVI Image

• Again, you are returned to your original image.  It looks the same, but there has been a new channel added.
• Select Processor, Display Image.., In the Set Display Specifications for: window, set the green gun to your new NDVI channel number.
• Because this is a new image, MultiSpec32 must build a new histogram file for it.  Proceed as before and save your histogram file.
• Your image now opens with the NDVI channel assigned to the green gun. Areas that are brightest green have the highest NDVI values, and the other colored areas have the lowest NDVI.