MVH Web
July 5, 2006

Study a Time Series of Landsat Images with the VegetationAnalysis Software

Purpose: Explore a time series of 2 or 3 Landsat images side by side. 

The software is designed to use the jpegs of Landsat images available at the USGS Earthshots website: http://edcwww.cr.usgs.gov/earthshots/slow/tableofcontents.

NOTE: The images that follow are not interactive. Please download the software (OSX OS9 PC) and follow along with the mini tutorial.

Background: Landsat

The Landsat satellite program has a long and significant history in monitoring our Earth. The first images of the Earth's land features were taken in 1972, and for over 40 years, the data has been used to study the many interconnected processes changing our land cover.

Background Information: Normalized Difference Vegetation Index

A Vegetation Index is a measure of the amount of healthy vegetation covering the Earth's surface. 

    Healthy vegetation absorbs visible light, especially red and blue light, and reflects much of the infrared.
    A simple Vegetation Index is based on the difference between these intensities. 

Another index, the "Normalized Difference Vegetation Index" or NDVI, provides a more consistent identification of vegetation as well as indicating the health of plants.

NDVI is the difference between the intensities of the reflected infrared and visible red light divided by the sum of the intensities of the two light measurements. 

    This mathematical manipulation compensates for areas experiencing hazy sunshine compared to those in clear skies as well as reduces shading and sloping topography.
    Dense, healthy vegetation produces NDVI values near +1.0.
    Bare soil and rock reflect similar levels of infrared and red light, so these surfaces produce NDVI values near 0.
    Clouds, water, and snow reflect more visible light than infrared, which is the opposite of vegetation, and so produce NDVI values near -1.0.

Useful articles on vegetation indices using satellite data:

Getting Started

First, TYPE the number of satellite images to work with in the white box above.

    This number must be either 2 or 3. 

Next, click 'Select Satellite Images'. 

    Pick the earliest (oldest) picture first, followed by the next oldest, and the most recent image last.
    You will not be allowed to continue if you pick images that are not of the same area or not in increasing year. 

After selecting an image, enter the numerical value of the map scale value visible in the lower left corner of the satellite image.  The program will automatically calibrate the analysis tools to measure distances and areas of features within the image.

    If you make a mistake, select the 'Calibrate' option in the 'File' menu.

Exploring Satellite Images

Use the 'Change Enhancement' button to toggle between an RGB picture and a NDVI enhancement. All satellite pictures will have the same enhancement. 

Analysis Tools: There are three tools to analyze the satellite images.  Information at identical locations within each satellite image will be automatically calculated and displayed.

1) Point Analysis Tool

    Explore the vegetation index (NDVI) at each pixel of the satellite images.
    Compare the values at the same location on each satellite image. 
    Move the cursor around a satellite image in three ways:

    1) Click the mouse on an area of interest,
    2) Click and drag the mouse to an area of interest,
    3) Use the small up and down arrows along the upper-right edge.  These arrows become visible when you select the Point Tool.

2) Line Analysis

    Place the cursor in a satellite image, click and drag the cursor to draw a line. 
    Adjust the end points of the line with the Use the small up and down arrows along the upper-right edge. 

    These arrows become visible when you select a Line Tool.
    The start and end of the line are color-coded (blue = start, red = stop). 

    NDVI values for pixels along the line are graphed. 

    Move the cursor to each satellite image to see individual lines of yearly data on the graph.
    An average NDVI of pixels along the line are calculated and color-coded by the year of each image.

3) Area Analysis

    Place the cursor in a satellite image, click and drag the cursor to draw a rectangle. 
    NDVI values for pixels inside the rectangles are used in calculations. 
    If min/max NDVI values are -1 and 1, respectively, a histogram of NDVI values within the rectangle are plotted. 
    As with the line tool, move the cursor to each satellite image to see individual lines of yearly data on the graph.
    By changing the min/max values, the percent of NDVI values between the selected range is displayed. 

Page Setup and Print: To print the images and graphs, first use 'Page Setup' in the File Menu. Select 'landscape' printing and experiment with the scale for a full-page print.  We have found that a scale of 75% works on our printers.

Getting Help: Check the 'Help" menu for information on software features when using the software.