Greenness of the Conterminous U.S.
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Explanation and Information
What are the Greenness Maps?
The greenness maps are representations of the Normalized Difference Vegetation Index (NDVI). NDVI is computed daily from two spectral channels observed by NOAA`s AVHRR sensor. The two channels are reflected sunlight in the red (RED) and near-infrared (NIR) regions of the electromagnetic spectrum. NDVI, which is the difference between near-infrared and red reflectance divided by the sum of near-infrared and red reflectance, is computed for each image pixel as follows:
NDVI = (NIR - RED) / (NIR + RED)
Each day new greenness data are computed and compared with the previous day`s greenness data. Pixel-by-pixel the highest (maximum) index value is retained and a new greenness image composite is produced. The use of maximum values effectively reduces cloud cover and errors due to missing data. At the end of a week period the greenness image is completed.
The greenness images presented at this web site are composites of two-weeks (biweekly composites) of maximum greenness values. Biweekly composites are used in order to achieve a nearly cloud free image. Over a region as large as the conterminous U. S. it is highly unlikely that the entire region would be cloud free during a one week period. Weather fronts, especially in the spring time, can take days to tranverse the continental U. S. However, it is more likely to get a cloud free image if you observe a two week period. Therefore, each pixel in a greenness image shows the maximum condition of vegetation for the 1x1-km segment of land for the 14-day, biweekly period.
However, experiences has shown that vegetation condition can change
quite rapidly during the growing season. These rapid changes are
often the most important. Therefore we have adopted an approach that
updates a biweekly image every week. Each week a new image is produced
of the past two weeks of data. In this way, each biweekly image contains
one week of old data, which is also in the previous image, and one week
of new data. For example, image #1 contains maximum greenness values for
weeks 1 and 2; image #2 contains maximum greenness values for weeks 2 and
3; image #3 contains maximum greenness values for weeks 3 and 4; and so
on.. The biweekly NDVI data and image maps are made available for analysis
The Electromagnetic Spectrum:
The electromagnetic spectrum, as shown in the left half of the diagram below, comprises a number of atmospheric "windows" that allow multispectral detection of reflected light and emitted thermal energy from space. It is within these atmospheric windows, specifically the red, near-infrared, and thermal regions, that AVHRR spectral data are collected. Only the red and near-infrared spectral regions, as detected by AVHRR Channels 1 and 2, are used to prepare the greenness maps.
Source: Coast Guard Academy`s Remote Sensing Curriculum: Studying the Environment from Space,
(http://satori.gso.uri.edu/satlab/, Last Updated: March 15, 1996)
What are Spectral Signatures?
The graph below shows the spectral signatures of three features typical of a High-Plains Rangeland ecosystem--green grass, dead grass, and dry soil. A unique characteristic of healthy green vegetation is the presence of the distinctive near-infrared "shoulder." The shoulder results from low red light reflectance caused by chlorophyll absorption and high near-infrared reflectance caused by the plant`s spongy mesophyll leaf structure; both of which are present in actively growing vegetation. This abrupt change in reflectance from red to near-infrared creates the unique spectral shoulder.
So what does this mean in terms of the greenness images? In a typical rangeland setting each image pixel will be a mixture of various types of green healthy vegetation, dead vegetation, and soil; each with their own unique spectral signature. However, the dominance of one over the others will determine how these pixels will appear (that is, what NDVI value the pixel will have). A pixel dominated by green grass, for example, will have a higher NDVI value of about 0.8 compared to a pixel dominated by dead grass or dry soil which will have lower NDVI values of about 0.3 to 0.12. It is this "averaging" characteristic of a satellite image pixel that creates the problem known as mixed spectra or spectral mixing because each pixel will record a mixture of spectral signatures unique to the various features present within the area of the pixel. The features that may be present within a single pixel could be numerous and include many types of vegetation, soils, and other natural or constructed features, or less numerous as in a agricultural setting dominated by a single crop. Also, keep in mind that large image pixels (for example, 1-km AVHRR images) will generally contain a greater variety of features than small image pixels (for example, 30-m Landsat Thematic Mapper images).
What Do The Image Colors Mean?
The colors displayed on the greenness and Departure From Normal image maps are shown in the color bars below. The colors on the greenness images range from medium brown to yellow and green for partially to fully vegetated surfaces having NDVI pixel values greater than 0.05. Bare surfaces, where rock or soil dominate the 1x1-km pixel area, have NDVI values less than 0.05 and are shown in dark brown. The Departure From Normal images are colored to indicate percentage change from normal (1989-present); where normal (average) is considered 100 percent and is colored grey. Areas of negative change (1-99 percent) are colored in shades of brown, orange, and yellow to indicate the presence of less vegetation during a period as compared to normal . Areas of positive change (greater than 100 percent) are colored in shades of light green to dark green to indicate the presence of more vegetation during a period. On both images, water is colored blue and clouds (or snow) are colored white. In general, the deepest (darkest) shades of green indicate areas of abundant healthy vegetation, whereas shades of brown, orange, and yellow indicate areas of barren to sparsely vegetated Earth.
|NDVI||NDVI Departure from Normal|
How do I Use these Images in a GIS?
Before you can accurately use these greenness images in a GIS you have to know something about the scale and map projection format of the images. This information is given below. Additionally, you have to know how your GIS handles images and image coordinate files. In ArcView, TIFF-formatted image files can be used together with a "World File" to accurately place these images into your GIS project file. Follow these five steps:
Save any of the full-resolution GIF-formatted image files at this site
Convert the GIF file to a TIFF-formatted image file with a "tif" file extension
Create a world file (see below) having the same file name as the "tif" image file above, but with the "tfw" file extension
Store all the files in the same directory on your local hard drive
Load the "tif" image file into ArcView. ArcView will automatically read the coordinate information contained in the world (tfw) file when the image is read. Remember to set the view`s projection paramters to the the greenness image (see below).
Creating a World File: Put the following information (no blank lines) into a World File using an ASCII text editor like Notepad. See ArcView help on World Files (or send email to me at email@example.com) for more information on what these image parameters mean.
What is the Display Scale of the Images?
The map scale of the images depends on the size of your computer display. The 17-inch monitor that I am currently using displays 1,024 pixels (1,024 km horizontally on the image) over a distance of about 12 inches (30.48 cm on screen). Therefore, my display shows the full-resolution NDVI and NDVI Departure From Normal images at scales of about 1:3,360,000. The calculation is as follows:
1024 km x 1000 = 1,024,000 meters (the actual ground distance across the display image)
30.48 cm / 100 = 0.3048 meters (the actual distance of the 1024 pixels across my display monitor)
1,024,000 / 0.3048 = 3,359,580 (map scale of the image; stated as a ratio 1/3,359,580 or 1:3,359,580; and rounded to 1:3,360,000)
What is the Map Projection of the Images?
The projection characteristics for the conterminous U. S. greenness and Departure From Normal images are as follows:
|Map Projection: Lambert Equal-Area Azimuthal (Equatorial)|
|Longitude of central meridian||100 00 00 W|
|Latitude of origin||45 00 00|
|Units of measure||Meters|
|Pixel size||1000 meters|
|Center of pixel (1,1)||( -2050000, 752000 )|
|Number of lines||2889|
|Number of samples||4587|
|LAZEA minimum bounding rectangle:|
|Lower Left||( -2050500, -2136500 )|
|Upper Left||( -2050500, 752500 )|
|Upper Right||( 2536500, 752500 )|
|Lower Right||( 2536500, -2136500 )|
|Lower Left||( -119.9722899, 23.5837576 )|
|Upper Left||( -128.5300591 48.4030555 )|
|Upper Right||( -65.3946489 46.7048989 )|
|Lower Right||( -75.4163527 22.4793919 )|
|Lower Left||( -119 58 20 23 35 02 )|
|Upper Left||( -128 31 48 48 24 11 )|
|Upper Right||( -65 23 41 46 42 18 )|
|Lower Right||( -75 24 59 22 28 46 )|
Tell Me More About The Satellites...
A brief description (press release) of the most recent NOAA 16 satellite is given below. For information about other NOAA satellites used to collect AVHRR data click on one of the following satellite links:
NOAA-16, launched from Vandenberg Air Force Base, Calif., on Sept. 21, will improve weather forecasting and monitor environmental events around the world. It is the second in a series of five polar-orbiting satellites with improved imaging and sounding capabilities that will operate over the next ten years.
The spacecraft has a rectangular shape (166" long by 74" high) and is powered by a 191" by 94" solar array. The satellite is Earth oriented, three-axis stabilized and weighed approximately 2200 pounds. NOAA-16 is the eighth operational satellite in the Advanced TIROS-N series (NOAA 13 never officially became operational as it failed during its 21 day checkout period). The satellite carried the AVHRR, TOVS, and the solar proton monitor. All of which were present on previous NOAA satellites. The ERBE instruments, the SBUV radiometer and the SARSAT systems were also flown on this satellite. NOAA-16 was placed in a near circular, (470nm) polar orbit. At this time the spacecraft is still in its checkout period, and APT transmissions appear to be normal.
Links to More Information on...
Satellite remote sensing and its application to vegetation and rangeland studies:
Links to Satellite Imagery: