EM 1110-2-2907
1 October 2003
avoidable. Again, a ratio of the bands of interest will limit shadowing and sun angle ef-
fects. It is therefore possible to perform a temporal analysis on data collected at different
times of the day or even at different seasons.
(b) A disadvantage of using band ratio is the emphasis that is placed on noise in
the image. This can be reduced, however, by applying a spatial filter before employing
the ratio function; this will reduce the signal noise. See Paragraph 5-20c.
(6) Create a New Band with the Ratio Data. Most software permits the user to
perform a band ratio function. The band ratio function converts the ratio value to a
meaningful digital number (using the 256 levels of brightness for 8-bit data). The ratio
can then be saved as a new band and loaded onto a gray scale image or as a single band
in a color composite.
(7) Other Types of Ratios and Band Arithmetic. There are a handful of ratios that
highlight vegetation in a scene. The NDVI (Normalized Difference Vegetation Index;
equations 5-1and 5-2) is known as the "vegetation index"; its values range from 1 to 1.
NDVI = NIR-red/NIR + red
(5-1)
where NDVI is the normalized difference vegetation index, NIR is the near infrared, and
red is the band of wavelengths coinciding with the red region of the visible portion of
the spectrum. For Landsat TM data this equation is equivalent to:
NDVI = Band 4- Band 3/ Band 4+ Band 3
(5-2)
In addition to the NDVI, there is also IPVI (Infrared Percentage Vegetation Index), DVI
(Difference Vegetation Index), and PVI (Perpendicular Vegetation Index) just to name a
few. Variation in vegetation indices stem from the need for faster computations and the
isolation of particular features. Figure 5-12 illustrates the NDVI.
5-22
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