A very useful image processing technique is band ratioing. For each pixel, we divide the DN value of any one band by the value of another band. This quotient yields a new set of numbers that may range from zero (0/1) to 255 (255/1) but the majority are fractional (decimal) values between 0 and typically 2 - 3 (e.g., 82/51 = 1.6078; 114/177 = 0.6440). We can rescale these resulting ratio values to provide a gray-tone image, in which we can reach 16 or 256 levels, depending on the computer display limits. One effect of ratioing is to eliminate dark shadows, because these have values near zero in all bands. This tends to produce a “truer” picture of hilly topography in the sense that the shaded areas are now expressed in tones similar to the sunlight sides. In other words, band ratioing has the effect of removing shadows.
Three pairs of ratio images can be co-registered (aligned) and projected as color composites. In individual ratio images and in these composites, certain ground features tend to be highlighted, based on unusual or anomalous ratio values. For example, an ore deposit may be weathered or altered so that the diagnostic surface staining or gossan develops. This stain consists of hydrated iron oxide (rust) that is normally yellow-brown. In Band 3, this material reflects strongly in the red but it is apt to be dark in Band 4. The ratio quotient values for this situation tend, therefore, to exceed 2-3, giving rise to a bright spot pattern in a 3/4 ratio image.
For example, rocks containing native copper would develop a distinct greenish tone due to the weathering of copper minerals to form malachite and azurite. To identify this kind of a surface effect, we may ratio TM Band 4 to Band 2, which should make the distinction. Vegetation in Band 4 should be quite bright whereas the copper stain will be greenish and rather bright. The ratio should be very light in tone or gray level. But, for the stain, its reflectance will be low in Band 4 and moderate (medium gray) in 2, so the ratio should be a medium-dark but probably at a higher gray level than if Band 3 had been used instead.
Clay ratio maps are invaluable in mineral exploration. Almost all air-borne and satellite imaging techniques do not directly model mineralization, but instead identify broad areas of alteration associated with the passage of mineralizing fluids. Movement of mineralized fluids through a rock mass causes significant changes to rock mineralogy and chemistry, frequently resulting in the formation of clay minerals due to alteration of feldspars (common in all rocks, basic and acidic igneous rocks as well as sedimentary rocks). The presence of clays is highlighted by 5/7 ratio.
NDVI (normalized difference vegetation index) is a simple formula using two satellite bands. If one of these band is in the visible region (VIS, for example Landsat Band 3) and one is in the near infrared (NIR, for example Landsat Band 4), then the NDVI is the ratio (NIR - VIS)/(NIR + VIS). The reason NDVI is related to vegetation is that healthy vegetation reflects very well in the near infrared part of the spectrum. Green leaves have a reflectance of 20 percent or less in the 0.5 to 0.7 micron range (green to red) and about 60 percent in the 0.7 to 1.3 micron range (near infra-red).
The possible range of NDVI values lie between -1 and 1, but the typical range is between about -0.1 (NIR less than VIS for a not very green area) to 0.6 (for a very green area). The values are stretched over the entire dynamic range to be displayed. NDVI provides a crude estimate of vegetation health and a means of monitoring changes in vegetation over time.
Fig 1 shows an true color (nearly) image of the area around Aligarh town combining Landsat 7 ETM+ Bands 3, 2 and 1 as RGB. Fig 2 is the same image with the red layer showing the NDVI ratio.
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