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Another image manipulation technique is 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 to provide a gray-tone image, in which we can reach 16 or 256 levels, depending on the computer display limits. 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 a diagnostic surface staining, called 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.Tthe ratio quotient values for this situation tend, therefore, to exceed 2-3, giving rise to a bright spot pattern in a 3/4 image. Section 5 includes some exceptional examples of the power of ratioing to discriminate alteration and other rock types.

1-18: Although this is a rare natural situation, rocks containing native copper could develop a distinct greenish tone (think on occasions where you saw weathered copper roofing or a penny that had been outdoors a long time). What two TM bands might you ratio to distinguish this surface indicator of copper from natural vegetation. ANSWER

IDRISI generates ratio images through a module called OVERLAY. These may need to be rescaled and converted to byte format for display. However, the new images are difficult to combine in composites. We show just one ratio image

Ratio image of Morro Bay, California.

made by dividing band 1 into band 4, to illustrate a characteristic product. Most of the image is moderately dark without much variation in gray levels. This implies similarities in DN values for equivalent pixels in the two bands. However, the tonally bright (high DNs) golf course fairways at (s) contrast sharply with their surroundings. Fields also stand out in this way. The grassy area at (v) has well-defined boundaries and is ligher toned.

1-19: Which Principal Component image does this ratio image most resemble? ANSWER

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Primary Author: Nicholas M. Short, Sr. email:

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