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 by stretching to provide a gray-tone image of each Band X/Y ratio, in which each can fill out 16 or 256 levels, depending on the computer display limits.
Still, while class-related tonal contrasts may smooth out, certain ratios can point to certain tonal anomalies that are diagnostic of special conditions. This first ratio image (in Wyoming), made from an MSS Band 5/MSS Band 4 data set, exemplifies this. A red surface would give a high DN value in Band 5 (MSS red band) and a low number in Band 4 (little green light), so that the ratio would be a large number that would produce a light tone in an image. In the image below, the bright tonal patch just below the center is an iron-rich anomaly (reddish on the ground) over a small oil field that has leaked petroleum which leached out iron oxide to form a tell-tale stain (the linear white pattern in the middle right is a red-colored geologic outcrop):
For a Landsat MSS data set, a total of six ratios (Bands 4/5. 4/6, 4/7, 5/6, 5/7, 6/7) and six reciprocals (5/4, 6/4....7/6) are possible. 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. The ratio quotient values for this situation tend, therefore, to exceed 2-3, giving rise to a bright spot pattern in a 5/4 image.
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 made by dividing Band 1 DN values into Band 4 DNs, to illustrate a characteristic product.
In this unstretched version, 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.
(Note: the above ratio 1/4 image was produced on a DOS version of IDRISI. The writer (NMS) tried to create new ratio images on IDRISIW, the Windows version, with no luck. But excellent examples of ratio images, and ratio color composites, developed on the IDIMS system at NASA Goddard while the writer was still there, are shown on page 5-3.) To preview this, look at a TM Band 3/Band 1 ratio image of a mineralized area in southwestern Utah, and then a color composite made from 1/7 = blue; 4/2 = green; 3/1 = red.
The lightest tones coincide with iron enriched surface material (reds and yellows in the field).
The yellows and reds in this composite denote areas of rock alteration and mineralization.