User Manual - pancroma

select ‘File’ | ‘Open’ and open your file. Now select ‘Pre Process’ | ‘GammaCorrection’ and either ‘Grayscale Image’ or ‘Color Image’. When you do so athumbnail version of your input image will be come visible. Using the slider,adjust the gamma coefficient until the image has the brightness and contrastcharacteristics you desire.Band Math - Image SubtractionAs its name implies, this utility subtracts one grayscale image from another on apixel-by-pixel basis. Select ‘File’ | ‘Open’ and open two files. PANCROMA TM willsubtract the second file from the first. If you wish to scale how much of thesecond file gets subtracted, select the ‘Activate Image Processing Routines’check box. When the image processing box appears, use the ‘Subtraction ScaleFactor’ slider bar to adjust the percentage of the second file to be subtractedfrom the first. This utility will use 16-bit math for Landsat 8 data.Band Math - Band RatiosBand ratios are often very helpful for highlighting different features on the earth’ssurface. The technique is especially useful for Landsat bands and there is quitea bit of information available regarding this technique. (See the ’BandCombinations’ link in the PANCROMA TM ‘Help’ menu for some useful ratios.)As its name implies, ratioing two Landsat bands just means dividing each pixelvalue in the numerator image by its corresponding pixel in the denominatorimage. There are some complications in practice however. The first problem isthat corresponding pixels tend to be not very different in magnitude. So althoughpixel values can theoretically range from 0 to 255 (disallowing division by zero),most of the values will be less than 6. Such a division would result in a verydark, low contrast image that would not be useful. As a result, PANCROMA TMuses the following algorithm to compute band ratios:• Convert each pixel value to type FLOAT• If the numerator and denominator are both zero, set the ratio equal to zero• (Or else) if the denominator is zero, set the ratio equal to 254• (Or else) f the denominator is greater than zero, compute the ratio as aFLOAT• Multiply the computed ratio by scaleFactor• Remove all values greater than the Upper Clipping Percent*Max Value• If the Histogram Stretch check box is checked, stretch the image so thatthe max DN is 254 and the min DN is 0• If the Histogram Equalize check box is checked, equalize the image• Convert the ratio to type BYTEIt is often necessary to clip the ratio histogram significantly if there are high flyersin the band ratio array. The portion of the histogram that is clipped is determined170