User Manual - pancroma

Since only a portion of EMR reaching the satellite sensor is actually reflectedfrom objects on the surface under observation, the result is a blurring effect, oftensignificantly limiting the view of surface features. The result of both types ofscattering is less of the incident light reaching the surface and more radiationreceived at the satellite sensor from the atmosphere.There are a few widely-used techniques used to reduce the effect of molecularand aerosol scattering. One technique is called the Dark Image Adjustment.This is a histogram-based method that determines the onset of the lower edge ofthe band histograms. The Dark Image algorithm then subtracts this value fromeach digital number (DN) of the image. If the subtraction causes the DN to beless than zero, it sets it equal to zero. In addition to reducing the hazecomponent of a band file, it of course decreases most of the DNs of the image,thus the name.PANCROMA TM has a preprocessing utility for haze reduction of an RGB imageusing the Dark Image method. To use it, open your three band files in the orderblue, green and red by selecting ‘File’ | ‘Open’ from the menu. When the threefiles are open, select ‘Pre Process’ | ‘Haze Reduction’ | ‘’Dark Image Technique’.After the files are opened, you will be prompted to input a cutoff value. The cutoffis the minimum number of DNs in a channel that determines the location of thethreshold. For example, if the cutoff is 500, and channel 44 of the histogram isthe first one to have at least 500 DNs in it, then the threshold and therefore theonset of the lower edge of the histogram will be determined to be 44. The figurebelow illustrates the idea.Cutoff=500Threshold=44You must specify the cutoff value using the ‘Dark Area Data Input’ form that willappear after you specify your band files. The form is shown below.142