Satellite Digital Photography: Preserving the Image in the Digital-to-Analog, Conversion

element (pixel) of the sensor array. The spatial resolution of Landsat Thematic Mapper data is 30 x 30 meters for each of six visual and infrared (IR) bands and 120 meters for one thermal IR band. SPOT delivers 10 meter spatial resolution from one panchromatic band and 20 meter data from three multispectral bands. Raw imaging data transmitted by the satellite typically undergoes a series of preprocessing routines to correct for geometric and radiometric corrections. Variations in the position of the satellite during data acquisition influence the geometric accuracy of the image structure. Algorithms based on tracking data from the ground station are written to mathematically adjust for such distortions before the image is released as a working database. Corrections to the response curves of the satellite’s sensors likewise restore radiometric accuracy to the raw digital data. Corrected data for any one spectral band received from the satellite rarely fill all 256 cells that define the radiometric resolution potential. The composited full-color image is built from the contributing data of three bands selected for a particular application of image interpretation. The compositing formula dictates which one of the additive primary colors – red, green, or blue – is assigned to each of the bands. Regardless of the medium selected for display of the full-color image, it is essential that the monochromatic or gray scale accommodate 256 discrete steps from 0 through 255 if the palette of 16,777,216 colors is to be achieved. Any reduction of that palette will indiscriminately eliminate original data acquired by the satellite and devalue the information content of the image. The challenge, therefore, is to ensure that the display medium retains the full range of data delivered by the satellite and that validation of the color signatures generated by that data be made available to photo interpreters. When the end product is to be a color photograph, an electro-optical film recorder is used to convert the digital image database into a latent analog image in a photographic emulsion. The digital value of each pixel represented in each of the contributing bands regulates the intensity of the light source in the film recorder to expose the emulsion pixel by pixel. The composite of three values generates a single color signature for each pixel in the photograph. When the recorder writes the data from each band onto a separate sheet of black-and-white film, the opacities of each Abstract