Photomechanical Density Modification of ERTS Imagery

Photomechanical Density Modification of ERTS Imagery Charles E. Ogrosky* Geographers frequently use conventional aerial photographs or other remotely sensed imagery such as that provided since 1972 by the successful Earth Besources Technology Satellites, EBTS-I and EBTS-2.1 In many instances, it is necessary to modify imagery so as to delete undesirable detail, enhance useful information , or adapt the imagery to a particular reproduction process.2 Such processing is evident, for example, in the orthophoto topographic quadrangle series being prepared by the United States Geological Survey, in which images, supplemented by overprinted symbols, provide map content that must be prepared rapidly and published in quantity.3 Automated systems for image manipulation are being developed , but their experimental nature and high costs are restric- * Mr. Ogrosky is an Assistant Professor of Geography at Rutgers University , New Brunswick, New Jersey 08903. He was a graduate student at the University of Washington when this paper was presented at the annual meeting of the Association in Corvallis in 1974. 1 For detailed information on the Earth Resources Technology Satellites, renamed "Landsat-1" and "Landsat-2" by the National Aeronautics and Space Administration in January 1975, see the Earth Resources Technology Satellite Data Users Handbook, prepared for NASA by the General Electric Company, Space Division, Valley Forge Space Center, Philadelphia, 1972. 2J. S. Keátes, Cartographic Design and Production (New York: Wiley, 1973), pp. 102-104. Also see E. A. Wingert, "Frequency Concepts in Cartographic Design," Proceedings of the American Congress on Surveying and Mapping, 34th Annual Meeting, St. Louis, Mo., March 1974. 3 D. Landen, "Progress in Orthophotography," Photogrammetric Engineering , Vol. 60, No. 3 (March 1974), pp. 265-270. 87 88ASSOCIATION OF PACIFIC COAST GEOGRAPHERS tivè.4 A more feasible technique at present consists of manipulating photographic exposure and processing3 by means of photomechanical masking, in order to alter the density relationships of a continuous tone original image. Because this technique requires only minimal equipment and materials, it will remain useful for many applications, although relatively few cartographers are yet aware of its potentials. Photomechanical Masking A mask is a photographic image which is used during exposure to modify densities in selected areas on a new image. In this respect, the term "mask" is akin to its familiar cartographic meaning; for example, using a solid area image, such as a "water mask," to add a gray tone or tint to selected areas of a map image. Photomechanical masking involves exposure of high contrast (litho) film through a negative but with modification of the image which would normally be obtained by including a positive film mask produced from the negative in the exposure sequence. Two products, phototones and photolines, may be prepared by masking. The character of the film negative and positive mask pair (i.e., continuous tone or high contrast) and variation of spacing between the negative and the positive mask during exposure of the litho film determine which product will be produced. Masking for Phototone Images Masking for a phototone yields a high contrast reproduction of a continuous tone original image such as the one shown in Figure 1. 4 For a discussion of computer-aided ERTS image interpretation, see National Aeronautics and Space Administration, Third Earth Resources Technology Satellite Symposium, Vol. II (Washington, D.C.: Government Printing Office, 1974). Also see Photogrammetric Engineering, Vol. 40, No. 10 (October 1974), which is devoted to automated imagery processing. 5 R. N. Weiler, "Photo Enhancement by Fi'm Sandwiches," Photogrammetric Engineering, Vol. 36, No. 5 (May 1970), pp. 468-474; and U. Nielsen, "Agfacontour Film for the Interpreter," Photogrammetric Engineering, Vol. 38, No. 11 (November 1972), pp. 1091-1098. YEARBOOK · VOLUME 38 · 197689 Figure 1. Multi Spectral Scanner (MSS) Band 5 (600 to 700 nanometres ) Puget Sound image reproduced in original 70 mm form. This image was used to produce the normal contrast negative shown in Figure 2a. Phototone production requires preparation of a continuous tone negative (Figure 2a) from the original positive image and then a low contrast continuous tone positive mask (Figure 2b) from that negative. Contact exposing the negative and positive mask in register to high contrast film creates a very different image from the one which would result from exposure...