Photomap Image Quality

Abstract

Both image and photomap scales are being reduced and consequently the retention of detail in the cartographic process becomes increasingly difficult. It is suggested, therefore, that a systematic method based on Modulation Transfer Function (MTF) theory may be of value for evaluating the transfer of detail when planning map products. In order to maximise photomap detail the original photograph should be enlarged to a point where the MTF of the enlargement will not degrade the printing MTF. As the printing process is often limited by the method of screening the image, screening techniques which result in the retention of spatial frequencies to beyond 5 1/mm are required. Based on theoretical analyses of different types of imagery, enlargement factors of 4 to 8x for high-altitude aerial photography, less than 7x for ERTS A imagery and from 10 to 20x for SKYLAB photographs will result in quality photomap products, provided suitable printing techniques are employed.