Photomosaic, Controlled

Photomosaic, Controlled 

an exact photographic plan of a locality, compiled chiefly for cartographic purposes.

A controlled photomosaic is mounted to a controlling framework of geodetic points on a nondeformable backing; it is compiled from photographs that have been referred to a given scale and horizontal orientation by using a special instrument to eliminate distortions due to inclinations of the camera axis during exposure and to the unevenness of the surface photographed. In general, it is the central sections of adjacent, overlapping aerial or satellite photographs that are used in assembling controlled photomosaics that are highly detailed and suitable for precise measurements. Large-scale controlled photomosaics may be assembled mechanically by cutting and mounting prints, or optical systems may be used to project corresponding sections from negatives onto base sheets. It is much more complicated to prepare controlled photomosaics of mountainous regions than of plain regions because of the wide range of elevations involved. For this reason photographs may be differentially corrected to obtain a special photomosaic called an orthophotomap. Still in the development stage are methods for compiling controlled photomosaics from photographs that reproduce the terrain from the display screen of scanning systems and from ground and underwater photographs.

Controlled photomosaics are prepared strictly within the framework of the map projection limits to topographic maps, for which they serve as the source material. They are often used without modification in planning and surveying operations, and they are a necessary source material for the compilation of photomaps.

L. M. GOL’DMAN