Topographic instructions, Book 3, multiplex procedure; Chapter 3 C7a-e

By direct projection of overlapping photographs, printed on glass plates, the multiplex produces an exact optical model, in miniature, of the terrain to be mapped. To create the model, the multiplex projectors must be properly positioned and oriented so that they duplicate the orientation of the aerial camera at the instant of each exposure. By means of a floating mark, horizontal and vertical measurements can be made in the model, and planimetry and contours can be drawn. The applicability of the multiplex to a given mapping project depends largely on the contour interval and compilation scale required, and also depends, to a lesser extent, on the vegetation and terrain cover as it may affect accuracy requirements. The steps in multiplex procedure are orientation, stereotriangulation, and compilation of detail. In orientation, the projectors are arranged so that the projected images form a stereoscopic model which can be adjusted to fit horizontal and vertical control points. In stereotriangulation, three or more multiplex projectors are oriented so that the consecutive models fit existing control, permitting the establishment of additional or intermediate control. In compilation, the features appearing in the model are delineated on the map manuscript.