of image parameters such as spacial frequency, contrast, correlation 
sensitivity, and terrain roughness. For each desired photograph area, 
the coordinates of the best pass-point are recorded, along with its quality 
weighing factor, for use in point-transfer operations or analytical ad- 
justment procedures. 
The most recently publicized development in the field of mapping 
that involves photo interpretation in one sense is the development of the 
Stereo Image Alternator i, While perhaps better described as a opto- 
mechanical improvement, its development has expanded the capability of the 
stereoplotter operator which is certainly one of the aims of automation. 
As is well known, direct-viewing double-projection stereo- 
plotters have relied on anaglyphic two-color projection for the presentation 
to the operator of a three dimensional relief model. To view this relief 
and subsequently interpret the contours of the stereo projection, the 
operator was required to wear anaglyphic spectacles. While certainly 
an adequate arrangement for many years, it had several inherent dis- 
advantages such as an extremely low light intensity of the projected model 
due to double filtering which required operation with a limited level of 
ambient light. Incomplete image separation was also evident due to in- 
effectiveness of the filter as was unequal sharpness of the filtered images 
due to the fact that the optimum projection distances for red and blue 
light are not the same. Perhaps most important, the anaglyphic technique 
was not compatible with color photography. 
(13) "The Stereo Image Alternator", J. William Knauf, U. S. Geological 
survey, Washington, D.C., Photogrammetric Engineering, Vol. 33, No. 10, 
October 1967.