Patent GB-841835-A (GB)
Abstract: 841,835. Topographical maps plotting devices. PICKARD & BURNS Inc. Oct. 25, 1956 [Nov. 1, 1955], No. 32605/56. Class 40(3). A device for plotting contour lines from stereographic pairs of photographs includes means for electro-optically scanning images of the two photographs, means for processing and comparing the two electric signals obtained and means for marking a surface to form a contour line in response to the difference between the processed signals. In Fig. 7, images of diapositives prepared from a stereographic pair of photographs are projected by projectors 22.1, 22.2 on to a scanning surface 23 divided into separate areas for each image by a septum 51. A slide 52 in the surface 23 is formed with a scanning aperture 23.1 in one image area and with a similar scanning aperture 23.2 (not shown in Fig. 9) in the other image area. Light passing through the apertures 23.1, 23.2  falls on to respective photo-tubes beneath the scanning apertures. The surface 23 is carried by a scanning head 50 which can be displaced in horizontal (x and y co-ordinate) directions and in a vertical (z co-ordinate) direction by lead screws 83, 86, 91. A carriage 85, which carries the scanning head 50, also carries a scriber 95 over a metal plate 61 on which the mapping sheet 62 is placed. The lead screws 83, 86 are driven by motors so that the scanning apertures 23.1, 23.2, execute coarse scanning movements to cover the whole area of the projected images and the slide 52 is simultaneously vibrated so that the apertures 23.1, 23.2 execute fine scanning of detailed features in the images. After a complete scan across the whole areas of  the projected images the vertical position of the  surface 23 is re-adjusted by means of the lead screw 91 so that the spacing between the apertures 23.1, 23.2 may bear a different ratio to the dimensions of the projected images whereby the scanning may yield information from features in the images corresponding to a different elevation. Each of the photo-tubes beneath the apertures 23.1, 23.2 is connected to a separate electronic circuit in each of which the photo-tube output is filtered of its d.c. component and is amplified by an amplifier having a.g.c. to hold the output amplitude to constant levels, the amplified signals from the two phototubes then being applied to a subtractor circuit which produces an output proportional to the difference of their instantaneous voltages. The subtractor output is amplified and rectified to obtain an absolute value, this rectified signal being integrated in two separate parallel branches over short and long time constants. The scriber 95 is energized when signals from both integrating branches fall below a threshold limit given for each. Thereby a mark is made on the mapping sheet 62 whenever identical features in the images are scanned simultaneously. In alternative arrangements both images are projected through filters or polarizers 231.1, 231.2 into the same area, as shown at 223 in Fig. 14 and the scanning is effected by oscillating galvanometer mirrors 231, 232 which direct the scanning beam through a thinly silvered mirror D.C. 233 which divides the beam into two components directed through collimator or microscope systems 71.3, 71.4 on to the photo-tubes P15, P16, having an appropriate filter or analyser 234.3, 234.4. In Fig. 13 the scanning is effected by an oscillating roofshaped mirror 225 operated by four actuating coils 227.1 to 227<SP>.</SP>4, and a fixed annular mirror 224. The fine scanning may be in the form of a self-intersecting closed curve such as the "daisy" pattern of Fig. 18.