Patent GB-870376-A (GB)
Abstract: 870,376. Inspecting stereo images for map making. HUNTING SURVEY CORPORATION Ltd. July 11, 1957 [Aug. 17, 1956], No. 21965/57. Class 40(3) In apparatus for deriving information to be used in the making of maps, from two stereo images by correlating in the two images corresponding points having similar information thereabout, the images are simultaneously and similarly scanned by a photo-electric system and the electrical outputs derived from the two images are multiplied together to obtain a multiplied signal which will be of maximum value at maximum correlation of information scanned on the two images, means being provided for moving one scanning pattern relative to the corresponding image until the multiplied signal is a maximum. Stereo pairs of photographs are each scanned individually by a circular scanning system, which may be as shown in Fig. 7, wherein a synchronously rotated eccentrically apertured lens 23 projects light from an illuminated photograph 25 on to a photo-cell 62 thereby effecting scanning by a spot 58 moving in a circular path 36. The scanning spot and scanning circle for one of the photographs are larger than the scanning spot and scanning circle for the other photograph. If the two scanning circles are similarly aligned with respect to the same feature XY in each photograph, a condition represented by the superimposed scanning circles of scanning spots 57, 58 in Fig. 17, then the information derived by the respective photo-cells in the scanning systems for the two photographs will show a maximum degree of correlation and the signals from the two photo-cells will contain in phase components at the scanning circle frequency. If the two scanning circles are misaligned in the manner represented in Fig. 19 then the information derived will show a lesser degree of correlation and the two photo-cells will contain components at the scanning circle frequency which have a phase difference depending upon the direction of misalignment of the two scanning circles. If the two scanning circles are completely misaligned, as represented in Fig. 23, both the degree of correlation between the two signals, and the scanning circle frequency component in the multiplied signal will be zero. The respective photo-cells 62, 63, Fig. 25, are each connected through a preamplifier and high pass filter 64, 66 and 65, 67 respectively to a ring demodulator 70 which multiplies the two signals together. The output of the demodulator feeds a band pass filter 72 operating at scanning circle frequency to derive a signal which by its phase represents the direction of misalignment between the two scanning circles as represented in Fig. 19. By means of a servo amplifier 74 and co-ordinate servo motors 76, 77 one of the scans can be adjusted until the maximum correlation condition of Fig. 17 is achieved. The output of the demodulator also feeds through a low pass filter 81 to obtain a D.C. signal which is of a maximum positive value when correlation between the two scans is a maximum and is zero when the correlation is zero, and which is fed to a D.C. differential servo-amplifier 82 to drive a direct current servo-motor 83. The servo-motor 83 is used to reduce the distance h in Fig. 7 to obtain the smallest scanning pattern possible at values above a predetermined correlation up to a maximum correlation and to increase the distance h to expand the scanning pattern under conditions of less than a predetermined degree of correlation.