Patent GB-893928-A (GB)
Abstract: 893,928. Apparatus for locating corresponding areas of stereoscopic photographic images. HUNTING SURVEY CORPORATION Ltd. April 21, 1958 [Nov. 14, 1957], No. 12500/58. Class 40(3). Apparatus for deriving information to be used for producing a map from a pair of stereoscopic photographic images by locating corresponding areas of said images having similar information therein comprises means for moving a scanning spot over a path of predetermined direction on each of said images means for independently sensing the information encountered in each image by each scanning spot and for generating information signals corresponding thereto, means for processing said information signals to establish a single order differential difference therebetween and means for multiplying said processed signals to derive a signal related to the misalignment of the corresponding areas relative to the scanning paths. In Fig. 1 stereographic transparencies 12, 13 of the aerial survey type are mounted on a base 15 which is movable in the directions of x and y axes. The transparency 13 is carried on the base 15 by a frame 11 which is movable relatively to the base 15 in the directions of the x and y axes by means of servomotors 16, 18. A scanning pattern produced on the screen of a cathode ray tube 20 is focused by lenses 25, 26 to scan elemental portions of the transparencies 12, 13 respectively, disposed about scanning axes 30, 31 directed on to photo-multiplier tubes 28, 29 respectively. The output of the photo-multiplier 28 is applied to a differentiating circuit 43, Fig. 2, the output of which passes to a multiplying circuit 45 where it is multiplied by a signal corresponding to the second-order differential of the output of the photomultiplier 29 as determined by a secondorder differentiating circuit 44. The output of the multiplying circuit 45 constitutes an error signal proportional to the difference in the displacement of similar features scanned in the transparencies 12, 13 with respect to the respective scanning' axes 30, 31. The error signal is applied to the servomotor to reduce the difference to zero. Since stereographic pairs of aerial survey images correspond in the direction of the y axis and embody differences in the direction of the x axis, the cathode ray tube 20 is first arranged to scan along the direction of the y axis and the frame 11 is adjusted by servomotor 18 in the direction of the y axis until zero error signal is obtained. The tube 20 is then arranged to scan a feature of known elevation along the x axis and the frame 11 is correspondingly adjusted in the x direction by servomotor 16 until zero error signal is obtained. The base 15 can now be moved so that the scanning axes 30, 31 pass through another feature in the transparencies and the error signal due to an x axis scan will indicate any difference in the displacement from the axes 30, 31 in the direction of the x axis of corresponding features scanned in the two transparencies. The distance through which the frame 11 has then to be moved in the direction of the x axis to reduce the error signal to zero is a function of the difference in elevation between the feature being scanned and the feature of known elevation. The size of the scan may be controlled automatically by feeding to the scan control circuit 21 a signal derived from a multiplication of single order differentials of the outputs of the photo-multiplier tubes 28, 29, so that the scan is reduced in size as the difference in displacement from the axes 30, 31 of corresponding features in the two transparencies is reduced. Specifications 870,376 and 893,930 are referred to.