Patent GB-1054047-A (GB)
Abstract: 1,054,047. Correlation techniques. T.R.W. Inc. Nov.28, 1963 [Nov.28, 1962], No.47150/63. Heading G1U. [Also in Divisions G4, H3 and H4] Automatic photogrammetric apparatus comprises means for scanning a pair of photographs to give two video signals, a computer to determine the correspondence between a space point and the points being scanned in the photographs and means moving the scanning means to maintain maximum correlation of the scanning means. Two stereo photographs D 1 , D 2  Fig. 4, (not shown) are mounted on a carriage 22 and lenses 32 and 33 focus the rasters of flying spot scanners 40, 41 on elemental areas of the photographs. The carriage is moved continuously in the y direction and stepwise in the x direction, the raster being made to follow the y movements during scanning of an elemental area, and to fly back to scan the next area (see "A sawtooth circuit", below). The x movements are counted by the overall control computer 30, and the y movements are reported back to the computer in a digital code by a shaft encoder. Correlation. The outputs of the photomultipliers through gates to a plurality of correlator channels, the gates being controlled so that different portions of the raster scan are effective for each channel. The channel related to the central portion of the raster derives a height error signal, and those related to peripheral portions derive slope information to enable the computer 30 to anticipate the probable height of the next elemental area to be scanned. The correlator channels, Fig. 11, comprise two square-law correlators 151, 152 which receive signals from the photomultipliers after suitable delay, 155, 156. If the signal from PM 1  is delayed relative to that from PM 2 , delay 156 improves the correlation, and 155 reduces it so correlator 152 gives a larger signal than correlator 151, and the output of amplifier indicates the error in height estimation of the computer. The error signal is digitized by feeding to an integrator 260, Fig. 14, (not shown) which controls and is reset by Schmitt trigger circuits 262 which operate a binary counter 265. Tracking homologous points. The error signal from the height correlator is noted in computer 30 and used to adjust the X, X', Y, Y' servos of the lenses 32, 33 so that the rasters are focused on homologous points. Inertia in the lens servo systems is counteracted by applying the error signal to the appropriate deflection circuits of the flying spot scanners 40, 41. Read-out maps. Cathode-ray tube 44 prints out on a negative N2 a shade indicative of the height signal derived from the computer, to produce a shaded contour map, and cathode-ray tube 43 prints out the video signal from one of the photographs D 1  D 2  to produce an orthophoto negative N 2 . Both prints are stopped by a blanking signal from the threshold circuit, 161, Fig. 11, when the correlation falls to a preset level. An alarm is given to an operator who realigns the scans so that tracking can continue. In addition to enabling the computer to anticipate the height of the next area to be scanned, the slope signals derived by the correlators are used to adjust the shape of the rasters on flying spot scanners 40, 41, so that the areas scanned correspond to rectangular areas on the orthophoto. Setting up. Photogrammetric parameters such as camera co-ordinates, lens aberrations and the co-ordinates of certain distinctive "passpoints" on the negatives are fed into the computer on punched tape or by a data writer. The outputs of the photomultipliers are fed to a stero-viewer 61 provided with cross-hairs which are aligned visually with the selected pass points in turn using the data writer. Markers for the passpoints may be inserted on the negatives on which the maps are to be produced. A Sawtooth circuit for controlling the scan of cathode ray tubes 40, 41 comprises two amplifiers 240A; 240B Fig. 13, (not shown) having integrating feedback capacitors 244A; 244B which integrate a tachometer signal to derive positional information. The capacitors are discharged by pulses from an oscillator 248 which generate a magnetic field in the vicinity of neon tubes 246A; 246B; connected across the capacitors. The amplifier outputs are thus interlaced sawtooth signals which are applied through the switch 249 to the tube deflection circuits.