987,854. Photogrammetry; stereophoto analysis. HUNTING SURVEY CORPORATION Ltd. Aug. 22, 1961 [Sept. 6, 1960], No. 30673/60. Headings G1A and G1F. GENERAL DESCRIPTION, Figs. 8, 18. In an electronic system whereby topographical information may be automatically obtained from a pair of stereoscopic aerial photographs and plotted in contour form or as a series of profiles along parallel axes, the two photographic plates 27 and 28, Figs. 8, 18. mounted above a plotting table surface 307, on which C.R.T. assembly 387 is mounted at the cross over point of movable orthogonal rails 33 5 and 339. Photo-cell assemblies 29 and 30 are mounted above each photographic plate and are coupled to the C.R.T. assembly by rod 414 such that they always point towards the screen 26a of the C.R.T. The C.R.T. displays a noise derived scan as shown in Fig. 8 embodying a 400c/s rotary motion. This random scan is used to analyse a small area of each photograph at a time depending on the position of the C.R.T. assembly on the plotting table. Assuming that the photographs are aligned in the Y direction, i.e. at right angles to the line joining their centres, any difference in the signals produced by the photo-cells represents a misalignment in the X direction, and in particular a lag of one signal with respect to the other represents an X parallex which in turn indicates that the feature being scanned is at a certain height. This parallex can be reduced to zero by adjusting the vertical position of the C.R.T. under the servo control of a signal representing the X parallex. If the C.R.T. is then moved along a line so that a corresponding line is scanned in each photograph, then the C.R.T. will automatically continually adjust its height whereby by suitable attachments a profile of the terrain represented along the said line, may be drawn. Alternatively the signals from the photo-cells may be analyzed to give the direction of maximum slope of the terrain portion momentarily scanned, and, by applying this information to the motors moving the rails 335 and 339, and thus the C.R.T., and by keeping the C.R.T. at a constant height, the C.R.T. will automatically trace out a contour of the region covered by the photographs at a height represented by the height of the C.R.T. The contour may be drawn on the plotting table by attaching a stylus to the base of the C.R.T. assembly. CIRCUIT DETAILS (a) Profiling mode. Figs. 1, 2, 4, 8, 12, 13. In the scanning generator 107, Fig. 1, a 400c/s signal from source 10 is split into quadrature components which are added at junctions 16 and 17 with noise signals from generators 14 and 15. The composite signals are applied via coupled variable gain amplifiers 18 and 19 to the X and Y deflection plates of the C.R.T. 26, to produce a random scan as in Fig. 8 having a fundemental 400c/s repetition rate. The random scanning is used so that every possible direction of slope in the portions of terrains scanned shall be investigated, whilst the 400c/s repetition is provided to produce phase information as to the direction of maximum slope. A small area of each photograph is scanned by the C.R.T. scan and the resulting output signals from the photo-cells passed to a registration discriminator 39. The operation of this discriminator will be explained with reference to the simpler registration discriminator of Fig. 12 comprises band pass filters 51 and 52, which effectively act as differentiators, lagging inducing means 58, leading inducing means 59, a point by point multiplier 60, and a low pass filter 61. When a light region is scanned in the photographs, corresponding pulses are produced on lines 37 and 38, which on passing through filters 51 and 52 produce the waveforms of R and S. The sets of waveforms are given for three conditions of alignment between the areas of the photograph being scanned. It is thus seen that no output is produced at point 62 when the images are aligned (i.e. conditions of no parallex), a positive output is produced when the pulse leaves photo-cell 29 before photo-cell 30, and a negative output is produced when the pulse leaves photocell 30 before photo-cell 29. The modification of the circuit of Fig. 12 into the discriminator 39 shown in Fig. 1 nearly changes the stepped waveforms W into corresponding pulse waveforms. This pulse waveform, having a D.C. component proportional to the misalignment of the photographs in the instantaneous direction of the scanning spot is passed on line 50 to a parallex analyzer 81 where it is divided in two by divider 72 and passed to multipliers 75 and 76. To separate the misalignment signal into components along the X and Y axes the X and Y C.R.T. deflection signals are differentiated at 77 and 78 and respectively passed to the multipliers 75 and 76, via delay networks 79 and 80 to compensate for the circuit delays produced in the signals from the photo-cells. The two inputs to each multiplier are multiplied together to give a signal representing the X parallex on line 83 and a signal representing the Y parallex on line 82. It is required to bring the misalignment of the photographs in the Y direction to zero and maintain it at this value. For this purpose each platform supporting a photograph is rotatable about three axis K, # and #, Fig. 4 under the action of motors 93 to 97. The Y parallex signal is applied to the motors as required via switches 88 to 92 until the misalignment in the Y direction is brought to zero. The X parallex signal is composed of at least three components (a) A D.C. component, being the D.C. component of the pulses produced by discriminator 39, and representing the height of the area of terrain being scanned, (b) A component at 400 c/s, being a component produced by the fundamental rotation of the CRT scanning spot and the presence of continuous sloping areas in the area of terrain being scanned and having a phase representing the direction of maximum slope, and (c) A component at 800c/s, being a component produced by areas of changing slope in the area of terrain being scanned, and thus representing the roughness of the terrain. These components are separated out by filter network 108 to appear on lines 113, 110 and 111 respectively. The D.C. component on line 113 is fed to the left hand and centre contacts of a three position switch 120, Fig. 2, being one of four such ganged switches 117, 118, 119 and 120, the left hand positions thereof giving profiling along a Y co-ordinate, and the right hand positions giving the contouring mode. Thus with the switches in the left hand or centre position the D.C. component signal is passed to a motor 114 (see Fig. 4) whereby the vertical position of the CRT is adjusted until the D.C. component, produced by the observed parallex, is reduced to zero. The CRT is then at a height above the plotting table proportional to the height of the terrain portions being scanned. If the outputs of the filters 40 and 41, i.e. waveforms R and S, Fig. 13, are multiplied together, as in multiplier 134 Fig. 1 a signal is produced (Fig. 10, not shown), which is a maximum when the photo-cell outputs register alignment. Since it is desirable that the CRT assembly should be moved only when such alignment is registered, i.e. the CRT is at its correct height, the output from multiplier 134 is used as a drive signal to power the motors moving the CRT assembly. The multiplier output is thus fed to the left hand contact of switch 118, connected to the X drive motor 130, and to the centre contact of switch 119, connected to the Y drive motor 141. The multiplier output is fed to the switches via a drive reverse switch 124, which causes the drive direction to be reversed on receiving a signal from a boundary limit switch 125 (in this case a contact is made between the base of the CRT carriage and a conductive strip round the periphery of the plotting area on the plotting table). In addition the boundary limit switch gives a pulse each time the edge of the plotting area is reached, which is passed to the centre contact of switch 118 and the left hand contact of switch 119. The CRT thus executes a zig-zag motion across the plotting table sideways stepping on to a new parallel path at the end of each path. Scribing means may be attached to the CRT (Fig. 25, not shown) to plot the profiles, on the motion of motor 114 may be transformed into a signal by synchronous transmitter 143, which signal can be used to actuate the pen of a recorder 154, driven with signals from synchronous transmitters coupled to the X and Y drive motors. (b) Contouring mode Figs. 1 and 2. For this mode, switches 117 to 120 are set to their right hand position and the CRT is fixed at a height corresponding to the contour to be traced. For the CRT carriage to trace out a contour both the X and Y drive motors must be actuated to give a motion to the CRT carriage having two orthogonal components, (a) along the contour, being provided by the tracing drive signal and (b) along the line of greatest slope, seeking the contour, and provided by the parallex signal on line 113. The parallex signal on line 113 is fed to stator winding 164 of resolver 161, whilst the tracing drive signal on line 163 is fed to a second orthogonal stator winding 162. The drive signals to the X and Y drive motors 130 and 141 are taken from orthogonal secondary windings 165 and 166 respectively and deliver signals A Cos 6 and B Sin # and A Sin # and B Cos # respectively, where A is the magnitude of the tracing signal, B is the magnitude of the parallex signal, and # is the angular direction of the contour and also the angular positions of the rotor of the resolver 161. The angle # is in quadrature with the angular direction of maximum slope, as represented by the phase of the 400c/s signal on line 110. To produce this quadrature relationship, a revolving 400c/s field is produced in resolver 172 by means of quadrature 400c/s signals on li