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Aviation History
1958
1958 - 0205.PDF
215 The Wild A7 Precision Autograph, left, with the 39inxj9in plotting table. Above, the R.C.A. RT-7/APN-1 radar altimeter which is widely used tor height control. The two aerials are in the foreground. Canadian Aero Service operators at Wild A8 plotters. These machines are kept working tor 18 hours a day. The Kelsh principal of three-dimensional viewing is illustrated in the lower picture on the opposite page. The operator is viewing the model on the small round platform. Two Wild RC7a automatic plate cameras installed in a Swiss-owned Hunting Survey Prince. , plus to R.A.F. requirements are sold through such organizationsas the Direct Photographic Supply Company, Harrow Road, London, W.2. There are three main types of plotter for photogrammetric work,classified according to their accuracy into first, second, third and fourth orders. Aerotriangulation obviously requires the extremeaccuracy of the first-order machines; and these are typified by the heavy and expensive stereoplotters such as the Wild A8 and theZeiss Stereoplanigraph C.8 They may cost something over £15,000 each. Once the ground control points have been plotted on thephotograph and further control points transferred along a row of stereo pairs with such a machine, the plotting of feature and con-tour can be taken over by the second-order machines, typified by the Kelsh plotter. In many cases this work is also handled withthe Multiplex type of machine, which is generally of a third order of accuracy. It is the task of the plotting machine to project the image of thediapositive (positive transparency of the air photograph) on to the viewing surface in such a way that the attitude of the camera rela-tive to the ground at the moment of exposure is accurately reproduced. The characteristics of the plotter lens must also bematched closely to those of the camera. Each plotter has holders for two diapositives, the images from which are viewed three-dimensionally by the operator. Superimposed on the image, or model, is a dot which can be raised or lowered until it appears torest on the surface. It is then steered about the model by hand and foot controls and the position of the dot is recorded in the formof x, y and h co-ordinates. Control points can thus be established. The dot can also be set to control the movement of a pantographlinkage which traces a line on the Astrafoil sheet following the movement of the dot. Once the diapositives have been adjusted to reproduce cameraattitude, the model is aligned to coincide exactly with a pattern of previously surveyed ground control-points. Ae'- tl triangulationentails setting up a model on such a pattern arid then plotting further points on its forward edge. One of the two diapositives is then replaced by a new one and the new points are transferredto it within the 60 per cent overlap which always occurs between successive exposures. Leapfrogging in this way, the operatorestablishes a row of points and notes their exact co-ordinates. The Kelsh plotter (made by the Instruments Corporation, Bel-fort Observatory, Baltimore, Maryland) is generally considered accurate enough only for the transfer of control points on toadjacent models, rather than along a whole row of models. It can, however, use the control established by the first-order machine andbase a map on this. A rather different viewing and plotting method is used in the Kelsh plotter. There are two holders as before andthe accuracy of the model is set by their adjustment, but the visual image is projected on a small round platform in the centreof which is a spot of light. When viewed through glasses with one red and one blue/green lens, the three-dimensional effect results.Instead of moving the spot by means of handwheels, the operator moves the platform with its spot of light horizontally or vertically.A small scale shows the height displacement of the dot and a pencil attached to the platform mounting directly underneath the spottraces its line on the surface of the table. A pantograph attach- ment may be fitted to remove the drawing process to one side ofthe table. The same principle is applied in the Multiplex, but instead ofhaving only two diapositive holders producing one model at a time, it has up to seven separate holders, each of which is used to setup one diapositive. These have to be specially printed to remove as far as possible the distortions originating in the camera, themachine's own adjustment being primarily intended to counteract any inaccuracy in the projector and for correcting for any tiltingof the camera at the moment of exposure. Diapositives are pro- jected alternately in blue/green and red, to give the three-dimen-sional effect. The first pair is then matched to project an accurate model and succeeding projections synchronized with it. Allow-ance is made for correct heights and plotting begins. Multiplex plotters are made by the Williamson Manufacturing Co., Ltd.,Willesden Green, London, N.W.10.
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