Patent GB-198497-A (GB)
Title: Improvements in or relating to photographic surveying and appliances or apparatus for use in connection therewith
IPC: G01C11/00
Filed: 1922-03-28
Abstract: 198,497. Gordon, J. W. March 28, 1922. Photographic surveying-instruments. - The area to be surveyed is photographed, using a camera of known focal length F tilted at a known angle X to the horizontal, the optical centre of the photograph is identified by a fiducial marking in the usual way, and certain basic lines in the photograph are determined, thus enabling the photograph to be placed in correct position with reference to a field centre on a plotting board, upon which the plan is drawn. The direction of the horizon line on the photograph is first determined by the following method, or by one of several slight variations thereof set out in the Specification. Six points are identified on the photograph as corresponding to the terminal and dividing points of two straight lines in a horizontal plane in the object-space or field. The lengths of the first line being in the objectspace a and b and in the photograph A and B, and the corresponding lengths for the second line being a<1>, b', A<1>, and B', lengths C and C<1> are calculated such that C = (a + b) AB / aB - bA and C<1>= (a<1> + b<1>) A<1>B<1> / a<1>B<1> - b<1>A<1>. The lengths C and C<1> are then measured along the corresponding lines on the photograph from the dividing points to define two points through which a straight line is drawn. This line is parallel to the horizon line. In a modification of this method three points are identified in the photograph as corresponding to three points disposed in a vertical line in the object space, and the length C is calculated according to the above equation and is measured from the dividing point on the photograph to define a point. The straight line joining this point to the optical centre is the prime meridian of the photograph, and the horizon line is at right angles to this meridian. The photograph is then placed on a plotting board provided, as shown in Fig. 9, with two lines XPX and HPh at right angles constituting respectively the prime meridian and the parameter parallel of the cartographic field. The prime meridian of the photograph is placed in line with the line XPX and the optical centre 0 is placed at a distance from the field centre P of (sec #-tan #) F. The various points on the plan are then plotted from the corresponding points on the photograph either by calculation or by the use of a perspective diagram or a counter-perspective diagram constructed in accordance with the mathematical principles of the perspective system which are set out in the Specification. For determining contour levels, two photographs of the area taken with the same compass bearing from two separate camera stations are compared by means of a parallacter diagram based on the reduced parallax of a point at datum level. Fig. 9 shows one construction of plotting board comprising a hollow table with a plate-glass top 12 mounted in a hinged frame 13, and fitted along its front edge with a slide-rule 15 for performing the calculations. The plotting paper 19 is pasted on the glass and is provided with an aperture to receive the photograph in correct position in relation to the field centre P. The photograph is illuminated from below by a lamp 17. In a modification, Fig. 11, the table top is mounted on a sloping frame 34 and the photograph is illuminated by light entering a rear opening 33 and reflected by a mirror 32 which is hinged to be turned into the table top when the board is not in use. The slide-rule 15 is secured by removable clamps 21 to a carrier 20 fitted with an adjusting-handle 23, and mounted to slide on a block 24. The carrier is guided by a tongue 25 engaging a groove 26 in the block. The slide part 15<a> is adjustably held against a spring 22 by a screw 22<a>, and the cursor 28 is mounted friction-tight on a rod 29 upon which it may be adjusted by a handle 30, while for fine adjustments the rod 29 is movable axially by a nut 31 acting in conjunction with an abutment spring 31<a>. Fig. 12 shows a plotting board fitted with devices for plotting a plan at the side of, instead of overlapping, the photograph. Parallel lines XPX and X<1>P<1>X<1>, drawn through the field centres P, P<1> of the photograph and plan respectively at right-angles to the line PP<1>, represent the two prime meridians, and serve to define the correct position of the photograph and of the counter-perspective diagram used in plotting the plan. Equidistant parallel lines 44 are drawn on the photograph parallel to the line PP<1> and the spaces between them are evenly subdivided. These parallels 44 are conjugate to corresponding parallels 46 on the counter-perspective diagram. An arm 47, carrying a transparent plate 49 marked with a central longitudinal line 60 passing over the point P, is secured to an arcshaped member 51 fitted to turn in a fixed circular guide 52 having its centre at P. An arm 48 with a straight-edge 68 passing over the point P<1> is secured to an arc-shaped member 56 fitted to turn in a fixed circular guide 57 having its centre at P<I>. The members 51, 57 are connected by a link 61 such that on moving the link by a handle 63, the edge 68 is maintained parallel to the line 60. To avoid parallax in placing the line 60 over a point on the photograph, the arm 47 is fitted with a sliding marked prism 65. Levelling-staves and the like.-Three points in vertical line in the object space for use in determining the direction of the horizon line on the photograph as above described may be constituted by bobs 73, Fig. 15, on a plumb-line 72 suspended from the rod 74 of a tripod stand or from a balloon.