Patent GB-727235-A (GB)
Abstract: 727,235. Photogrammetry. THOMPSON, E. H. June 25, 1952 [July 6, 1951], No. 16140/51. Class 97 (2). A method of making a map of an area shown on a pair of overlapping photographs comprises determining by means of the Porro principle the distances between a predetermined basal plane and points whose images are shown on the overlapping photographs, determining by means of the Porro principle the projections of the said points on the predetermined basal plane, thereby establishing the positions of the points in space and marking the said positions on the map. In Fig. 1, S, S<SP>1</SP> are two points from which photographs of a point P in a basal plane inclined at an angle # to a vertical plane through S, S<SP>1</SP>, and the method is based on the fact that the position of the point P will be defined if the angles #1, #<SP>2</SP> and #<SP>21</SP> can be found. One form of photogrammetric plotter for fixing the position of point P on a map by reproducing these angles is shown in Figs. 2 and 4. This apparatus includes projectors 1, 1<SP>1</SP> mounted on a beam 2 rotatable about its axis (the polar axis) relatively to sighting means Z comprising two telescopes, the light from each of a pair of overlapping photographs in the projectors being directed into the telescopes by rotatable and slidable mirrors and fixed prisms (see Fig. 3, not shown). The collimation axes of the telescopes lie in a plane containing the polar axis 2. In order to sight the image of the point P, the projectors are rotated through angle # and the mirrors are adjusted, the angles between the lines of sight and the polar axis being #1 and #1<SP>1</SP> respectively. The angles #2 and #2<SP>1</SP> are reproduced by movement of links 15, 15<SP>1</SP> about axes separated by a distance S1, S1<SP>1</SP> representing in the instrument the photographic base SS<SP>1</SP>, each of the links being coupled to the corresponding mirror in the manner shown in Fig. 4 for the link 15 and one of the mirrors 7. This mirror is pivotally mounted at M on a slide 23 constrained to move parallel to the polar axis 27 and spaced therefrom by a distance p equal to the distance from axis 27 of the pivot 19 of a plate 18. This plate has a pin and slot connection 21, 22 with a link 20, parallel to the polar axis and movable towards and away therefrom. Link 20 is slidably pivoted at E to a link 25 pivoted to the slide 23 at B and pin and slot connected at C to a link 24 which extends perpendicularly to the mirror 7, the distance BC being equal to LB so that if the angle between the polar axis 27 and the line of sight 26 is #<SP>1</SP>, the link 25 cuts the polar axis at the same angle. Link 15 is slidably pivoted at B and is connected at D to a point on the polar axis 27. It is shown in the Specification that if the plate 18 is rotated through an angle #, the link 15 is inclined to the polar axis at an angle #<SP>2</SP>. The links 15, 151 are interconnected by a slide 17 mounted on a bridge 14 which also slidably supports a slide 13 which is also slidable on a link 9, rotatable with the projectors through an angle # by bevel gears 11 coupled to the beam 2 through a coupling 12 which is adjustable to ensure that when # is zero, link 9 is perpendicular to the bridge 14. The movements of the slides 13, 17 are transmitted to the pencil of a rectangular co-ordinatograph whereby the position of point P is plotted. To allow for inclination of the photographic base SS<SP>1</SP> to the horizontal, there may be used a slide 28 interconnected to slide 17 and movable along a bridge 29 extending between arms 30, 30<SP>1</SP> which can be rotated to a position in which bridge 29 is inclined to bridge 14 at an angle equal to the inclination of the photographic base to the horizontal. A further form of apparatus is shown in Fig. 8, this form being based on the fact that the position of the point P can be plotted if the position of its rabatement on the vertical basal plane is known or can be deduced, if NP and # can be found and if means are provided for moving the rabatement of P into co-incidence with the projection of P on the vertical basal plane. In this form, the projectors are mounted in gimbals 39, 39<SP>1</SP> in a frame 38 rotatable about polar axis R-R, and the lines of sight to telescope eye-pieces 40, 40<SP>1</SP> are defined by optical elements including prisms 70, 70<SP>1</SP> carried by links 42, 42<SP>1</SP> rotatable about axes B1 and B2 normal to the polar axis and slidably engaging pins 47, 48 on blocks 49, 50 connected to a carriage 51 slidable on a range bridge 54 by rods 52, 53 whose effective lengths are separately adjustable. The bridge 54 is parallel to the polar axis and is movable along a pillar 56 which is perpendicular to the horizontal plane containing axes R-R, B1 and B2. A ground frame 57 having a slight tilt about a horizontal axis H-H normal to the axis R-R is provided to compensate for a corresponding tilt of the photographic base with respect to the horizontal. A carriage 58 is slidable on frame 57 and is actuated by a screw (not shown) for plotting the x co-ordinate of the point P. A height screw 59, calibrated to measure height of the point P, is carried by the carriage 58 and has threaded thereon a nut 60 having projecting pins constrained to engage orthogonal arms 63, 64 which are slidable on the pillar 56 and connected to the carriage 51 respectively. A right angle bell-crank lever 28, 29 is slidably connected at 65 to the frame 38 and pivoted at 30 to a carriage 31 slidable on an arm 67 secured to the arm 63, the carriage 31 being connected to means (not shown) for plotting the y co-ordinate of the point P. A pin 34 on arm 29 and a pin 35 on the carriage 31 are equally spaced from the pivot 30 of the bell-crank lever 28, 29 and are slidably engaged by a lever 33 pivoted at 36 to the range bridge 54.