Patent GB-286824-A (GB)
Title: Improvements in or connected with means for determining geometrical constants appertaining to photographs
IPC: G03B27/68
Filed: 1927-01-01
Abstract: 286,824. Barr & Stroud, Ltd., Barr, A., and Stroud, W. Jan. 1, 1927. Photographic surveying.- Relates to apparatus for determining geometrical constants of photographs, particularly those taken from aeroplanes, to facilitate the use of these photographs in constructing maps &c. The photograph, which includes the images of at least three points on the ground having known co-ordinates, is mounted in a projector and light is directed through these points on to a screen provided with a number of adjustably mounted point-screens which are set to represent to a chosen scale the co-ordinates of the known points. By adjustment of the projector and screen the images are brought into coincidence with the point screens and the required data are then obtained from the settings of the projector and screen and their distance apart. The photograph 5 is mounted in a projector 2 having a diaphragm 3 in front of which is preferably mounted a lens 4, the distance from the photograph to the diaphragm being adjustable in accordance with the focal length of the camera objective, and the principal point 6 of the photograph being on the axis of the projector. A number of rods 8 are arranged to be movable around the rim of the projector and on each rod is mounted a slider 9 carrying a prism 10, the arrangement being such that each prism is adjustable along a line passing through the axis of the projector. The prisms can thus be set to direct light from a source 7 through the three (or four) control points on the photograph. The light 7 is arranged on the axis of the projector and at the same distance from the plane of the prisms 10 as this plane is from the diaphragm 3, so that all the rays reflected by the prisms pass through the diaphragm. The projector is carried by a mounting 11 in which i; can be rotated about a horizontal axis at right angles to its own axis, and this mounting can be rotated around a vertical axis 12 on a plate 15, which in turn can be displaced along a base-plate 1 by a screw 16. The screen 20 is carried by a ring-shaped frame 19 which can be rotated about a horizontal axis in a bracket 17 which can be rotated about a vertical axis on the base-plate. The screen 20 is provided with a horizontal slide-bar 21 and two pivoted slide bars 24, 25, on which are mounted slides 23', 26', 27' having bearings for tubular members 23, 26, 27, the parts being so disposed that the axes of the tubular members always lie along radii of the screen. A similar tubular member 22 is mounted at the centre of the screen. Each tube carries at one end a ground glass &c. disc marked with a cross &c. and at the other end a lens for viewing the disc. The screens can be adjusted so as to correspond with the ground positions of the control points to a chosen scale and they can also be adjusted towards or away from the plane of the screen so as to make the heights of the points to the same scale. In using the apparatus, one of the control points is chosen as a centre and the apparatus is adjusted until the image of this point falls on the centre of the screen 22. The projector and the screen have then to be adjusted relatively to each other both as regards their angular disposition and their distance apart until the images of the other points (three in the case shown) fall on the centres of the screens 26, 27 and one of the screens 23. The required data can then be obtained, ether by direct measurement from the apparatus, or preferably by calculation from the obseived angular settings of the projector and screen, and their distance apart.