Patent GB-1401534-A (GB)
Title: Ortho projector to make photo maps form aerial photographs
IPC: G01C11/00
Filed: 1972-08-02
Abstract: 1401534 Photogrammetry WILD HEER BRUGG AG 2 Aug 1972 [6 Aug 1971] 36164/7 Heading H4D In an orthoprojector for converting an aerial photograph into an orthographic map and of the type wherein the photograph is projected on to a film through a slit aperture scanning in a raster along lines perpendicular to the line of the slit and the photograph is moved vertically in correspondence with the height of the terrain in the photograph being instantly scanned, distortion can occur between adjacent exposed strips of the map-e.g. a straight road crossing the strips will appear broken and crooked-if the terrain has a marked slope along the line of the slit. This is due to the vertical position of the photograph only being correct for part of the terrain in the slit area. To overcome this, the photograph 2 is placed as one of a stereo pair, in an automatic stereo photogrammetric or autograph machine, Fig. 6b, e.g. one commercially available for deriving terrain height information, i.e. a model, form a stereo pair of photographs. The autograph machine simulates the model in an X, Y, Z co-ordinate plane by the movement of the junction point of the tracking rods (in some cases a projection table is used). The junction point (or projection table) is scanned through the model 12, Fig. 7, is a series of parallel equi-spaced profiles in a path F, (X, Y). The variation in height of the model and correspondingly of the junction point, causes the tracking point 17 (Fig. 6b) on the photograph 2, e.g. one element of a binocular stereo viewer (or the path of the beam of projected light) to describe a distorted version 39 of the path F(X, Y), viz f(x, y). In path 39, a line element, which in the model 12 is perpendicular to the profiles and joins to points of different heights, becomes a' line element 3, turned (angle r) and changed in length (u) in relation to the original line element. The values of path 39, f(x, y) for two successive profiles are stored, after digitizing, at 27 and then fed to a computer 38. The x and y values (x n-1 , y n-1 ), (x n , y n ) for the two ends of successive perpendicular line elements in the path F(X, Y) are therein used to determine the average co-ordinates X on , Y on  of the corresponding lines 3, together with the co-ordinate differences x on , y on , where:- The calculated values are interpolated at 23 to enable scanning of the orthoprojection film 6 at a variable speed. The photograph, now in the orthoprojector, is moved vertically in accordance with the height signals produced by the autograph while the slit aperture describes the path F(X, Y). The slit aperture therefore scans the photograph in the path f(x, y). The image of the photograph passes to the slit aperture through an optical arrangement 25 enabling rotation and enlargement of the image according to signals r and v derived in computer 24 from the values x on , y on , x on , y on . The perpendicular line element in the mode is thus projected to give a correct perpendicular image 14 in the orthomap. An exposure control 26 may be controlled by scanning speed, film speed, height data. The values f(x, y) can be recorded and read out of computer 38 for production of the orthophoto at a later time.