Patent GB-266044-A (GB)
Title: Improvements in and connected with apparatus or appliances for use in making measurements and plotting plans in systems of photographic surveying and for other purposes
IPC: G01C11/30
Filed: 1925-11-16
Abstract: 266,044. Gordon, J. W. Nov. 16, 1925. Photographic surveying.- Apparatus for use in making measurements and plotting plans in systems of photographic surveying i n which use is made of two photographs of substantially the same tract taken from different view-points comprises a comparator enabling the two photographs to be accurately superposed, with means for shifting one photograph relatively to the other in any desired direction, indicators being provided for measuring the amount of shift. One of the photographs is a negative and the other a diapositive, so that by suitable shifting of the supports a part of one can be made to cancel the corresponding part of the other when viewed by transmitted light. The apparatus comprises a block a mounted on a base b and having a circular aperture o, Fig. 4, in which is a glass bed-plate d mounted in a ring e having teeth h so that it can be rotated by a pinion i operated by a knob j. One of the photographs l is mounted on the plate d the upper surface of which is slightly proud of the block a to facilitate removal of the photograph by sliding. The second photograph w is mounted in a frame p having a flange s serving to support it in an aperture in an arm t which is mounted on a sliding member 1 of a mechanical stage or slide rest 2 supported by the base b. The stage comprises two sliding members 4, 1 arranged to move on guides 5, 6 at right-angles to each other and adapted to be displaced by traversing-screws 7, 8. The displacements are indicated on scales 11, 12, in conjunction with which verniers 13, 14 are provided, the verniers being separately displaceable within limits by rack-and-pinion gears 17, 18 for adjustment of the zero from which any given reading is to be taken. To facilitate adjustment of the arm t into a position in which the photographs w, l are exactly parallel, the inner end of the arm is formed as a triangular plate 26 provided with adjustment screws 23, 24, 25 bearing on the slide 1 and with a binding or locking screw 27. This adjustment enables the film surfaces of the two photographs to be brought almost into contact to minimise errors due to instrumental parallax, and in order further to avoid such errors the photographs may be observed through an eyepiece 31 carried by a foldable lever 32 on a column 33. To permit the arm t to be moved from its position over the bed d it may be made in two parts connected by a hinge 35, the holder portion 3 having an extension in the form of a pointer 36 co-operating with a scale 37 on the main part of the arm and a clamping-screw 39 being provided for locking the two parts together. To enable the angular positions of points on a photograph to be determined, an arm 40, Fig. 3, is adapted to be mounted on a pivot 42 on the block a and is provided with teeth 48 engaging with a pinion 49. The part 43 is slotted and contains a transparent plate 44 marked with a longitudinal centre line 45. A slide 46 mounted on the arm 43 carries a pointer 47 vertically above the line 45, so that parallax can be avoided by bringing the pointer and line into apparent coincidence. A uniformly-divided scale 52 on the block a indicates the tangents of angles made by the line 45 with the centre line C-C. Specification 198,497 is referred to.