229,276. Brock & Weymouth, Inc., (Assignees of Brock, A., and Holst, L. J. R.). Feb. 16, 1924, [Convention date]. Photographic surveying.-In the production of maps from overlapping aerial photographs, the dia-positive photographs are arranged in pairs, as shown in Fig. 2, such that the point which is at the centre of one photograph also appears on the other, and also such that both photographs contain a number of points the co-ordinates and altitudes of which on the ground are known. By comparing the two photographs in a stereoscopethe inclination to the horizontal of one or of each plate at the time of exposure can be roughly estimated to facilitate the re-projection of the photographs to obtain their equivalents reduced to horizontal focal planes. These re-projected photographs are then compared in the stereoscope to enable contour lines to be traced. A method is also described for converting the conical projections represented by the photographs to orthographic projections for producing mosaic maps. In Fig. 2, C<1>, C<3> represent the centres of the two dia-positive photographs and c<1>, c<3> the corresponding points on the conjugate photographs. The datum points A<1> - - A<4> are seen in both photographs. Co-ordinates of points in the photographs are measured along the line xx representing the line of flight of the aircraft, and the lines yy, y<1>y<1> at right-angles thereto. The stereoscope, Fig. 5, comprises a bed E having guides E' for a carriage F, which is formed with guides F<1> for a carriage G on which are mounted supports H, H<1> rotatably carrying the photographs. The support H can be adjusted in guides G' towards or away from the support H<1>, the extent of such independent adjustment being indicated by a micrometer H<3>. The photographs are oriented until the axes xx are parallel to the guides F' and are adjusted until the points c<3>, C<3> are seen at the centres of the eyepieces J, J<1>. By then measuring the ordinates and parallaxes of the datum points an approximate estimate can be obtained of the inclination of one or of both photographs. If one of the photographs is found to be approximately horizontal it is used to make a template to assist in re-projecting the inclined photograph. To make the template, a sheet of non-actinically coloured transparent paper is attached to the positive plate and is marked with the points C', c<3> and also with the datum points &c., the paper being perforated around these points to form a stencil. The plate so prepared is placed in a unit magnification printing device, Fig. 4, comprising a reflecting prism O<1> and projecting lens O, the adjustment being such that the point c<3> on the prepared plate 1 appears at the centre of the template, the point C<1> at the centre of the plate 1 being consequently displaced from the centre of the template. This template T is mounted in the re-projecting apparatus, Fig. 6. comprising a frame M<1> mounted on a vertical pivot M<2> on a support M and carrying on horizontal pivots M<3> a frame M<4> having guides M<5> in which is adjustably supported a frame M<6> having guides M<7> supporting a frame M<8> on which the template is rotatably carried by a turn-table M<10>. The frame M<8> can be displaced within; the guides M<7> by a rack M<9> and pinion M<14>, the extent of such displacement being indicated by a micrometer M<13>. The template T is adjusted until its centre is on the axis of the projecting lens N and until the point c<3> lies at the intersection of the axes of the pivots M<2>, M<3>. The negative of the inclined photograph 3 is supported in an adjustable device similar to that above described and the apparatus is then adjusted until the ordinates of the datum points A<1> - - A<4>, projected from the negative, agree with those on the template and until the parallax measurements for these datum points, as indicated on the micrometer M<13>, agree with the calculated values. The template is now removed and replaced by a sensitive plate so that on exposing this plate the corrected positive for the photograph 3 is obtained. The two corrected plates are now placed in the stereoscope which is adjusted to give the correct parallax displacement for a contour line of a given height, the contour line being then obtained by joining up points which are seen in coincidence with the crosswires. Further contour lines can be drawn by re-adjusting the stereoscope. For producing mosaic maps in orthographic projection the corrected plates are used to produce diagrams on tracing paper &c. such as those shown in Fig. 9 in which the centre C<1> &c. of each plate is connected by straight lines with the datum points A<1> &c., the positions of two of these points being correctly marked on the first diagram. The second diagram is then placed over the first in such a position that the lines C<1>, c<3> and c<1>, C<3> coincide and the lines C<3>A<1>, C<3> A<2> pass through the marked points A<1>, A<2>, as shown in Fig. 10. This enables the positions of the points C<3>, A<3>, A<4> to be obtained in orthographic projection and by proceeding similarly with the other corrected positives the field can be completely mapped.