Mapping from aerial photographs is the best mapping procedures yet developed

Mapping from aerial photographs is the best mapping

This preview shows page 59 - 62 out of 99 pages.

wherein the photographs are taken by a camera mounted in an aircraft flying over the area. Mapping from aerial photographs is the best mapping procedures yet developed for large projects, and are invaluable for military intelligence. The major users of aerial mapping methods are the civilian and military mapping agencies of the Government. 3. What is plane table photogrammetry? The plane table photogrammetry consists essentially in taking a photograph of the area to be mapped from each of the two or three stations. The photograph perpendiculars may be oriented at any angle to the base, but usually from an acute angle with the latter. The main difficulty arises in the identifications of image points in a pair of photographs. In the case of homogeneous areas of sand or grass, identification becomes impossible. The principles of stereo photogrammetry, however, produced the remedy.
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4. Define terrestrial stereo photogrammetry. In terrestrial stereo photogrammetry, due to considerable improvement of accuracy obtained by the stereoscopic measurement of pairs of photographs, the camera base and the angles of intersection of the datum rays to the points to be measured can be considerably reduced since the camera axes at the two stations exhibit great similarity to each other. The image points, which are parallactically displaced relative to each other in the two photographs, are fused to a single spatial image by the stereoscopic measurement. 5. What is shoreline survey? The shoreline surveys consist of: (i) Determination or delineation of shore lines, (ii) Location of shore details and prominent features to which soundings may be connected, (iii) Determination of low and high water lines for average spring tides, The determination or delineation of shorelines is done by traversing along the shore and taking offsets to the water edge by tape, or stadia or plane table. If the river is narrow, both the banks may be located by running a single line of traverse on one bank. For wide rivers, however, transverse may be run along both the banks. The traverse should be connected at convenient intervals to check the work 6. What is sounding? The measurement of depth below the water surface is called sounding. This corresponds to the ordinary spirit levelling in land surveying where depths are measured below a horizontal line established by a level. Here, the horizontal line or the datum is the surface of water, the level of which continuously goes on changing with time. The object of making soundings is thus to determine the configuration of the sub aqueous source. 7. What are the uses of soundings? The object of making soundings is thus to determine the configuration of the sub aqueous source. Soundings are required for: (i) Making nautical charts for navigation;
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(ii) Measurement of areas subject to scour or silting and to ascertain the quantities of dredged material; (iii) Making sub-aqueous investigations to secure information needed for the construction, development and improvement of port facilities.
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