Aerial photogrammetry 05

Mosaic,
Mirror stereoscope,
Parallax Bar

1. Advance Surveying Aerial Photogrammetry by Prof. Rajguru R.S. Civil Engineering Department ([email protected]) Sanjivani College of Engineering, Kopargaon , MH, India

2. Lecture Outline Mosaic Mirror stereoscope Parallax Bar

3. Mosaic: Mosaic Series or the assembly of overlapping aerial photograph is called as mosaic. It is prepared by cutting and removing the common area in one of the 2 adjacent photographs taken during the aerial photography, then joining them together. Compared to photograph a mosaic represents large amount of area. Mosaics are mainly classified into 2 types as given below. Controlled Mosaic Controlled mosaics contain ground control points. These points are marked at various locations on the photograph and are transferred into the mosaic while preparing. These ground control points help in understanding or estimating the distance between various locations on the mosaic. Uncontrolled Mosaic Uncontrolled mosaics doesn’t contain any control points. The photographs are simply joined together for the sake of continuity only. It is normally used for understanding or visually inspecting various features of the terrain. Measures cannot be made from this type of uncontrolled mosaics.

4. Mosaics: Other types of mosaics as given below . Semi-controlled mosaic: When rectified points are available and ground control point of the original photographs are matched with the plotted original photographs are matched with the plotted ground control points to find the direction and distances then mosaic so formed is called Semi-controlled mosaic Strip mosaic: When a strip of terrain is photographed, as for highways, canals, railways etc. Wet mosaic: When photograph are placed in proper sequence with overlap portion out, and no glue is pasted, it is known as a wet mosaic Index mosaic: Photo number and flight number of each photograph is kept in sequential order, it is known as a index mosaic Use of mosaic - i) Completed mosaic is photographed and then print ii) pictorial view of the terrain and culture is desired with moderate degree of accuracy

5. Mosaics: Sr. No. Map Mosaic 1 Map contains uniform scale through out. Whereas mosaic doesn’t have a fixed scale, it varies with location. 2 Map is created using orthogonal projection of rays As mosaic is photographic product, the rays drawn in perspective fashion 3 Map contains symbols and labels. A raw mosaic doesn’t show any labels or symbols. It shows terrain as a whole 4 Map contains coordinates and assumed data. Mosaic shows only real features of the area. 5 Map preparation take a time Mosaic can be prepared in very less time 6 Map preparation is costly Mosaics are less costly 7 Map are not easily understood A mosaic can be easily understood

6. Mirror stereoscope: Arrange the selected pair of photos in such a way that the shadows on them generally appear to fall toward the viewer. It is also desirable that the light source enters the side away from the observer during the study of the photographs Place the pair of photographs on a flat surface so that the detail on one photograph is directly over the same detail on the other photograph Place the stereoscope over the photographs so that the left lens is over the left photograph and the right lens is over the right photograph Transfer the P1 principal point of first photograph on second photograph and mark as P’1 as conjugate principal point and measured P2 P’1

7. Mirror stereoscope: Transfer the P2 principal point of second photograph on first photograph and mark as P’2 as conjugate principal point and measured P1 P’2 Air base distance = Photo base distance x scale Air base distance = ( P2 P’1 X SCALE 2 + P1 P’2 X SCALE1 )  

8. Parallax Bar: Photographs are adjusted below stereoscope and then parallax bar is so placed that graticule dots fuse together forming floating mark, which appears to move vertically or float by adjusting micrometer. The float mark can be adjusted at the level of selected point say1 and reading is noted, which gives parallax at this point. Procedure is repeated to parallax of point say2 The difference of parallax in the reading taken at the point1 &2 gives the parallax difference Parallax bar has great application in determining the flying height

9. Parallax Bar: Δ h = h = Elevation of lower point H = altitude above m.s.l . Δ h = Difference in height = Mean principal base Δ P= Difference in parallax between two point Approximate formula, when Δ P is very small h is also small dh=  

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