Mosaic, Mirror stereoscope, Parallax Bar

1. Advance Surveying
Aerial Photogrammetry
by
Prof. Rajguru R.S.
Civil Engineering Department
(rajgururajeshcivil@sanjivani.org.in)
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 =
1
2
(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:
• Δh =
(H−h)
2
ΔP
ΔP(H−h)+ 𝑏 𝑚H
• h = Elevation of lower point
• H = altitude above m.s.l.
• Δh = Difference in height
• 𝑏𝑚 = Mean principal base
• ΔP= Difference in parallax between two point
Approximate formula, when ΔP is very small h is also small
• dh=
(H−h)
2
𝑏 𝑚H
ΔP

10. THANK YOU