stereoscopic principles, calculation of height with parallax and relief displacement, mosaic and maps, mosaic vs maps

1. Topic: Stereoscopy
By
T.SrinivasaRao

2. Relief displacement

4. from the above 2 equations,
d = rxh/H or rxh/(A-E).

6. Definition
Stereoscopy (also called stereoscopics) is a
technique for creating or enhancing the
illusion of depth in an image by means of
stereopsis for binocular vision.
(source: Wikipedia)

7. ➔ The science of stereoscopy deals with the
stereoscopic effects and techniques which
produces stereoscopic effects.
➔ The effect of stereoscopy is the stereoscopic
vision which enables the observer to view an
object in a pair of aerial photographs in the
form of a 3 dimensional model or
stereoscopic model.

8. ➔ Stereoscopic vision is more conveniently
produced with the aid of an optical instrument
called stereoscope.
➔ In practice stereoscopic vision is produced by
viewing 2 photographs of same area taken from
different angles.
➔ This results in 3 D model due to mental
superposition and fusion of 2 images.

9. Human Eye and Stereoscopic vision
➔ Eye ball is globular in form and consists of
pupil which controls the intensity of light
ray.
➔ cornea and lens forms the image on the
retina
➔ retina consists of millions of cones and
rods which are sensitive to light.

10. ➔ when light falls on retina, a photochemical
change occurs within them, which in turn,
stimulates the optical nerves and sends
electrical signals to the brain.
➔ Since, 2 eyes are present depth
perception is possible.

11. ➔ The monocular vision permits the perception
and determination of distances and position
of objects in their relative direction in the field
of view.
➔ A false depth perception is produced due to
varying object sizes with varying scales at
varying distances and overlapping of rear
objects with the front objects.

12. ➔ It is impossible to distinguish objects in depth, if
the difference of angle of convergence is less
than 20 seconds of an arc.
➔ Depending upon interpupillary distance of a
person and for angle of 20 seconds the max.
distance limit of depth perception varies from
1700 to 2450 ft.

13. The limiting distance is called as radius of
stereoscopic perception.

14. Stereoscopy - observations till now
- Deals with the use of binocular vision for
achieving 3-D effects.
- It enables us to view an object
simultaneously from 2 different
perspectives.
- 2 aerial photographs taken from 2
different points are normally used in RS.

15. Stereoscopic pair
➔ Stereoscopic pair of photographs consists of 2 adjacent,
overlapping photos in the same flight line.
➔ Stereoscopic view is observed only in the overlapped
area.
➔ A minimum overlapping of 50% is required for complete
stereoscopic coverage.
➔ As a safety factor additional 10% is considered usually.

16. Stereogram
➔ In a stereopair, if we cut common area
(overlapped) in each photograph, and
then properly orient and place them side
by side a stereogram will be formed.

17. Stereogram

18. Stereoscope
➔ To achieve stereoscopic vision the
binocular optical instrument used is called
as stereoscope.
◆ lens stereoscope
◆ mirror stereoscope
◆ scanning stereoscope
◆ zoom stereoscope

19. Geometry of Stereoscopy
➔ In order to stereoscopic vision, it is
important to have properly oriented
photographs that are aligned in the
geometry when the images were taken.
◆ coordinate axes
◆ absolute parallax
◆ flight line location

20. Coordinate axes
➔ Geometry of overlapping photographs is
different from single vertical photograph.
➔ In single vertical photograph x, y axes are
defined by fiducial marks.
➔ For 2 successive photos in the flight line x-axis
is determined by the line joining principal point
and its conjugate principle point.

21. cont..
➔ y-axis is the line that passes through the
principal point and is perpendicular to the
x-axis.
➔ This is called as the flight line system of the
of coordinates for stereoscopic pairs of
photographs compared to the fiducial
system used in single vertical photographs.

22. cont...
➔ If we take a stereo triplicate, the center
photograph will have 2 sets of coordinate
systems if the aircraft does not fly in the
straight line.

23. Absolute Parallax
➔ Stereoscopy is possible because of the
displacement of objects of different
elevations differently in the successive
photographs along the x-axis.
➔ This difference in displacement is called
as absolute parallax (dP).

24. cont...
➔ The absolute parallax of a point on a pair
of overlapping vertical photographs is
equal to the x-coordinate of the point
measured on the left hand photograph
minus the x-coordinate of the same point
measured in the right photograph.

25. cont...
Definition: The absolute parallax is the
algebraic difference, measured parallel to the
line of flight (x-axis) from the corresponding
y-axis to the 2 images of the point on a
stereoscopic pair of aerial photographs
assuming they are taken at the same altitude.
(source: David P.Paine)

26. Flight line location
➔ flight line passes through the principal
point
➔ for stereoscopic vision minimum 50%
overlap is required.
➔ principle points of adjacent photos should
be imaged on each photograph.

28. cont...
➔ Mark principal point and conjugate
principle points on each vertical
photograph.
➔ By drawing a line between the principal
point and conjugate principal point we can
mark the flight line for that vertical
photograph.

29. Theory of stereoscopy (ToS)
Single eye alone cannot perceive depth
accurately.
perception is because of association unlike
the formation of stereoscopic model in
binocular vision.

30. Accommodation and convergence
➔ The relationship of accommodation
(change of focus of the eye for distance)
and convergence is very important in
stereoscopic vision.
➔ when our eyes focus on a nearby object
they also converge, hence lines of sight
from eyes intersect at the object.

31. ➔ This is a problem in case of stereo pairs.
➔ For getting stereovision, lines of sights
should be parallel.
➔ This is achieved our eyes on an object at
infinity.
➔ Many stereoscopes works by making use
of this principle.

32. Depth perception

33. Floating Dot Principle

34. Vertical Exaggeration (V.E)
➔ V.E is an important factor which makes
the topography of an area look more
steeper and higher.
➔ It is caused due to the differences in the
ratio of the base height with that of the
viewing height while viewing stereo
photographs.

35. ➔ V.E increases with the ratio of the distance (air
base) between exposure stations over the flying
height above the ground.
➔ V.E can be calculated by multiplying the air
base by flying height ratio with inverse of eye
base to apparent viewing distance ratio.

36. AVD is estimated as 17 inches, and EB as
2.625 inches for calculations.

41. 1. Two ground points A and B appear on a pair of overlapping photographs,
which have been taken from a height of 3650 m above MSL. The base
lines as measured on the two photographs are 89.5 and 90.5 mm
respectively. The mean parallax bar reading from A and B are 29.32
mm and 30.82 mm respectively. If the elevation of A above MSL is
230.35 m, compute the elevation B.
2. In the above problem if the lengths of base lines are not known and
the absolute parallax of A is measured to be 89.80 mm, compute the
elevation of B. Also, find the height of another point C whose parallax
bar reading is 32.32 mm.