This document discusses photogrammetry and the use of a mirror stereoscope for viewing stereo pairs of aerial photographs. It defines stereoscopy and explains that overlapping aerial photos of the same ground area form a stereo pair that can be viewed three-dimensionally. The mirror stereoscope allows a wider viewing area compared to other stereoscopes. It consists of a double optical system mounted on a frame to create a virtual image and stereoscopic vision without eyestrain. Procedures for interpreting aerial photos using a stereoscope are also outlined, including locating principal points and viewing the stereo pair to see relief displacement effects from varying ground heights.

1. Unit 3
Photogrammetry:
Mirror Stereoscope
P.A.Nadgouda
Civil Engineering Department

2. Definition of Stereoscopy
• Stereoscopy, sometimes called stereoscopic
imaging, is a technique used to enable a three
dimensional effect, adding an illusion of depth to
a flat image.
• In aerial photography, when two photographs
overlap or the same ground area is photographed
from two separate position forms a stereo-pair,
used for three dimension viewing. Thus obtained
a pair of stereoscopic photographs or images can
be viewed stereoscopically.

3. • It is based on Porro-Koppe’s Principle
that the same light path can be
generated in an optical system if a light
source is projected onto the image
taken by an optical system.

4. A stereoscope
facilitates the stereo
viewing process by
looking at the left
image with the left
eye and the right
image with the right
eye.

5. Types of stereoscopes
• There are two types of stereoscopes:
Lens (or pocket) stereoscope and mirror
stereoscope.
• Lens (or pocket) stereoscope has a limited
view and therefore restricts the area that can
be inspected where as in mirror stereoscope
has wide view and enables a much larger area
to be viewed on the stereo-pair.

6. Mirror stereoscope
A stereoscope is used in
conjunction with two
aerial photographs taken
from two different
positions of the same
area, (known as a
stereo-pair) to produce a
3-D image.

7. Construction
• A stereoscope (Fig. 8.3)
consists of a double optical
system (lenses, mirrors,
prisms, etc.) mounted on a
rigid frame supported on
legs. In this way distance d
is fixed and kept the focal
distance. Thus the optical
system creates a virtual
image at infinity and
consequently stereoscopic
vision is obtained without
eyestrain.

8. Procedure for aerial photo
interpretation
• Select overlapping photographs.
• Locate the principle point & mark on each photo
corresponding principle point of the other.
• Draw a straight line across each photo, passing
through the principle points
• Place the photograph on the desk, one
overlapping the other.
• Place the stereoscope over the photograph. Look
at the photos and if necessary rotate the
stereoscope until a single line is seen.

9. Relief Displacement
• Due to varying heights of different ground
points, every point on the photograph is
displaced from its original position.
• It is the displacement of the objects due to
their heights on a vertical aerial photograph
from their true horizontal positions.
• The direction of relief displacement is always
radial from the principal point, and therefore
it is also known as radial displacement.

10. Relief Displacement
• The relief displacement will be
more when the focal length of
camera lens is increased, and
vice versa.
• The taller the object, the
greater the relief displacement.
• The relief displacement is zero
at the Principal Point
• and increases towards the
border of the photograph.
• Relief displacement is one of
the main reasons, a
• photograph is not directly used
as map substitute.