Aerial Photogrammetry

1. Aerial
Photography
By
Pramoda G
Faculty in Geology
YCM

2. Aerial Photography
• Science of making photographs from the air, for studying the surface of
the earth.
Uses
• pictorial representation (Mosaics),
• preparation of the base maps,
• photo interpretation,
• photogrammetric surveys,
• economizing and expediting natural resources surveys in the fields of
geology, soils, land use, civil engineering and town planning.

3. Basic Requirements of Aerial Photography
faithful image
image should be sharp, bright and
clear
Tilt and crab should be within
tolerable limits
continuous with sufficient
overlapping of successive
photographs
Scale variation should be within
tolerable limits
distortion free

4. Factors Influencing
Atmospheric factors
Reflectivity of the object
Aircraft
Camera
Processing and printing
Negative and positive base and emulsions

5. Stages of Aerial Photography
Planning for
photography
Planning and
execution of
photographic
flights
Processing of
negatives and
production of
positive copies

6. Planning for
Photography
Area to be
photographed
Purpose of
photography
Type of
photography
Vertical aerial
photograph
Oblique aerial
photographs
Scale of
Photography
Inclination of
optical axis
Optical
photographic
deficiencies
Topographic relief of the
terrain photographed

7. Extreme growth in the technology and techniques used for acquiring aerial
photographs. Aerial photographs are classified according
• Attitude of camera axis,
• Lens system,
• Types of cameras,
• Types of films and filters or
• Some special equipment employed in the camera or techniques to record
some special effect on the film.
Types of Aerial Photographs

8. Vertical photographs
• Two distinct axes are formed from the camera lens centre. One towards the
ground plane and the other towards the photo plane.
• Perpendicular from the camera lens centre to the ground plane is termed as
the vertical axis. Line drawn from the lens centre to the photo plane is
known as the photographic/optical axis.
• When the photo plane is kept parallel to the ground plane, the two axes also
coincide with each other - vertical aerial photograph (Figures).
• Very difficult to achieve perfect parallelism between the two planes.
• Photographic axis deviates from the vertical axis, Deviation is within the
range of plus or minus 3 degree, the near-vertical aerial photographs are
obtained.
• Photography with an unintentional deviation of more than 3 degree in the
optical axis from the vertical axis is known as a tilted photograph.
Types of Aerial Photographs - Based on the Position of the Cameral Axis

10. Aerial photograph taken with an intentional deviation of 15° to 30° in the camera axis
from the vertical axis is referred to as the low oblique photograph. This kind of
photograph is often used in reconnaissance surveys.
(ii) Low oblique photographs

11. (iii) High oblique photographs
The high oblique are photographs
obtained when the camera axis is
intentionally inclined about 60°
from the vertical axis. Such
photography is useful in
reconnaissance surveys.

12. Scale of Photography
Ratio of photograph image distance to ground distance. This ratio is same as that
of camera focal length to camera height - (f/H).
The scales generally used in natural resource surveys vary between 1:5,000 and
1:50,000 depending upon the purpose for which the photographs are used. Two
scales commonly preferred are 1: 15,000 and 1: 25,000. For general mapping
purpose in the field of geology, the scale 1:50,000 or1: 60,000 are suitable. These
scales have the advantage of corresponding to the scale of modern top sheets.
Selection of scale often depends on relief and other considerations. The higher
the relief of the terrain and higher the density of vegetation, the smaller should be
the scale selected. While selecting the scale of photography for geological
interpretation, advantages and, disadvantages of large and small scale should be
considered.

13. • Cameras are framing systems which acquire a near-
instantaneous "snapshot" of an area (A), of the surface.
Camera systems are passive optical sensors that use a lens
(B) to form an image at the focal plane (C), the plane at
which an image is sharply defined.
Aerial Cameras

14. FRAME
• lens
fixed
relative
to the
focal
plane
• film
being
moved
between
exposure
s
STRIP
Film is moved
continuously
along the focal
plane and a
narrow slit
shaped aperture
kept open
constantly
geophysical and
other non-
imaging surveys
and for low
altitude
PANORA
MIC
• Ground
areas are
covered by
either
rotating the
camera lens
or rotating a
prism
infront of
the lens.
MULTISPEC
TRAL
• used to
simultaneou
sly image
the terrain
in different
spectral
bands
Types

24. Normal or
Standard angle
camera:
Lens with an
angle of coverage
upto 75º and the
focal length
ranging from 200
to 300 mm.
Precision of the
planimetry is the
highest.
Super wide-angle
camera:
Lens with an angle of
coverage greater than
100º and the focal
length ranging from
45 to 90 mm (f/8).
Precision of height
measurement is the
highest.
Wide angle
camera:
Lens with an angle
of coverage
between 75º &
100º and focal
length ranging
from 100 to 150
mm. Precision of
height
measurement is
higher.
Lenses of 11.5 cm
and 15cm focal
length are referred
to as Wide angle
lenses,
lens of 21cm focal
length is referred
to as Normal
angle lens
lens of 30cm focal
length is referred
to as Narrow
angle lens.
Based on the angle of coverage
Based on Focal Length

25. • Generally Cardinal direction
• E-W & N - S direction.
• The direction along the length of
the area is commonly decided upon
to keep the number of strips to
minimum.
• For geological interpretation flight
direction across the strike of the
formations (cross stripping) is
preferred in highly folded areas to
ensure sufficient overlap across the
strike. In high mountainous areas
where relief displacement is more.
Flight Direction

26. Forward and Lateral Overlaps
Forward Overlap: The forward overlap (fig-
2a) generally chosen is 60% ~ 5%. In no case
it should be less than 53%. In mountainous
areas it is safer to have overlap of 65%.
Lateral Overlap: In general, a lateral overlap
(fig-2b) of 20% + 5% is specified. In areas of
high relief such as Himalayas a lateral overlap
of 35% is specified to cater for relief
displacements.

28. At high altitudes because of strong wind currents called sidewinds
influence the aeroplane in maintaining predetermined direction and
straightness of run. This deviation from the original intended flight
path is called as drift.
Pilot tries to maintain original path of the aeroplane slightly against
the wind, this makes the aeroplane to rotate on its vertical axis. In
this case, original path is maintained but the area covered by
photograph is much different than that planned. The aerial
photograph is rotated in the direction opposite to wind direction. This
defect is called as crab. The above 2 defects causes reduction in
stereoscopic coverage of the terrain.
Drift
Crab

29. Time of Photography
• Decided as to avoid long shadows and haze conditions. Long
shadows hide the detail and bring down the interpretational
value of the photographs.
• Normally the time is confined to the period when the sun is
between 30º and 60º (8 to 10 AM and 2 to 4 PM are preferred).
• In mountainous areas however the period around noon is
preferred to avoid shadows of the hills.
• In tropics where the time is limited to 1.5 to 3 hrs after the
sunrise.

30. • Selection of the season depends on various factors such as
seasonal changes in light reflection, seasonal changes in vegetation
cover, seasonal changes in climatological factors.
• The flying season in India is generally from September to October
and March to April.
• For the photogrammetric, geological and soil surveys the ground
should be visible as clearly as possible. In forested areas such a
time will be when the trees shed their leaves. Thus, for these
purposes early spring to beginning of summer is most suitable.
Land use surveys, preferable to have the photography when the
crops are standing. Therefore, for these end of rainy season to the
beginning of winter is suitable.
Season of Photography

36. Glossy, matt, semi-matt, single weight or double weight should be
decided.
Type and Number of Prints
Planning and Execution of Photographic Flights
• Aerial photography is a delicate operation and demands painstaking
preparation and professional execution.
• The purpose of the project largely determines the scale.
• The proper camera, necessary filters, suitable film, the photographic
plate and other equipment will be selected.
• Predetermined parallel flight lines.
• The lines are equally spaced and generally run along the length of
the area.
• The spacing between the flight lines are predetermined according to
the requirement of lateral overlap.
• Interval between successive exposures is determined taking into
account the speed of the aircraft and the forward overlap required
on successive photographs.

37. • The timing of successive
exposures is regulated
by an instrument called
"intervalometer" which
is set to trigger the
camera at proper
intervals of time.
• The exposure time,
aperture opening are
controlled by the
photographer. Cross or
oblique winds may force
the pilot to head the
plane diagonally into
the wind to keep it
moving along the
predetermined flight
line.

38. Aerial photography in India is controlled and coordinated by the
Survey of India and information about the existing photo coverage is
available with the Surveyor General.
Numbering of photographs
• Job Number: Every photographic task is allotted a job number by
Surveyor General of India for easy reference and handling. The
task carried out by I.A.F. is given a number 'suffixed by letter 'A'
while that carried out by A.S.Co. is suffixed by 'B', and that by the
NRSA is suffixed by 'C', EX:346-A, 331-B.
• Strip Number: If the strips are flown E-W, numbering of the strips
is given from N to S. If they are flown N-S, the numbering is given
from W to E.
• Photo Number: If the strip is flown E-W, the photos of the strip are
numbered from W to E. If the strip is flown N-S, the photos are
numbered from S to N.
Procurement of Aerial Photographs

39. To show the position of any photo relative to it's approximate
geographical position on a published map. These are of two types:
(1)Photographic Index, and (2) Line Index.
Photographic Index: It is normally prepared for areas where no
reliable map coverage exists and for operations such as
reconnaissance. The index is carried out by using photos on a smaller
scale than the actual aerial photography.
Line Index: It is a line map showing the photo layout in a mosaic as it
covers the terrain. The layout is with flight lines, photo numbers etc.
It is prepared on 1 inch: 4 miles or 1:2,50,000 scale and the
longitudes and latitudes are marked at intervals of 15 minutes.
Preparation of Photo Index

40. Information Recorded on Aerial Photographs
Fiducial marks: Fiducial marks or
collimating marks for the
determination of the principal points.
Altimeter reading: Recording of
Altimeter reading for knowing the
flying height of 'the aircraft above
mean sea level (msl) at the time of
exposure.
Time: Recording of time at the
moment of exposure.
Level bubble: To indicate the tilt of
the camera axis at the moment of
exposure (not very accurate).
Principal distance: For determining
the scale of the photograph.
Number of the photograph : e.g.
342-A

41. Elements of aerial photograph

42. • A systematic study of aerial photographs and satellite
imageries usually ,involves several characteristics of
features shown on an image and it depend upon field of
application.
• Most of the application consider the following basic
characteristics or variation in them ,which aid the visual
interpretation process of satellites imagery
• Although there is a difference of opinion on the number
of elements ,there is namely tone, size, shape, texture,
pattern, location, association, shadow and resolution
Image interpretation keys

43. Shape:- refers to the general form, structure, or outline of
individual objects. Shape can be a very distinctive clue for
interpretation. Straight edge shapes typically represent urban or
agricultural (field) targets, while natural features, such as forest
edges, are generally more irregular in shape, except where man
has created a road or clear cuts. Farm or crop land irrigated by
rotating sprinkler systems would appear as circular shapes

44. Size:- of objects in an image is a function of scale. It is important to
assess the size of a target relative to other objects in a scene, as well as the
absolute size, to aid in the interpretation of that target. A quick
approximation of target size can direct interpretation to an appropriate
result more quickly. For example, if an interpreter had to distinguish zones
of land use, and had identified an area with a number of buildings in it,
large buildings such as factories or warehouses would suggest commercial
property, whereas small buildings would indicate residential use.

45. Tone:- refers to the relative brightness or colour of objects
in an image. Generally, tone is the fundamental element for
distinguishing between different targets or features.
Variations in tone also allows the elements of shape, texture,
and pattern of objects to be distinguished.

46. Pattern:- refers to the spatial arrangement of visibly discernible objects.
Typically an orderly repetition of similar tones and textures will produce
a distinctive and ultimately recognizable pattern. Orchards with evenly
spaced trees, and urban streets with regularly spaced houses are good
examples of pattern.

47. Texture:- refers to the arrangement and
frequency of tonal variation in particular
areas of an image. Rough textures would
consist of a mottled tone where the grey
levels change abruptly in a small area,
whereas smooth textures would have very
little tonal variation. Smooth textures are
most often the result of uniform, even
surfaces, such as fields, asphalt, or
grasslands. A target with a rough surface and
irregular structure, such as a forest canopy,
results in a rough textured appearance.
Texture is one of the most important
elements for distinguishing features in radar
imagery.

48. Shadow:- is also helpful in interpretation as it may provide an idea of the
profile and relative height of a target or targets which may make
identification easier. However, shadows can also reduce or eliminate
interpretation in their area of influence, since targets within shadows are
much less (or not at all) discernible from their surroundings. Shadow is also
useful for enhancing or identifying topography and landforms, particularly
in radar imagery.

49. Association:- takes into account the relationship between other recognizable
objects or features in proximity to the target of interest. The
identification of features that one would expect to associate with other
features may provide information to facilitate identification. In the
example given above, commercial properties may be associated with
proximity to major transportation routes, whereas residential areas
would be associated with schools, playgrounds, and sports fields. In our
example, a lake is associated with boats, a marina, and adjacent
recreational land.
Site:-refers to the vocational
characteristic of object such as
topography, soil, vegetation and
cultural features

50. AERIAL MOSAICS
• Mosaic is an array of overlapping aerial photographs
systematically assembled to form a continuous pictorial
representation of a terrain.

52. • Planning purposes.
• Mosaic provides an over view of the terrain, which helps in
forming an initial impression about the nature and distribution of
the materials and features occupying the terrain.
• From the mosaic it will also be known how much scale variation
exists from photo to photo, or if there is any crab or drift causing
gap in the strip.
• A mosaic annotated with the locational information on rivers,
villages, townships, roads and other similar features taken from
the topographic map helps in readily knowing about the
geographic position of the area being interpreted.
Uses of mosaic

53. Based on the method of compilation, the mosaics are classified in to
three types
Un-controlled mosaic: It is a compilation of photographs without
regard to any horizontal control positions. The photographs are
oriented in to position by matching corresponding images on
adjacent photographs.
Semi-controlled mosaic: It is a compilation of photographs without
using rectified photographs but using control for positioning of each
photograph.
Controlled mosaic: It is a compilation of scaled and rectified
photographs (tilt corrected photographs) assembled to fit plotted
control points.
Types of mosaics

54. • The task of compiling of mosaic is generally entrusted to an air
surveyor photogrammetric organization.
• Mosaics are generally compiled on the scale of available aerial
photographs and then reproduced by enlargement or reduction to
the desired scale.
• More than double size enlargement is not recommended as it not
only reduces the image quality and resolution but also becomes
unwieldy in size for handling.
Compilation of mosaic
• For the mountainous terrain where relief displacement is much in
the photographs, the compilation of the mosaic is done using
orthophotographs. Such a compilation is called orthomosaic.
• This method involves considerable amount of work in the field as
well as in the laboratory and as such is resorted to rare cases.
Orthomosaic

55. • The scale is decided based on the intended use.
• Small scale (1:25,000 or smaller) mosaic, generally compiled on
the scale of photographs chosen for interpretation, is used for
geology, survey, forestry, flood control, terrain evaluation and
other studies of extensive areas.
• Medium scale (1:10,000 to1:20,000) is used for town planning,
road and railway alignment and other geotechnical studies.
• Large scale (larger than 1:10000) is used in detailed investigations
for any of the purposes stated above.
Scale of mosaic:

57. PHOTOGRAMMETRY
• Coined by the geographer KERSTEN in 1855.
• Photos (light) + Gramma (something drawn or written) + Metron, (to
measure).
• Art and Science of making measurements from imagery.
• In a broader sense, it is the method of determining the shapes, sizes
and positions of objects using their imaged positions.
• the art, science, and technology of obtaining information about
physical objects and the environment by photographic and
• electromagnetic images.
it includes;
(i) Photographing an object,
(ii) Measuring the image of the object on the processed photographs.
(iii) Reducing the measurements to some useful form such as a
topographical map or a numerical cadaster.
deals with the geometrical aspects of aerial photographs

58. • Photographs taken by a lens system are of central projection. This
photographs fall under the category of central perspective, which is
that all straight lines joining object and image points pass through a
point called the perspective centre.
• An aerial photograph is a central perspective picture. For
understanding the geometry of an aerial photograph, it is necessary
to consider a low oblique photograph.
• Focal plane is the flat surface where film is held.
• Focal length is the distance from the focal plane to approximately
the center of the camera lens. Thin lens equation is:
Geometry of Aerial Photographs

64.  Isocenter (i) :-The point that falls on a
line halfway between the ‘Principal
Point’ and the ‘Nadir’. ( dotted line ---
---- in sketch).
 Nadir: The point vertically beneath the
camera at the time the photograph was
taken. That plumb line extended up to
ground gives Ground Nadir Point (N)
 Principal plane:- Plane defined by
exposure station (O), Ground nadir
point (N) and ground principal point
(P) ( i.e. plane NOK)
 Principal line : Line of intersection of
principal plane with photograph plane
–nk

66.  Principal point: It is geometric center of the photograph, and the
intersection of the X and Y axes. Camera axis extended up to ground,
the point obtained on ground is called Ground Principal point (K)

67.  Exposure station (o) : The point in the atmosphere occupied by
center of camera lenses at instance of photography.
 Flying height : Vertical distance between exposure station and
mean sea level.
 Flight line: Line traced by exposure station in atmosphere ( track
of aircraft)
 Horizon point (h) : It is point of intersection of horizontal line
through center of lenses and principal line (np) on photograph.
 Azimuth : (A) : Clockwise horizontal angle measured about
ground nadir point from true north to the principal plane of
photograph.(Ф )
 Swing (S) :-Angle measured in plane of photograph from +y axis
clockwise to photo nadir point.

68. Geometry of Low Oblique Photograph
• The point of intersection of the optical axis of the camera with the
photo plane and ground plane are referred to respectively as photo
principal point (p) and ground principal point (P).
• The point vertically below the camera lens (perspective centre) on
the ground is called the ground nadir point (N) and the
corresponding image point on the photo is called the photo nadir
point (n).
• The points that falls on a line halfway between PP and Nadir –
Isocenter. The isocenter vertical with the photo plane and ground
plane are referred to respectively as photo isocentre (i) and ground
isocentre' (I).
• The distance along the optical axis from the perspective centre to
the photo plane is the focal length (f) and the, vertical distance
from the ground to the perspective centre is the flying height (H)

70. In the vertical aerial photograph the optical axis coincides with the
vertical dropped from the perspective centre. As a result, the principal
point, nadir point and isocentre coincide. The relationship of the camera
lens, positive print and the ground in' a vertical aerial photograph is
shown in fig.
Geometry of Vertical Photograph

71. • The geometry of a stereo pair of vertical aerial photographs having
60% overlap.
• Overlap of 60% results in the appearance of the principal point of the
left photo on the right photo and vice versa.
• Line joining the principal point and the image of the principal point of
the adjoining photo indicates the direction of flight and this distance
on the photo is called the photo base (b). Corresponding distance in the
air between two successive camera stations is known as air base (B).
• Fixing the principal point on a photograph four index marks known as
the "fiducial marks", one in each corner of the photograph or about the
middle of each edge of the photograph are given.
• The intersection of lines joining opposite fiducial marks define the
principal point on the photograph. In some photographs the position of
the principle point itself is indicated by a cross.
Geometry of a stereopair of Vertical Photographs

74. • Image distance divided by ground distance.
• vertical photograph - focal length divided by flying height i.e. f/H.
• From this relation vertical photograph the scale varies with relief
variations on the ground, ‘f ’ being constant.
Scale in Vertical Photographs
• Tilted photograph the scale is not constant even if the terrain is flat.
• The scale is constant only along any particular line on the photo,
parallel to the axis of tilt. Such lines are called “plate parallels”.
• Perpendicular to the axis of tilt the scale varies. It can be
geometrically shown that in the case of a flat terrain the scales
along plate parallel passing through the principal point, the
isocentre and the nadir point are as follows, where θ is the angle of
tilt, ‘f ’is the focal length of the camera and 'H' is the flying height.
Scale in Oblique Photographs:

76. Scale along the plate parallel passing through the
principal point
f cos θ/H
Scale along the plate parallel passing through the
isocentre
Scale along the plate parallel passing through the nadir
point
f/H
f/H cos θ
• The above relations show that the scale in an oblique photograph
of a flat terrain is the same as that of a vertical photograph (f /H).
• only along the plate parallel passing through the isocentre of the
oblique photograph. This plate parallel is called the "isometric
parallel".
• No photograph, however, exists which has covered a terrain that
is ideally flat allover except in the case of large water bodies. As
such, the scale variations in an oblique photograph are due to the
accumulative effect of tilt and relief.

77. • Relief variations of the
terrain cause shifting of
images from their correct
planimetric positions.
• Due to the height ‘h’ of an
object at B, its planimetric
position on the photograph is
displaced by a distance of a –
b. It can be shown that the
displacement due to relief =
r.h/H,
• where 'r' is the displacement
measured on the photo from
nadir along the base, 'h is
height of the object and 'H' is
flying height.
Image displacement due to
Relief

78. Image displacement due to tilt: In the case of a tilted photograph
of a flat terrain the image displacement equals to
for a point which is on the principal line (line through the
principal point and nadir point), where, 'i a' is the distance from
isocentre to the image on the photo and Ie' is the angle of tilt and
If', the focal length.
If the image is not on the principal line and if the line joining
isocentre and image makes an angle Ø with the principal line then
the tilt displacement is equal to

79. STEREOSCOPY
• A pair of photographs taken
from two camera stations
covering some common area
constitutes a stereo-pair which
when viewed in certain manner
gives the impression as if a
three-dimensional of the
common area is being seen.
• The stereoscopic vision is the
capacity of the brain to perceive
the objects in three dimensions
by physiologically fusing the
two different views of the
object by the right and the left
eye.

80. The optical axes of the camera must be approximately in one plane
though the eyes can accommodate to a limited degree.
1. The ratio of the distance between the exposure stations and the
flying height or the base – height ratio (B/H) must have an
appropriate value. If this value is < 0.2 the depth perception is no
stronger than if only one photograph is used. The ideal value, though
not exactly known is about 0.25.
2. The scale of the two photographs should be approximately the same.
Differences upto 15% may be successfully fused. For continuous
observations, however, differences > 5% may be disadvantageous.
3. Each photograph of the pair should be viewed with one eye only.
4. The brightness of the photographs should be similar.
5. While viewing the photographs should be given the same relative
position as they had during the time of exposure.
Conditions

81. Pseudoscopy
For normal vision the left eye should see only the left-hand photo and
the right eye should see the right-hand photo. If this condition is
reversed the depressions appear as elevations and the elevations as
depressions. Such a condition is known as pseudoscopy.
Distortions in a stereo model
The natural relief in a stereo model is obtained only when the base
height ratio is maintained at the ideal value of 0.25. The base height
ratio used in aerial photography vary from 1:3 to 1:1 or even 1:0.6.
The stereoscopic image obtained from photographs is, always
different and distorted.

82. 1. As the eye base changes from the photograph stations but the
view remains similar in all other respects.
2. As the photographs are observed at a distance which is not equal
to the principal distance (for easy understanding this term may be
taken as synonym of focal length though it is not exactly so).
3. The magnification, and more importantly the ratio between X, Y
scale against the Z (height) scale changes. We get a flattened
model, if this distance is smaller than the principal distance and
exaggerated if it is greater than the principal distance.
4. Deformation in the model is produced because the eyes are moved
away from the vertical through the principal points.
5. Shape of the object, shadows and association of other features
influence the depth perception.
Factors which influence the stereo model

83. Stereoscopic Parallax in a Stereopair of Photographs
Stereopair of aerial photographs the shift in the camera position
between two successive exposures causes apparent displacement in
the position of an object, This is called parallax.
Calculation of Heights using Parallax Difference
The parallax difference due to an elevation difference of Δh above a
reference plane is diagrammatically represented in fig. It can be
shown geometrically that the elevation Δh above the reference plane
is given by the formula:
It must be kept in mind that the formula gives correct result when
the photographs are truly vertical

84. Δh = Height of the object AR
PR = Parallax of point R
PA = Parallax of point A
ZR = Flying height above the reference plane
Δ P = Difference in parallax due to relief (PR – PA )

85. • It is an instrument designed for use with a mirror stereoscope that
has a stereo base of ten inches or less.
• It measures the relative position of identical points in both
photographs, that is their parallax difference.
• The parallax difference can then be used to compare the relative
heights.
• Parallax Bar consists of two glasses engraved with measuring marks
(dots) connected by a bar whose length can be changed by a
micrometer screw.
• When seen through the stereoscope, the dots are fused
stereoscopically and appear as a single dot having a fixed position
in space.
• By proper adjustment it can be placed on any feature on the stereo-
model surface and reading obtained from the micrometer drum.
Measurement of Parallax difference with Parallax Bar

86. • The micrometers are numbered increasingly as distance between
corresponding points is decreasing. This means the point with a
larger parallax gives a higher reading corresponding with a point of
higher elevation.
• Deducting this value from the value obtained of a point on the
reference plane we get a minus value of ‘Δ p’ for points above the
reference plane and plus value for points below the reference plane
for substitution in the above formula.

87. It is the ratio between the height of an object as seen in the stereo-model and the
actual height of the object, relative to the horizontal dimensions. The factors that
influence the stereoscopic exaggeration are:
Air base (B) Focal length (f) Flying height (H) – Photo Variables
Viewing distance (d) Separation of photographs (s) Eye base (E) – Viewing
Variables
It can be shown that the relationship of these variable with the stereoscopic
exaggeration is :
Stereoscopic Exaggeration

88. Where,
‘f’ is the focal length and 'PR' is the parallax of the bottom point of
the slope and ‘d’ is the horizontal distance of the slope on photo
scale. PR is photobase of the photo.
Estimation of slope and dip
The slopes and dips we observe in a stereomodel vary from the true
slopes and dips. The true slopes and dips are estimated from the
photographs by measuring the parallax difference Δp between the top
and bottom points of a given slope with a parallax bar and
substituting it in the formula: