Department of Geography
The science or art of obtaining reliable measurements
by means of photography (American Society of
The art, science and technology of obtaining reliable
information about physical objects and the
environment by recording, measuring and
interpreting photographic images (American Society
for Photogrammetry and Remote Sensing, 1987)
The use of photography for accurate measurement is
called photogrammetry (simple definition)
Aerial photographs areAerial photographs are notnot mapsmaps
Aerial Photographs are single-point
perspective views of the Earth’s surface,
whereas maps are orthogonal representations
of the surface
Sizes shapes and positions of objects are
distorted in aerial photographs
However, aerial photographs can be used to
construct maps and to accurately measure
distances, heights and elevations.
Orthogonal representations of theOrthogonal representations of the
surface (MAP)surface (MAP)
Figure - 1
Single-point perspective views of theSingle-point perspective views of the
Earth’s surfaceEarth’s surface (AERIAL PHOTOGRAPH)(AERIAL PHOTOGRAPH)
Figure - 2
If the terrain is completely flat and
horizontal, the vertical photograph is
same as map. If difference in height
occur, then relief displacement becomes
appearance. Pole AB appears as a point
on map and as line on aerial photo. So
aerial photo cannot be used as a map.
Then how to prepare maps from aerialThen how to prepare maps from aerial
To overcome relief displacement, which is shown above,
two photographs are taken of the same area at different
positions. These photographs are overlapped and
viewed stereo viewing devices and using this principle a
map can be prepared.
Figure – 4
There are two kinds of photographs
used in photogrammetry, aerial and
◦ Aerial photographs are usually acquired from
aircraft but can also come from satellites, hot air
balloons or even kites.
◦ Terrestrial photographs come from cameras
based on the ground (Photo Theodolites), and
generally are used in different applications from
There are two main data extraction
methods used for analyzing these
◦ Quantitative: that is size, length, shape,
height, area etc.
◦ Qualitative : geology, vegetation, drainage,
land use etc.
This section is primarily concerned with the
quantitative evaluation of survey photographs.
The rays of light pass through one central point in the lens of
a survey camera, and the distance between this point and the
film is calibrated to decimals of a millimeter. This distance is
known as the focal length.
The film plane is as flat as possible, often using flattening
devices to achieve this.
The camera body also carries reference marks known as
fiducial marks that define a coordinate measurement axis and
allow film stretch to be determined.
The camera is also very large, the size of the negatives is
23cm by 23cm which is somewhat bigger than the average
The cameras are constructed in this manner so that
measurements of a high accuracy and precision can be
Fiducial marks: Marks on the photograph margins used to locate
principal point in photo.
Nadir point:-The vertical line passing through the perspective
centre (camera lens) intersects the negative plane of at nadir
Principal point: The orthogonal projection of the perspective
centre on the negative (photo) is the principal point. In other
words it is the geometric centre of photograph.
Principal distance:- The distance from projection centre to
negative plane (c).
Iso centre: The angle α is called the angle of the maximum tilt.
The point where the line bisecting this angle intersects the
negative plane is the isocentre (i).
Principal axis:- The perpendicular line from the perspective
centre on the negative plane.
Principal line:- The line joining the nadir point and principal point.
Principal plane:- The plane containing perspective centre , nadir
point, iso centre, principal point, principal axis and principal line.
Iso line:- A line representing the intersection of the plane of a
vertical photograph with the plane of an over lapping oblique
Axis of tilt :- A line through the perspective centre perpendicular
to principal plane.
Conjugate principal point: Point in overlapping photo that is
equivalent to principal point of adjacent photograph.
Photo base: Distance between principal point and conjugate
principal point measured on a single photograph.
Ground (air) base: Ground (air) distance between principal points
of overlapping photographs.
Parallax: Apparent shift in relative positions of objects when
viewed (photographed) from different vantage points.
1. On photo, the distance between nadir point
and principal point is 0.5cm. Focal length is
200mm. Determine the tilt.
Tan a = 5/200 a = 1° 25’ 55.6”
2. The distance between principal point &
nadir point 20mm. C=210mm. Where is
the iso center?
Comparison of aerial photographs
vertical, oblique and high oblique
Photographs High oblique Low oblique Vertical
Characteristic Horizon on
Tilt smaller than 4
Coverage Greater (wide) Less (normal) Least (narrow)
Area (shape) Trapezoidal Trapezoidal Rectangular
Scale Decrease from
for ground to
Like as high
Uniform for one
Advantage Economic Easiest to map
The Geometry of the Aerial Photograph
The average scale of the aerial
photograph can be computed
either by taking the ratio of the
flying height above the ground and
the focal length (f/H), or by taking
the ratio of a known distance in the
photograph and the distance on the
As the flying height above the ground is not usually known
accurately, the second method is employed where a more
reliable scale is needed.
1. Focal length = 6” Flying height = 12000ft
Scale = 6”/ 12000’ = 1:20000
2. c = 210mm and Z = 4200 m
Scale = 210mm / 4200m = 1:2000
3. The measured distance between points A & B is 215mm.
The given co-ordinates are XA =205346.39 m,
YA = 10793.16m ; XB =205100.11m, YB = 11587.98 m.
Find the scale
Neither of the two scale calculation methods
give an accurate scale as there are two
distortions that affect measurements made on a
single aerial photograph.
a. Height distortion:
Because a lens and a photograph give a perspective or
central projection, objects that are above or below the
plane will be shifted by an amount approximately
b. Tilt distortion:
Although a lot of care is taken in the flying of aerial
photography the photographs are rarely taken exactly
horizontal. The tilts that occur in the aircraft, although
kept to a minimum by the levelling of the camera
system, do affect the position of objects on the
There are other distortions such as film shrinkage,
earth curvature, refraction effects and so on but these
are only significant when dealing with precise
Determining the height of an object from shadow length
h = L tanα
where H = flying height
h = height of the vertical object
L = length of the shadow
α = sun elevation angle
f = focal length
The shadow on which the ground falls is level.
The object is vertical
Sun elevation angle:Sun elevation angle:
The sun angle above local horizon can be
derived using a solar ephemeris table,
given the latitude and logitude of the site
and photograph acquisition date and time.
Relief displacementRelief displacement
Any objects that are higher or lower than the
principal point are displaced from its true planimetric
(x,y) location on a vertical aerial photograph. This
displacement is referred to as relief displacement or
The relief displacement is outward from the principal
point for objects whose elevations are above the local
datum, and toward the principal point for objects whose
elevation are below the local datum.
The direction of relief displacement is radial from the
Height from Relief DisplacementHeight from Relief Displacement
Simple definitionSimple definition::
Relief displacement is the distance
between the position of a point on the
photograph if it was on the reference
plane and its actual position due to relief.
Conditions for Relief DisplacementConditions for Relief Displacement
Method to find object heightMethod to find object height
Aerial photographs must be vertical or near
vertical (<3° of tilt). Namely, the principle point
is the photo nadir.
The top and the bottom of the object are
The object is on level base and vertical.
1. On flat horizontal terrain there is erected a flag pole of
25 m. The distance from the nadir point of the vertical
photograph to the pole is 300 m. The flying height is
1000 m. Principal distance is 200 mm. Compute the relief
2. On a photo of flat terrain there is an isolated hill 200m
high. Flying height is 2000m. What is the greatest value
of relief displacement which we may find ?