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Chapter two

• TYPE OF AERIAL PHOTOGRAPHS

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Introduction to Aerial Photographs

• Aerial photogrammetry is the oldest and most
  widely used method of remote sensing.
• Photography from aircraft received a big
  attention in the interest of military
  reconnaissance during World War I.
• After World War I, former military photographers
  founded aerial survey companies, and widespread
  aerial photography of the United States began.
• In 1934, the American Society of Photogrammetry
  (now the American Society for Photogrammetry and
  Remote Sensing) was founded as a scientific and
  professional organization dedicated to advancing
  this field.

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TYPES OF AERIAL PHOTOGRAPHS

• Aerial photographs may be divided into two major
  types
• Vertical (taken with the camera axis aimed
  vertically downward.
• Oblique (made with the camera axis intentionally
  inclined at an angle between the horizontal and
  vertical).

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Vertical.

• A vertical photograph is taken with the camera
  axis pointed as straight down as possible .
• Allowable inclination is usually 3 from the
  perpendicular (plumb) line to the camera axis.
• A vertical photograph has the following
  characteristics
• The lens axis is perpendicular to the surface of
  the earth.
• It covers a relatively small area.
• The shape of the ground area covered on a single
  vertical photo closely approximates a square or
  rectangle.
• Distance and directions may approach the accuracy
  of maps if taken over flat terrain.
• Relief is not readily apparent.

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Relationship of the vertical aerial photograph
with the ground.
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Types of aerial photograph Vertical is most
important as it has minimum distortion and can be
used for taking measurements
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• Types of aerial photograph
• Vertical

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Vertical photograph.
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SCALE OF A VERTICAL PHOTOGRAPH

• Scale is ordinarily interpreted as the ratio of a
  distance on a map to that same length on the
  ground.
• The scale of a vertical photograph is the ratio
  of a photo distance to the corresponding ground
  distance.
• If L is the exposure station of a vertical
  photograph taken at an altitude H above datum.
• The camera focal length is f and o is the
  photographic principal point.

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Cont

• Points A , B , C , and D ,which lie at elevations
  above datum of hA , hB , hC and hD respectively.
  
• Are imaged on the photograph at a , b , c and d.
• The scale at any point can be expressed in terms
  of its elevation, the camera focal length and
  flying height above datum.
• From the similar triangles Lab and LAB the
  following formula can be written.

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Cont
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Example
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Class work

• Calculate the flight height above average terrain
  that is required to obtain vertical photographs
  at an average scale of 18000 if the camera focal
  length is 152.4 cm.
• 10 Minute

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Low Oblique.

• This is a photograph taken with the camera
  inclined about 30 from the vertical .
• It is used to study an area before an attack, to
  substitute for a reconnaissance, to substitute
  for a map, or to supplement a map.
• A low oblique has the following characteristics
• It covers a relatively small area.
• The ground area covered is a trapezoid, although
  the photo is square or rectangular.
• Parallel lines on the ground are not parallel on
  this photograph therefore, direction (azimuth)
  cannot be measured.
• Relief is discernible but distorted.
• It does not show the horizon.

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• Types of aerial photograph
• Low oblique (no horizon)

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Relationship of low oblique photograph to the
ground
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oblique photograph
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High Oblique

• The high oblique is a photograph taken with the
  camera inclined about 60 from the vertical .
• A high oblique has the following characteristics
• It covers a very large area (not all usable).
• The ground area covered is a trapezoid, but the
  photograph is square or rectangular.
• The view varies from the very familiar to
  unfamiliar, depending on the height at which the
  photograph is taken.
• Relief may be quite discernible but distorted as
  in any oblique view. The relief is not apparent
  in a high altitude, high oblique.
• The horizon is always visible.

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Relationship of high oblique photograph to the
ground
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• Types of aerial photograph
• High oblique

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High oblique photograph.
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Image Parallax

• Parallax is defined as the apparent displacement
  of the position of an object with respect to a
  frame of reference due to a shift in the point of
  observation.
• For example, a person looking through the view
  finder of an aerial camera in an aircraft as it
  moves forward sees images of objects moving
  across the field of view.
• Points closer to the camera (of higher elevation)
  will appear to move faster and have greater
  parallaxes than lower ones.
• It is also possible to compute X and Y ground
  coordinates from parallax.

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Cont .

• Suppose an overlapping pair of vertical
  photographs exposed at equal flight heights H
  above datum.

The distance between exposure stations L1 and
L2is called B, the air base. The inset figure
shows the two exposure station
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Cont ..
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The parallax displacements occur only parallel to
the line of flight
The line of flight for any given stereo pair
defines a photo coordinate x axis for use in
parallax measurement
The parallax of any point, such as A in the above
Figure, is expressed in terms of the flight line
coordinate system as
Where Pa parallax of point A Xa measured x
coordinate of image a on the left photograph of
the stereopair Xa x coordinate of image a on
the right photograph    
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Example
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RELIEF DISPLACEMENT ON A VERTICAL PHOTOGRAPH

• Relief displacement on a vertical photograph is
• the shift or movement of an image from its
  theoretical datum location caused by the objects
  relief that is, its elevation above or below
  datum.
• Relief displacement on a vertical photograph
  occurs along radial lines from the principal
  point and increases in magnitude with greater
  distance from the principal point to the image.

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Cont

• The concept of relief displacement in a vertical
  photograph taken from a flying height H above
  datum. where the camera focal length is f and o
  is the principal point.
• Points B and C are the base and top,
  respectively, of a pole with images at b and c on
  the photograph.
• A is an imaginary point on the datum plane
  vertically beneath B with corresponding imaginary
  position a on the photograph.
• Distance ab on the photograph is the image
  displacement due to the elevation of B above
  datum, and bc is the image displacement because
  of the height of the pole.

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Relief displacement on a vertical photograph.
From similar triangles an expression for relief
displacement is formulated. First, from
triangles Figure 27.9
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Cont .
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Example
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Homework
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