Photogrammetry

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Photogrammetry
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Introduction

• Photogrammetry the science, art, and technology
  of obtaining reliable information from
  photographs.
• Two major areas metric, and interpretative.
• Terrestrial and aerial Photogrammetry.
• Uses of Photogrammetry topographic mapping,
  determine precise point coordinates, cross
  sections, deflection monitoring, and many other
  applications.
• Why Photogrammetry?

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Aerial Cameras

• For precise results, cameras must be
  geometrically stable, fast, have efficient
  shutters, sharp lenses
• Single-lens frame cameras figure 27-2
• most used format size is 9, focal length 6 in
• components lens, shutter, diaphragm, filter,
  focal plane, fiducial marks.
• shutters can be operated manually or
  automatically.
• The camera could be leveled regardless of the
  plane orientation.
• Exposure station and principal point.
• Camera calibration.

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Aerial Photographs

• True Vertical if the camera axis is exactly
  vertical, or near vertical.
• Tilted Photographs
• Oblique photographs high and low
• Vertical Photos are the most used type for
  surveying applications

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Geometry of Vertical Photographs

• Figure 27-6
• Define image coordinate system (right handed),
  principal point, exposure station.
• Measurements could be done using negatives or
  diapositives, same geometry.
• Strips and Blocks.
• Sidelap (about 30), and Endlap (about 60), why?

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Scale of a Vertical Photograph

• Figure 28-6
• Scale of a photograph is the ratio of a distance
  on a photo to the same distance on the ground.
• Photographs are not maps, why?
• Scale of a map and scale of a photograph.
• Orthphotos
• Scale (s) at any point

f
S
H - h
f

• Average scale of a photograph

Savg
H - havg
If the f, H, and h are not available, but a
map is available then
photo distance
X map scale
Photo Scale
map distance
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Ground Coordinates from a Single Vertical
Photograph

• Figure 27-8
• With image coordinate system defined, we define
  an arbitrary ground coordinate system.
• That ground system could be used to compute
  distances and azimuths. Coordinates can also be
  transformed to any system
• In that ground system
• Xa xa (photograph scale at a)
• Ya ya (photograph scale at a)

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Relief Displacement on a Vertical Photograph

• Figure 27-9
• The shift of an image from its theoretical datum
  location caused by the objects relief. Two
  points on a vertical line will appear as one line
  on a map, but two points, usually, on a
  photograph.
• In a vertical photo, the displacement is from the
  principal point.
• Relief displacement (d) of a point wrt a point on
  the datum

r h
d
H
where r is the radial distance on the photo to
the high point h elevation of the high point,
and H is flying height above datum

• Assuming that the datum is at the bottom of
  vertical object, H is the
• flying height above ground, the value h will
  compute the object height.

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ra/R f/H
Or ra H R f ----(1)
rb/R f/(H-h)
Or rb (H-h) R f ---(2)
Then from (1) and (2) Or ra H rb (H-h)
then D rb - ra rb hb /H
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Flying Height of a Vertical Photograph

• Flying height can be determined by
• Readings on the photos
• Applying scale equation, if scale could be
  computed
• Example what is the flying height above datum if
  f6, average elevation of ground is 900ft, scale
  is 1100ft? Is it 1500?
• Or, if two control points appear in the
  photograph, solve the equation
• L2 (XB - XA)2 (YB - YA)2
• then solve the same equation again replacing the
  ground coordinates with the photo coordinates.