Photogrammetry ' Lecture 14

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• Lecture 14 Content

• Flight preparation
• Equipment
• Weather conditions
• Flying height
• Coverage

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• Flight preparation
• a single flight of an aircraft includes a take
  off to touch down
• To secure satisfactory air cover, the following
  must be considered
• Proper equipment must be used
• Weather conditions, including height and sun
  intensity must be favorable

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• The flying height is so chosen that, with the
  given lens, contact prints of a suitable mean
  photo scale are produced
• The accuracy for contouring with certain
  instruments in the United States is often defined
  by
• C-factor Flying Height (H) / Contour Interval
  (CI)
• Used for vertical wide-angle photography with
  nominal focal length of 150mm

Scale of photograph f / (H-h)
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(No Transcript)
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• The area of interest must be completely and
  systematically covered so that every bit of
  ground appears on at least two photographs
• In addition, the optical axis must be nearly
  vertical at the time of exposure
• Considerations in the flight planning are covered
  in the following slides
• Equipment
• Weather conditions
• Flying height
• Coverage

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• Equipment
• The aircraft
• The aircraft must be able to carry the aerial
  camera
• The aircraft must be capable of flying at the
  height required (high or low)
• The camera
• Mounted on the gimbal to keep the camera
  stationary, ensuring that the optical axis passes
  through the center of the lens
• Free to rotate about the optical axis to
  facilitate different camera orientation

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• Weather conditions
• The following affect the stability of the
  aircraft and should be avoided
• Dense, whitish, turbulent clouds
• Cumulus clouds are turbulence
• Air pockets, in which there is a sudden downward
  current of air
• Photographs cannot be normally taken through
  cloud or fog, smoke, dust or haze.
• High sun intensity implies that the shadows of
  the photograph become too dense, and obscure
  detail
• Rainy conditions must be avoided

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• Shadows obscure details, so that the sun should
  be fairly high in the sky to give a reasonably
  short shadow
• Necessary for aircraft, camera and crew to stand
  by for many days before the desired weather
  conditions is obtained
• Cost of flying is very high

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• Flying height
• The mean average scale of the air cover should be
  approximately the same as the desired scale of
  the compilation map
• Given focal length of the camera lens, and a
  given compilations scale, the necessary height of
  the aircraft can be calculated using
• Scale of photograph f / (H-h)

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Scale of photograph f / (H-h)

• An example
• The photo scale 1/10,000 is required, focal
  length is 150mm and h is 100m above mean sea
  level

a
b
f
S
Horizontal plane Representing mean ground level
H
B
A
h
Mean sea level
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• Applying the formula
• 1/10,000 0.150 / (H 100) m
• i.e.
• H 100 1500m
• H 1600m
• Therefore the flying height must be 1600m
• Given the same focal length and the same size of
  format, the higher the aircraft flies the greater
  the area of ground covered, and the less the
  number of photographs required to cover any
  particular area of land
• Typical aerial photo format is 9inx9in
  (230mmx230mm)
• Therefore the greater the flying height, the more
  economical the coverage

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• Alternatively, the less the flying height, the
  greater the scale, the more detail that can be
  readily discerned and the greater the plotting
  accuracy
• Greater the flying height increases the thickness
  of haze and dust that light rays must penetrate,
  with consequent loss in the photograph quality
• Normal flying heights maybe between 1000m to
  4500m above ground level, therefore we expect
  flying cost to increase in mountainous areas

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• Coverage
• Systematic coverage is obtained by flying the
  aircraft at a fixed height, in a series of
  straight lines, and taking photographs at regular
  intervals
• The photographs taken during the time that the
  aircraft flies along any one of these straight
  lines are known collectively as a run

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• Each photograph in a run should overlap its
  predecessor by 60

Aircraft
Aircraft
1st
2nd
1st
2nd
3rd
60 overlap
double overlap

• Double overlap usually is 20of each of the three
  photographs

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• Having flown one complete run the aircraft must
  turn around and fly back another straight line,
  again taking photographs at regular intervals
• The second strip of photographs must also overlap
  the first run, this is known as the lateral
  overlap
• Lateral overlap is usually between 20 to 30
• Runs will normally flown in a direction parallel
  with the longest dimension of the land to be
  covered

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1st run

2nd run

Lateral overlap
3rd run

4th run
Plan showing later overlaps
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Overlap between Runs or Swaths
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Exposure for 1st strip
Exposure for 2nd strip
Exposure for 3rd strip
Exposure for 4th strip
A typical flight plan
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• Other factors to be considered in planning
  aerial photography include
• The intended use of photography (quantitative or
  interpretive)
• The desired product (Map, orthophoto, mosiac,
  etc)
• The specified accuracy
• The size and shape of the interested area
• The amount and disposition of relief
• The scale of photography (influences the final
  scale of the end product, camera to be used, and
  so on)

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• Computational steps for flight planning
  (refer to text book)
• Choose flight stripes which will minimize the
  number of aircraft turns and realignment.
• Compute the flying height using (Hf/S) or the
  C-factor equation
• Add the mean site elevation to the computed
  height (H)
• Determine the ground coverage per image from the
  film format size and photo scale
• Determine the ground separations between photos
  (60 overlap)
• Determine the time between exposures
• Calculate the distance between photo centers.
• Compile the number of photos per run
• Calculate the separation between runs
• Determine the number of runs required to cover
  the area
• Calculate the spacing of flight lines on the map
  and the number of photos needed.

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• The End