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Airborne Platforms and SensorsAirborne Platforms and Sensors
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Airborne Platforms and Sensors
�Development of aerial photography�The airborne platforms�The types of aerial photographs�Cameras, films and filters�Photogrammetry�Acquisition of aerial photographs
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Current remote sensing systems
�Classified by technology�Active remote sensing�Passive remote sensing
�Classified by platforms�Airborne systems�Spaceborne systems
�Classified by sensors�Photographic remote sensing�Digital remote sensing
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Types of platforms and sensors
PhotographicAerial photography
space mission photography
DigitalAirborne scanner
Satellite imagery
Airborne Spaceborne
Digital Airborne radar Satellite radar
Platforms
Senso
rs
Active
Passive
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Development of aerial photography
�The photographic camera is the oldest and the most frequently used remote sensing instrument.
�1859: Tournachon obtained balloon photographs of a small village near Paris.
�1909: Wright and a Pathé news cameraperson took motion pictures of Centotelli, Italy.
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1860 picture of Boston Harbour
This 1860 picture of Boston Harbour is thought to be the first aerial photograph taken in the US. The exposure was made from a balloon at an altitude of about 365m above the ground
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Old air-borne platforms
Above: Stalwart pigeon photographers prepare to work. The tiny pigeon cameras were designed in 1903 and weighed about 70g. Right: Peering down a camera viewfinder from the open cockpit of a Curtiss Jenny, a flier practices the early techniques of aerial photography.
�Mid-altitude platforms: normal aircraft�Low altitude platforms: light aircraft,
remotely piloted aircraft (RPA)�Perspective views�Mission-based data acquisition
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High-altitude airborne platforms
Left: high-altitude balloon that can reach the top edge of the atmosphere; Above: U-2 military spy aircraft that flies near 30,000 meters.
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Remote sensing aircraft
Mid-altitude remote sensing aircraft.12
Low-altitude platformsLeft: a remotely piloted aircraft (RPA) flying at a very slow speed and low altitude of 30 meters.
Right: a light two-seater aircraft that is capable of flying at a slow speed and low altitude of 100 meters.
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Types of aerial photographs
Depending upon the orientation of the camera’s optical axis with respect to the earth’s surface, airphotos are classified as:�Vertical airphotos�Oblique airphotos
�High-oblique�Low-oblique
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Orientation of an aerial camera
Orientation of an aerial camera for vertical, low-oblique, and high-oblique photography. Also shown is the shape and relative size of the ground area associated with each type of photograph.
Vertical Low oblique High oblique
Film plane
Lens
Horizon line
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Vertical airphoto
Vertical airphoto of Maipo wetland of Hong Kong and
adjacent Shenzhen urban built-up area (1997)
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High-oblique airphoto
High-oblique airphoto (horizon included) of Yuen Long and rural area of Hong Kong, by Lands Department of HKSAR (1999).
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Low-oblique airphoto
Low-oblique airphoto (horizon not
shown) of Aberdeen, Hong Kong (1999).
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The framing camera
� Camera body� lens - focal plane and focal length
� shutter - control exposure speed
� diaphragm - control apertures
� Film exposure� film exposure - the quantity of light that is
allowed to reach the film
� F/number = f / d (progressed by √2, ~1.4)� Lens speed: light-gathering power of a lens =
the F/number of the full aperture.
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Camera lens
A classical Carl Zeiss camara lens with 180mm focal length, note marks of diaphragm and focus range.
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Multispectral camera
A nine-lens multispectral camera.
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The framing camera (cont.)
Angular field of view (AFOV)
=f
LL 2
arctan2θ
=f
WW 2
arctan2θ
+=f
WLD 2
arctan222
θ
Classification of Lens Angles
Range of Angles Name< 60° Narrow angle
60 - 75° Normal angle
75 - 100° Wide angle> 100° Super-wide angle
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Lens angles in 3-dimensional space
W
L
DLens
Image format
θDθW
θL
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Ground distance
=2
tan2 LL HD
θ
=2
tan2 WW HD
θ
•
•
H’
H
D’D
θθθθ’
θθθθ
•
θ = 40°θ = 70°
θ = 90°θ = 110°
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Films and filters
�Black and white films�panchromatic or infrared
�Colour films�natural colour and colour infrared
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A mapping camera
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Aerial survey cameras
Aerial survey cameras and other equipment mounted inside an airplane.
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Generalised diagram of a black-and-white film
Spectral-sensitivity curves for panchromatic,
extended-red panchromatic, and infrared films 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-1
0
1
2UV Blue Green Red Infrared
Wavelength (µm)
Log s
ensi
tivity
Panchromatic(extended red)
Panchromatic Infrared
Black-and-white filmSilver halide grains
Gelatin
Emulsion
Base
Backing
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Panchromatic aerial photograph
A panchromatic aerial photograph of Tweed region, North NSW, Australia
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A black-and-white infrared photo
Source: Ross Alford(www.pibweb.com/ross)
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Normal colour and colour infrared films
Backing
Base
Red-sensitive layer
Green-sensitive layer
Yellow filter
Blue-sensitive layer
Haze filter
Backing
Base
Red-sensitive layer
Green-sensitive layer
Infrared-sensitive layer
Yellow filter
Normal colour film. A haze filter is placed over the camera lens to stop ultraviolet radiation from reaching the blue-sensitive layer.
Colour infrared film. A yellow filter must be used to stop ultraviolet and blue radiation from reaching the three emulsion layers.
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D. Resulting colours
C. Film after processing
B. Film after camera exposure
A. Original scene reflectance
Colour formation with a colour-reversal film
Blue Green Red White Black
Activated Activated
Activated Activated
Activated Activated
Blue Green Red White Black
B G RB G R B G R B G R B G R
White light
B G RB G R
Blue-sensitive layer
Green-sensitive layer
Red-sensitive layer
Yellow filter
Yellow dye layer
Magenta dye layer
Cyan dye layer
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Normal colour aerial photograph
A high-resolution (20cm) normal colour aerial photograph of a residential area in Berlin.
(Courtesy GeoContent GmbH: www.geocontent.de)
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A. Original scene reflectance
False-colour formation with a colour infrared-reversal film
D. Resulting colours
C. Film after processing
B. Film after camera exposure
Blue Green Red Infrared
Activated
Activated
Activated
Black Blue Green Red
B G RB G R B G R B G R
White light
B G
Infrared-sensitive layer
Green-sensitive layer
Red-sensitive layer
Yellow filter
Cyan dye layer
Yellow dye layer
Magenta dye layer
R
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Colour infrared aerial photograph
A high-resolution (20cm) colour infrared aerial photograph of a residential area in Berlin.
(Courtesy GeoContent GmbH: www.geocontent.de)
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�Marginal information of an aerial photograph:�obvious: name of the place, date of photograph, flight height, photo index, and producer�not-so-obvious: focal length of the camera lens, time of the photograph, position of the principal point of the photo
Using aerial photographs
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Name of the place date of
photograph, flight height, photo index, and producer
Focal length of the camera lens
Marginal information
Clock to show
time of the photograph
Notch to find
principle point
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Example of paired photographs that produce a 3-dimensional image when viewed through a stereoscope.
3-dimensional photography
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3-dimensional photograph that was produced from a photograph pair that is coloured as cyan for the left photo and red for the right photo. The 3-dimensional vision can be viewed using a coloured spectacles with cyan on the left and red on the right.
Courtesy Dr Zhang Yun Department of Geodesy and Geomatics Engineering,
University of New Brunswick,
Canada
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Flight mission for stereo coverage
Aerial camera stations are spaced to provide for about a 60% forward overlap of aerial photographs along each flight line and a 20-30% sidelap for adjacent lines.
20 to 30 % sidelap
60%
Forward overlap
1 2 3 4
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Stereoscopes
Desktop (left) and pocket (top) stereoscopes for stereo view of aerial photographs.
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Photogrammetry
�Photogrammetry: the technique of obtaining reliable measurements of objects from their photographic images.
�Determination of scale (RF: representative fraction)� from focal length and altitude� from photo-map distance� from photo-ground distance
H
fRF =
( )( )MSMD
PDRF =
GD
PDRF =
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Geometry of a vertical airphoto
D
B
C
A
E
Lens
Film plane
Principal point
Nadir
Optica
l axis
f
H
DCEACBAB
DE
H
f
∠=∠
=
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Understanding distortion
�Perspective view�Systematic distortion controlled by the
field of view that is determined by� focal length of the camera lens�size of the film media (e.g. 9 x 9 inches)
�Random distortion specified as crab and tilt of aerial photographs
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f
H
FOV
sidelap
overlap
Flight line
30%
60
%
crab
Tilt
Factors to be considered
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Computing heights
� Using object displacement� d = object length from base to top
� r = radial distance from nadir to top
� H = flying height
� Using stereoscopic parallax� P = absolute stereoscopic parallax at the base� dP = differential parallax
� Using shadow length� α = sun’s elevation angle
� s = shadow length
Hr
dh =
HdPP
dPh
+=
αtansh =
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Computing heights using object displacement
Wan Chai urban area of Hong Kong. The Building marked A shows great displacement because it is far away from the nadir.
A
nadir
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Computing heights using stereoscopic parallax (cont.)
P + dP
P
Stereo photo pair of Wan Chai urban area of Hong Kong. Note that the photo pair must be correctly aligned for stereo view before computing heights using parallax.
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Computing heights using shadow length
Height
Shadow
α
αtan×= shadowHeight
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Acquisition of aerial photographs
� Flight altitudes and focal lengths� Seasonal consideration� Time-of-day considerations� Availability of existing photography
� In Hong Kong, the territory has been covered at least once a year since late 1980’s.
� Since 1993, the coverage has been made in natural colour photographs.
� The airphotos can be purchased from the land department.
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Seasonal consideration
Summer (leaf on) and winter (leaf off) airphotos for the same ground area in western Pennsylvania
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Hotspot
HotspotNadir
Lens
Hotspot problem often occurs with high sun angle for vertical photos, so that summer mid-day flight mission should be avoided.