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GEOG3610 Remote Sensing and Image Interpretation

Photographic Remote Sensing 1

Copyright, 1998-2013 © Qiming Zhou GEOG3610 Remote Sensing and Image Interpretation

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.

(Courtesy Strain and Engle, 1992)

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Airphoto

– Hong

Kong

1945



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Airborne platforms

�Airborne platforms include:�High altitude platforms: high-altitude

balloons, U-2 reconnaissance aircraft, etc.

�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


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


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


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.

3-dimensional photography (cont.)

3D Visualization

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3-D visualisation 2D Visualization3D Visualization

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.



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Effects of 8 different illumination angles

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