chapter 4 Flashcards
aerial photograph
any photograph taken from the air
An aerial photograph, in broad terms, is any photograph taken from the air. Normally, air photos are taken …………………… using a ……………………………………..
vertically from an aircraft
highly-accurate camera.
There are several things you can look for to determine what makes one photograph different from another of the same area including
type of film, scale, and overlap
Other important concepts used in aerial photography are
stereoscopic coverage, fiducial marks, focal length, roll and frame numbers, and flight lines and index maps.
Optics refer to
lenses and the geometry of light retrieval in a camera
Optics refer to lenses and the geometry of light retrieval in a camera. The lenses in a camera are responsible for
focusing and zooming on an object
Before light reflected from an object ………………………, it must pass through
strikes the film
one or more lenses
As light passes through a lens, it is
bent to focus the imaged object on the film
To minimize distortions associated with the use of ………………
single lenses
To minimize distortions associated with the use of single lenses, most camera lenses are actually composed of …………………………. that work in concert to
multiple lenses
form an image onto the film
most air photo missions are …………. using ………………
flown
black and white film
most air photo missions are flown using black and white film, however ……………………………………………..are sometimes used for special projects
colour, infrared, and false-colour infrared film
Film in a camera is used to record the image that passes through the
lens
Photographic film is composed of
a durable base
Film in a camera is used to record the image that passes through the lens. Photographic film is composed of a durable base, which is coated with a ………………………….. known as the …………………..
light-sensitive layer know as the emulsion
During the short time that a shutter is open
light strikes the film and leaves a latent image on the emulsion
. During the short time that a shutter is open, light strikes the film and leaves a latent image on the emulsion. This image can be made visible by
the process of developing and printing.
Emulsions are made of
materials sensitive to particular regions of the electromagnetic spectrum
Emulsions are made of materials sensitive to particular regions of the electromagnetic spectrum. For example,
some film is only sensitive to visible light, whereas other film is sensitive to near-infrared light
In color film, the emulsion is composed of
three layers, with each being sensitive to different wavelengths of light,
In color film, the emulsion is composed of three layers, with each being sensitive to different wavelengths of light, normally
blue, green and red light.
With black and white film, the emulsion is sensitive to
to a broad spectrum of light
Film emulsions are generally limited to recording wavelengths between
0.4 to 0.9 micrometers
Black and white film sensitive to
visible light
black and white film sensitive to
infrared wavelengths
Black and white film sensitive to visible light and black and white film sensitive to infrared wavelengths can also be used for
remote sensing purposes
Film speed is another quality of
emulsions that is important for aerial photography
Film speed refers to
the quantity of light that is needed to expose the emulsion
Fast film requires
less light than slow film to record the same image
If the camera platform is moving, one would want to use ………………………. to …………………………………………….
a high speed film to reduce the blurring effects of the moving camera
If the camera platform is moving, one would want to use a high speed film to reduce the blurring effects of the moving camera. Unfortunately, there is a tradeoff between
film speed and image quality
the faster the film speed, the
grainier the image
, there is a tradeoff between film speed and image quality – the faster the film speed, the grainier the image. Because of this tradeoff, it is necessary to
carefully choose a film speed that will meet the requirements of the end user
Some sophisticated camera mounts have an
image motion compensator
Some sophisticated camera mounts have an image motion compensator that
reduces the blurring effect of the moving platform
Some sophisticated camera mounts have an image motion compensator that reduces the blurring effect of the moving platform, which potentially
allows the use of slower film
Focal length
the distance from the middle of the camera lens to the focal plane (i.e. the film)
As focal length increases,
image distortion decreases
The focal length is precisely measured when
the camera is calibrated
The amount of image detail that can be recorded on film is directly related to
the distance between the lens and the film
The amount of image detail that can be recorded on film is directly related to the distance between the lens and the film (referred to as …………………)
focal length
As the focal length increases, the …………………………………………………. increases
detail that can be seen on the film
Increasing the focal length is commonly called
zooming in on an object.
Scale:
the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground
Scale: the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground (i.e. …………………………….)
1 unit on the photo equals “x” units on the ground
If a 1 km stretch of highway covers 4 cm on an air photo, the scale is calculated as follows:
Another method used to determine the scale of a photo is to find the
ratio between the camera’s focal length and the plane’s altitude above the ground being photographed
A
camera
B
focal length
C
film (focal plane)
D
lens
E
altitude above ground level (AGL)
F
Altitude above sea level (ASL)
If a camera’s focal length is 152 mm, and the plane’s altitude Above Ground Level (AGL) is 7600 m, using the same equation as above, the scale would be:
Scale may be expressed three ways:
- Unit Equivalent
- Representative Fraction
- Ratio
A photographic scale of 1 millimetre on the photograph represents 25 metres on the ground would be expressed as follows:
- Unit Equivalent - 1 mm = 25 m
- Representative Fraction - 1/25 000
- Ratio - 1:25 000
Large Scale
Larger-scale photos (e.g. 1:25 000) cover small areas in greater detail
A large scale photo simply means that ground features are at
a larger, more detailed size
The area of ground coverage that is seen on the photo is …………………..smaller scales
less than at
Small Scale
Smaller-scale photos (e.g. 1:50 000) cover large areas in less detail
A small scale photo simply means that ground features are at
at a smaller, less detailed size
The area of ground coverage that is seen on the photo is …………………………………. larger scales
greater than at
In many remote sensing applications, it is important to restrict
the light entering the camera by the use of filters
Color filters work by
absorbing a range of wavelengths while allowing other wavelengths to pass through
Another filter type, known as neutral color filters,
do not alter the spectral composition of light, but instead reduce the amount of light of all wavelengths that pass through.
Perhaps the most common color filter is an
antihaze filter
Perhaps the most common color filter is an antihaze filter. These are ……………………………. filters
clear or yellow
antihaze filter. These are clear or yellow filters, which
absorb out the shorter ultraviolet and blue wavelengths
antihaze filter. These are clear or yellow filters, which absorb out the shorter ultraviolet and blue wavelengths that are substantially
scattered by particulates in the atmosphere.
Another filter used for monitoring vegetation is
an infrared filter
Another filter used for monitoring vegetation is an infrared filter, which
absorbs visible light and only allows infrared light to pass through
Fiducial marks
small registration marks exposed on the edges of a photograph
The distances between fiducial marks are
precisely measured when a camera is calibrated
The distances between fiducial marks are precisely measured when a camera is calibrated, and this information is used by
cartographers when compiling a topographic map.
A
fiducial marks
B
Optical axis
C
Principal point
D
Lens
Overlap
is the amount by which one photograph includes the area covered by another photograph, and is expressed as a percentage.
The photo survey is designed to acquire ……………………………………… and ………………………………….
60% forward overlap (between photos along the same flight line) and 30% lateral overlap (between photos on adjacent flight lines).
A
60 % overlap between photos (forward overlap)
B
20 - 40 % overlap between flight lines (lateral overlap)
Stereoscopic Coverage
the three-dimensional view which results when two overlapping photos (called a stereo pair)
Stereoscopic Coverage: the three-dimensional view which results when two overlapping photos (called a stereo pair), are viewed using a
stereoscope.
. Each photograph of the stereo pair provides a
slightly different view of the same area
Each photograph of the stereo pair provides a slightly different view of the same area, which
the brain combines and interprets as a 3-D view.
Roll and Photo Numbers
each aerial photo is assigned a unique index number according to the photo’s roll and frame
For example, photo A23822-35 is the
35th annotated photo on roll A23822
For example, photo A23822-35 is the 35th annotated photo on roll A23822. This identifying number allows you to
to find the photo in NAPL’s archive, along with metadata information such as the date it was taken, the plane’s altitude (above sea level), the focal length of the camera, and the weather conditions.
Flight Lines and Index Maps
at the end of a photo mission, the aerial survey contractor plots the location of the first, last, and every fifth photo centre, along with its roll and frame number, on a National Topographic System (NTS) map
Photo centres are represented by
small circles
straight lines are drawn connecting the circles to show
photos on the same flight line.
Photo centres are represented by small circles, and straight lines are drawn connecting the circles to show photos on the same flight line.
This graphical representation is called
air photo index map
This graphical representation is called an air photo index map, and it allows you to relate
the photos to their geographical location
Small-scale photographs are indexed on …………………… scale NTS map sheets
1:250 000
larger-scale photographs are indexed on ………………………… scale NTS maps.
1:50 000
A
flight line roll number
B
photo number photo centre
Aerial photography is one of the …………….forms of remote sensing
oldest
Aerial photography is one of the oldest forms of remote sensing and it is
still used extensively today
Aerial photography is one of the oldest forms of remote sensing and it is still used extensively today. It is usually the choice if
great spatial detail is needed
Aerial photography records the
ever-changing cultural and natural features on the Earth’s surface.
It shows geographical features such as:
- mountains
- canyons
- flatlands
- rivers from the source to the mouth
It reveals the earth’s resources, such as:
- lakes
- forests
It captures:
- residential and industrial areas
- road and rail networks
It has many practical applications such as:
- map-making
- urban and rural planning
- environmental impact studies
- civil law cases
- real estate evaluations
- and even wall art
vertical aerial photograph
A vertical photograph is one which has been taken with optical axis of camera approximately perpendicular to the horizontal plane
A vertical photograph is one which has been taken with optical axis of camera approximately perpendicular to the horizontal plane. A deviation up to …………is acceptable
4°
A deviation up to 4° is acceptable. This gives the map of the earth detail somehow in same
scale
optical axis of camera
Oblique Aerial Photographs
An oblique photograph is one which has been taken with the optical axis of the camera intentionally tilted from perpendicular position obliquely.
The degree of tilt from the perpendicular further classifies oblique photographs into
high oblique photograph and low oblique photograph.
A high oblique photograph is one which is taken with
the optical axis of the camera making an angle >30° with the vertical axis and which shows the apparent horizon on the photograph.
optical axis of camera
