AERIAL PHOTOGRAPHS// DEFINITION , CHARACTERISTICS// TYPES
What is Aerial Photographs?
Photographing from air is basically known as
aerial photography. The word ‘aerial’ derived in early 17th century from Latin
word aerius , and Greek word aerios . The term "photography" is
derived from two Greek words phos meaning "light" and graphien
meaning "writing" means "writing by light".
Aerial Photography
: Art, science and technology of taking aerial photographs from an air-borne
platform.
Aerial Photograph : A photograph taken from an
air-borne platform using a precision camera.
Aerial photography
comes under the branch of Remote Sensing. Platforms from which remote sensing
observations are made are aircraft and satellites as they are the most widespread
and common platforms. Aerial photography is a part of remote sensing and has
wide applications in topographical mapping, engineering, environmental science
studies and exploration for oil and minerals etc. In the early stages of
development, aerial photographs were obtained from balloons and kites but after
the invention of aircrafts in 1903 aircrafts are being used widely for aerial
photographs.
Aerial
photography is a fascinating niche that the pros use for many different
purposes, and there are many different types of aerial photography. The term aerial
photography refers to taking photographs from an elevated position, often using
an airborne craft, including such equipment as rockets, airplanes, hot air
balloons, and more recently, drones.
The sun provides the source of energy
(electromagnetic radiation or EMR) and the photosensitive film acts as a sensor
to record the images. Diversifications observed in the images of photographs
shows the different amount of energy being reflected from the objects as
recorded on the film. Nowdays aerial photography also become digital where
values of reflected electromagnetic radiation is recorded in digital numbers.
USES OF AERIAL
PHOTOGRAPHS
· Aerial photographs are used in topographical
mapping and interpretation.
· These two different uses have led
to the development of photogrammetry and photo/image interpretation as two
independent but related sciences.
· Photogrammetry: It refers to the
science and technology of making reliable measurements from aerial photographs.
The principles used in photogrammetry facilitate precise measurements related
to the length, breadth and height from such photographs.
· Hence, they are used as the data
source for creating and updating topographic maps.
Characteristics of
Aerial Photographs:
1. Synoptic view: Recording or taking aerial
photographs spatially over large area is like birds eye view from the top.
These technologies allows discriminating and detecting small scale features and
spatial relationships among them.
2. Time freezing
ability: They
are defined as virtually permanent records of the existing conditions on
Earth’s surface at one point of time, and further can be used as past document.
3. Capability to stop
action:
They provides a stop action view of dynamic state and are used in studying the
variable/dynamic phenomena such as flooding, moving wildlife, traffic, oil
spills, forests fires, changing dynamics in natural phenomenon etc.
4. Three-dimensional
perspective:
Aerial photographs provide a stereoscopic view of the Earth’s surface where one
can make horizontal and vertical measurements.
5. Spectral and spatial resolution: Aerial
films are susceptible to electromagnetic rays in wavelengths ((0.3 µm to 0.9
µm) beyond spectral sensitivity of the human eye (0.4 µm to 0.7 µm).
6. Availability: Airborne photographs can be taken
on user specific time and make permanent record at a range of scales for any
area.
Factors that influence Aerial Photography
Scale:
Scale is define as the ratio of distances
between two images on an aerial photograph and the actual distance between the
same two points/objects on the ground, in other words the ratio f/H (where f is
the focal length of the camera lens and H is the flying height above the mean
terrain), Change in scale from photograph to another is because of the
variations in flying height other factors that further affect the scale
variations are tilt and relief displacements.
Camera/Film/Filter Combinations Aerial Cameras:
Aerial
Cameras are special cameras that are built for mapping which have high
geometric and radiometric accuracy. Airborne camera are built with exactness
and purposely designed to expose a large number of films/photographs in speedy
succession with the ultimate in geometric fidelity and quality. Aerial cameras
generally have a medium to large format, with good quality lens, a large film
magazine, a mount to hold the lens, the camera in a vertical position and a
motor drive. There are various types of aerial cameras such as Aerial mapping
camera (single lens), Reconnaissance camera, Strip camera, Panoramic camera,
Multi- lens camera, multiband aerial cameras, Digital camera.
Aerial Films:
Aerial film is multi layer emulsion laid on a stable anti-halation base. Generally
aerial films are available in rolls that has cross section of about 10 inch in
wide and 200 to 500 ft in length.
Panchromatic:
Panchromatic, more often termed black and white, is the most commonly
encountered film employed for photogrammetry. The sensitive layer consists of
silver salt (bromide, chloride, and halide) crystals suspended in a pure
gelatine coating which sits atop a plastic base sheet. The emulsion is
sensitive to the visible (0.4- to 0.7-µm) portion of the electromagnetic
spectrum.
Colour:
Natural colour also known as true colour film.. The multilayer emulsion is
sensitive to visible region of electromagnetic spectrum. There are three layers
of gelatine containing sensitized dyes, one each for blue (0.4–0.5 µm), green
(0.5–0.6 µm), and red (0.6–0.7 µm) light. Green and red layers are also
sensitive to blue wavelengths. Visible light waves first pass through and react
with the blue layer and then pass through a filter layer which halts further
passage of the blue rays. Green and red waves pass through this barrier and
sensitize their respective dyes, causing a chemical reaction and thus
completing the exposure and creating a true colour image.
Infrared: Current aerial infrared film is
offered as two types: black and white infrared and colour infrared. Black and
White Infrared have the emulsion sensitive to green (0.54–0.6 µm), red (0.6–0.7
µm), and part of the near infrared (0.7–1.0 µm) portions of the spectrum and
renders a grayscale image.
Flight
Direction: It is advisable that aerial photography is flown in tiles to cover
the chosen area in designated flight line. For easiness in handling, it is
prudent to keep the number of tiles to minimum. The flight direction of the
strips/tiles is therefore kept along the length of the area.
Time:
The time at which aerial photograph taken is very important, as long, deep
shadows tend to doubtful details, where as undersized/small shadows tend tomark
out some details effectively and are generally fruitful in improving the
interpretational values of a photograph.
Season:
Factors
such as seasonal variations in light reflectance, seasonal changes in the
vegetation cover and seasonal changes in climatological factors are the tip
points for choosing the suitability of season.. The purpose for which aerial
photography is flown also dictates the season. For example, for photogrammetric
mapping, geological or soil survey purposes, the ground should be as clearly
visible as possible.
Atmospheric Conditions:
The
presence of particles (smoke or dust) and molecules of gases in the atmosphere
tends to reduce contrast because of scattering, especially by the heavier
particles; therefore the best time for photography is when the sky is clear,
which normally in India is from November to February. The presence of dust and
smoke during the pre monsoon summer months and of clouds during the monsoon
months forbids aerial photography during these periods.
Classification of Aerial Photograph:
There
are different criteria to classify aerial photographs. Different criteria are
scale, tilt angle, angular coverage, type of film and spectral bands. Depending
upon these criteria aerial photographs can be classified as:
A. Scale: The aerial photographs may also be
classified on the basis of the scale of photograph into three types.
✓ Large scale: When the scale of an
aerial photograph is 1 : 15,000 and larger, the photography is classified as
large-scale photograph
✓ Medium scale: : The aerial
photographs with a scale ranging between 1 : 15,000 and 1 : 30,000 are usually
treated as medium scale photographs
✓ Small scale: The photographs with
the scale being smaller than 1 : 30,000, are referred to as small scale
photographs
B. Camera Orientation:
Vertical:
A vertical photograph is taken with the camera pointed as straight down as
possible. Allowable tolerance is usually + 3° from the perpendicular (plumb)
line to the camera axis. The result is coincident with the camera axis.
A
vertical photograph has the following characteristics:
(1) The lens axis is perpendicular to the
surface of the earth.
(2)
It covers a relatively small area.
(3)
The shape of the ground area covered on a single vertical photo closely
approximates a square or rectangle.
(4) Being a view from above, it gives an
unfamiliar view of the ground.
(5) Distance and directions may approach the accuracy
of maps if taken over flat terrain.
(6) Relief is not readily apparent
Oblique:
a.
Low oblique:
This
is a photograph taken with the camera inclined about 30° from the vertical. It
is used to study an area before an attack, to substitute for a reconnaissance,
to substitute for a map, or to supplement a map .
A low oblique has the following
characteristics:
(1) It covers a relatively small area.
(2) The ground area covered is a trapezoid,
although the photo is square or rectangular.
(3)
The objects have a more familiar view, comparable to viewing from the top of a
high hill or tall building.
(4)
No scale is applicable to the entire photograph, and distance cannot be
measured. Parallel lines on the ground are not parallel on this photograph;
therefore, direction (azimuth) cannot be measured.
(5) Relief is discernible but distorted. (6)
It does not show the horizon.
b.
High oblique: The high oblique is a photograph taken with the camera inclined
about 60° from the vertical. It has a limited military application; it is used
primarily in the making of aeronautical charts. However, it may be the only
photography available.
A high oblique has the following
characteristics:
(1) It covers a very large area (not all
usable).
(2)
The ground area covered is a trapezoid, but the photograph is square or
rectangular.
(3)
The view varies from the very familiar to unfamiliar, depending on the height
at which the photograph is taken.
(4)
Distances and directions are not measured on this photograph for the same
reasons that they are not measured on the low oblique.
(5)
Relief may be quite discernible but distorted as in any oblique view. The
relief is not apparent in a high altitude, high oblique.
(6) The horizon is always visible
c. Trimetrogon: This is an assemblage of three
photographs taken at the same time, one vertical and two high obliques, in a
direction at right angel to the line of flight. The obliques taken at an angel
of 600 from the vertical, sidelap the vertical photography producing composites
from horizon to horizon.
d. Convergent Photography: It is a sequential
pair of low oblique in which the optical axes converse towards one another. in
this kind of photography both the photographs cover the same area but from
different locations
C.
Angular Coverage: Angular coverage is a function of focal length and format
size.
✓ Narrow Angle: Angle of Coverage
Less than 200 (Large Focal length) Used for General interpretation,
intelligence and mosaics.
✓ Normal angle: Angle of coverage
between 500 - 750 used for general interpretation, mapping, ortho-photography,
and mosaics.
✓ Wide angle: angle of coverage 850
- 950 used for general interpretation, general purpose photography for normal
terrain, resource mapping and mosaics.
✓ Super-wide angle: angle of
coverage more than 1100 Used for General purpose mapping of flat areas
D.
Film
✓ Black and white panchromatic: This
is most broadly used type of film for photogrammetric, mapping and
interpretation.
✓ Black and white infrared: This is
used interpretation and intelligence and in hazy environment as IR can
penetrate through haze.
✓ Colour: This is used for
interpretation and mapping.
✓ Colour infrared/ false colour:
This is used for vegetation studies, water pollution, and crop studies.
E.
Spectral Coverage/Response: Multispectral: Depending upon the number of
spectral bands.
|
Comparison
between Vertical and Oblique Photographs |
|||
|
Optical Axis |
Tilt < 3° i.e. exactly Deviation is
<300 Deviates by or nearly from the Vertical axis >30 degree coincides with
the axis. from vertical Vertical axis. |
Deviation is <300 Deviates by or nearly from the Vertical axis ><
30 degree coincides with the axis. from vertical Vertical axis |
Deviation is <300 Deviates by or nearly from the Vertical axis >>
30 degree coincides with the axis. from vertical Vertical axis |
|
Characteristics. |
Horizon does not appear |
Horizon appears |
Horizon appear |
|
Coverage |
Small area |
Relatively larger Area |
Larger area |
|
Shape of the area |
Square |
Trapezoidal |
Trapezoidal |
|
Photographed |
Uniform, if the terrain is flat |
Decreases from scale foreground to the
background |
Decreases from scale foreground to the
background |
|
Difference in comparison to map |
Least |
Relatively greatest |
Greatest |
|
Advantage |
Useful in topographical and thematic
mapping |
Reconnaissance Survey |
Illustrative |
|
|
|
|
|
ADVANTAGES
OF AERIAL PHOTOGRAPHY T
he
basic advantages that aerial photographs offer over ground based observation are
:
a.
Improved vantage point: Aerial photography provides a bird’s eye view of large
areas, enabling us to see features of the earth surface in their spatial
context.
b.
Time freezing ability: An aerial photograph is a record of the surface features
at an instance of exposure. It can, therefore, be used as a historical record.
c.
Broadened Sensitivity: The sensitivity of the film used in taking aerial
photographs is relatively more than the sensitivity of the human eyes. Our eyes
perceive only in the visible region of the electromagnetic spectrum, i.e. 0.4
to 0.7 µm whereas the sensitivity of the film ranges from 0.3 to 0.9 µm.
d.
Three Dimensional Perspective: Aerial photographs are normally taken with
uniform exposure interval that enables us in obtaining stereo pair of
photographs. Such a pair of photographs helps us in getting a three-dimensional
view of the surface photographed.
Aerial
photography in India
Aerial
photography in India goes back to 1920 when large-scale aerial photographs of
Agra city were obtained. Subsequently, Air Survey Party of the Survey of India
took up aerial survey of Irrawaddy Delta forests, which was completed during
1923–24. Subsequently, several similar surveys were carried out and advanced
methods of mapping from aerial photographs were used. Today, aerial photography
in India is carried out for the entire country under the overall supervision of
the Directorate of Air Survey (Survey of India) New Delhi. Three flying
agencies, i.e. Indian Air Force, Air Survey Company, Kolkata and National
Remote Sensing Agency, Hyderabad have been officially authorised to take aerial
photographs in India. The procedure for indenting aerial photographs for
educational purposes could be made with APFPS Party No. 73, Directorate of Air
Survey, Survey of India, West Block IV, R. K. Puram, New Delhi
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