Aerial Photographs: Factor Influence, Types and Characteristics.

Aerial Photograph Types and Characteristics
Aerial Photograph Types and
Characteristics

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 graphene meaning "writing" means "writing by light".
• 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.

• 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.
• Now days aerial photography also become digital where values of
reflected electromagnetic radiation is recorded in digital numbers.
An aerial photograph is any photograph taken from an airborne
vehicle (aircraft, drones, balloons, satellites, and so forth).The
aerial photograph has many uses in military operations; however,
for the purpose of this manual, it will be considered primarily as a
map supplementer map substitute.

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),
shown in figure 1.
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.
Aerial photograph, the image should be of the highest quality. To
guarantee good image quality, recent distortion-free cameras are used.
Some latest versions of cameras have image motion compensation
devices to eliminate or reduce the effects of forward motion. Depending
upon the requirements, different lens/ focal length/film /filter
combinations can be taken in use.

• 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.
Types of Film:
Depending upon the suitability for different
purpose and unique situations variety of
films are available that are used.
Panchromatic and natural color films are the
two most commonly utilized films. These
two films along with infrared and false
colour form the basic media used in aerial
photography. As shown below in fig.2.
Fig: 2. Types of film photographs

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 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 gray- scale image. (Fig.3)
Fig No: 3. Visible Spectrum

Colour Infrared:
Colour Infrared film is commonly
termed as false colour. The
multilayer emulsion is sensitive to
green (0.5–0.6 µm), red (0.6–0.7
µm), and part of the near infrared
(0.7–1.0 µm) portions of the
spectrum. A false colour image
contains red/pink hues in vegetative
areas, with the colour depending
upon the degree to which the
photosynthetic process is active
(Fig:4).
Fig No: 4. Vegetative areas

Flight Direction
It is advisable that aerial photography is
flown in tiles to cover the chosen area in
designated flight line(shown in fig 5).
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. This direction may be any suitable
direction along a natural or man-made
feature and should be clearly specified.
The further transmission process and data
collection is shown in fig 6.
Fig No: 5. Flight Line

Fig No: 6. Flight direction and signal receiving process

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
to mark out some details effectively and are generally fruitful in improving
the interpretational values of a photograph. Based on experience, aerial
photography should be flown when the sun's elevation is 30 degrees above
the horizon or three hours before and after the local noontime.
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:
As mentioned before, 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.
Stereoscopic Coverage:
To examine the Earth's surface in three dimensions, aerial photography is normally flown
with a 60 % forward overlap and a 25 % side lap, to provide full coverage of the area
(Fig.7a and b). This is an essential requirement from the photogrammetric mapping point
of view to obtain data both on planimetry and heights using the stereoscopic principle of
observation in 3-D and measurement techniques with stereo plotting instruments.
Stereoscopic viewing also helps in interpretation, as the model is viewed in three
dimensions.

Fig 7(a) Overlap required to get the full coverage of area

Fig 7(b) Overlap required to get the full coverage of area

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 follows (fig 8a, 8b):
• A. Scale:
✓ Large scale: between 1:5,000 and 1:20,000
✓ Medium scale: between 1:20,000 and 1:50,000
✓ Small scale: smaller than 1:50,000

Fig 8(a) Small scale and large-scale difference
Fig 8(b) Difference in levels of scale

B. Camera Orientation
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. (Fig 9a)
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

Fig 9(a) Camera orientation for various types of photograph

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 (Fig 9b). 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 60
degree from the vertical, sidelap
the vertical photography producing
composites from horizon to
horizon.
Fig. 9(d) Trimetrogon

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 (Fig. 9(d).
Fig. 9(d) Convergent Photography

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.

Aerial Photographs: Factor Influence, Types and Characteristics.

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