component-i(a) - personal detailsepgp.inflibnet.ac.in/.../1517207018aerialphotography(2.pdf ·...
TRANSCRIPT
-
Component-I(A) - Personal Details
Component-I (B) - Description of Module
Role Name Affiliation
Principal Investigator Prof.MasoodAhsanSiddiqui Department of Geography,
JamiaMilliaIslamia, New Delhi
Paper Coordinator, if any Dr. M P Punia Head, Department of Remote
Sensing, Birla Institute of
Scientific Research, Jaipur
Content Writer/Author (CW) Dr. M P Punia BISR, Jaipur
Content Reviewer (CR) Swati Katiyar Senior Research Fellow, Birla
Institute of Scientific Research,
Jaipur
Language Editor (LE)
Items Description of Module
Subject Name Geography
Paper Name Remote Sensing, GIS, GPS
Module Name/Title AERIAL PHOTOGRAPHY
Module Id RS/GIS-19
Pre-requisites
Objectives Student will get to know how aerial photography is done.
Student will acquire skill how to study data and apply aerial
photography data
1.
Keywords
-
AERIAL PHOTOGRAPHY
Photography means the art, hobby, or profession of taking photograph,
a) measuring the photographs,
b) reducing the measurement to some usable form such as map,
Outline
Aerial photography is a branch of remote sensing technology.
Aerial photographs presents "bird's-eye" view of the Earth in central
projection and are proved to be commanding tool for studying the Earth's
feature.
They are useful for cartographers and planners to gather detailed
measurements from aerial photos in the preparation of maps.
Skilled interpreters make use of aerial photos to determine land-use and
other phenomena on the Earth.
Aerial photos have high degree of radial distortion. The topography is
distorted, and until corrections are made for the distortion, measurements
made from a photograph are not accurate.
Aerial photographs are first-rate data source for many types of projects,
particularly those require spatial data from the same location at periodic
intervals over a length of time.
This module discusses concepts of aerial photography, their properties,
types of photography.
-
c) developing and printing the film or processing the digitized array image. „
Photography is construction of permanent images by means of imposing light
on sensitized surfaces (film or array inside a camera), which ultimately giving
rise to a new form of visual art.
1.1 Introduction
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 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. Nowdays aerial
photography also become digital where values of reflected electromagnetic
radiation is recorded in digital numbers.
Characteristics of Aerial Photographs:
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.
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. „
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. „
Three Dimensional perspective: Aerial photographs provide a
stereoscopic view of the Earth’s surface where one can make horizontal
and vertical measurements.
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). „
Availability: Airborne photographs can be taken on user specific time
and make permanent record at a range of scales for any area.
1.2 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.
Fig 1. Scale of photograph
Source- http://www.globalsecurity.org/military/library/policy/army/fm/3-25-
26/ch8.htm
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.
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
-
Source- http://www.globalsecurity.org/military/library/policy/army/fm/3-25-
26/ch8.htm
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
gray-scale image. (Fig.3)
-
Fig 3. Visible Spectrum
Source:http://www.harrisgeospatial.com/Learn/WhitepapersDetail/TabId/802/A
rtMID/2627/ArticleID/13742/Vegetation-Analysis-Using-Vegetation-Indices-
in-ENVI.aspx
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 4. Vegetative areas
Source:https://www.researchgate.net/publication/221915805_Introductio
n_to_Remote_Sensing_of_Biomass
-
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 5. Flight Line
Source:http://www.sonoma.edu/users/f/freidel/techniques/exer/rem_sens/RemS
en_a.html
-
Fig 6. Flight direction and signal receiving process
Source-http://www.seos-project.eu/modules/laser-rs/laser-rs-c07-p01.html
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 fruitfull 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
Source- http://hosting.soonet.ca/eliris/remotesensing/bl130lec4.html.
http://hosting.soonet.ca/eliris/remotesensing/bl130lec4.html
-
Fig 7(b) Overlap required to get the full coverage of area
Source- http://www.nrcan.gc.ca/earth-sciences/geomatics/satellite-imagery-air-
photos/air-photos/about-aerial-photography/9687
1.3 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
Source- http://www.physicalgeography.net/fundamentals/2a.html
Fig 8(b) Difference in levels of scale
Source- http://giscommons.org/chapter-2-input
B. Camera Orientation
Vertical: When the vertical photograph is taken it evident that optical
axis of camera should be vertical or nearly vertical. (Tilt is within 3°).
-
Fig 9(a) Camera orientation for various types of photograph
Source- https://www.e-education.psu.edu/geog480/node/444
Oblique:
a. Low oblique: Photograph is taken with strongly tilted optical axis but
not to the extent that horizon appear in the photograph (horizon does not
appear but tilt is more than 3°). (Fig 9b)
Fig. 9(b)Low oblique
Source- Source-
ttp://www.engr.usask.ca/classes/GEOE/218/notes/airphoto_reading/
apg.htm
-
b. High oblique: Photograph is taken with deliberately tilted optical axis
enough from the vertical to show the Earth's horizon (horizon appears in
the photograph).
Fig. 9(c) High oblique
Source:http://www.engr.usask.ca/classes/GEOE/218/notes/airphoto_readi
ng/ apg.htm
Horizontal or terrestrial: Photograph is taken with camera axis horizontal.
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.
1.4 Obtaining Aerial Photography
As per the existing policy of the Government of India, all types of aerial
photographs are classified documents (secret or restricted), depending upon the
location and its strategic importance. The Surveyor General of India coordinates
all activities relating to the execution of aerial photographic tasks for all civilian
needs. The coordinating authority performs the following functions :
Design and issue of the specifications for photographic tasks.
Layout and priorities, clearance from various agencies and distribution of
tasks among the three flying agencies.
Flight planning and evaluation for suitability of the executed tasks.
Distribution of photographs to the indenter.
Accounting for the above.
-
1.5 Project planning for Aerial Photography
Project planning for doing aerial photography operation involve 3 basic
phases
A. Flight Planning
B. Planning for ground control
C. Estimation of cost
A. Flight planning: Basic elements of flight planning are finalisation of flying
height, ground distance between successive exposures, ground spacing between
flight lines. Several factors must be taken into consideration in planning the
flight map, important are the followings:
I. Purpose of photography
II. Photographic scale
III. Allowable scale variations
IV. Relief Displacement
V. Photographic tilt
VI. Crab and Drift
VII. Flying height
VIII. Orientation of topography
Many of the factors are closely interrelated like scale, focal length, flying
height.
I. Purpose of photography
The aerial photography is generally conducted for specific purpose. All the
specifications are set to fulfil the purpose. The majority of photogrammetric
activities involve the compilation of topographic maps in a stereoscopic plotting
-
instrument. For such purpose wide-angle photography is required to get
appropriate base-height ratio that enhances vertical accuracy. If the topography
is very flat, a super-wide angle camera is used. Standard 60% forward overlap
and 15 to 30% sidelap is satisfactory for topographic mapping as it provides
complete stereoscopic coverage of the area without any gap. Orientation of the
flight line is generally kept along the length of the area, which also satisfy
economic criteria.
A Photography for aerial mosaics should ideally be from highest feasible
altitude and should contain overlap as per the topography. If the ground is fairly
flat, then 60% overlap and 15 to 30% sidelap is satisfactory. If the terrain is
rugged, both overlap and sidelap should be increased. The main objective here
is to hold the effects of relief displacement to a minimum. (Fig. 10)
Fig. 10 How photography is done and its purpose
Source-http://b-29s-over-korea.com/aerial%20photography/images/Flight
_plan.jpg
-
Photography taken for the production of ortho-photos, favourable orientation of
the flights lines are in a direction normal to the general trend of the topography.
If ortho-photos are to be pieced together to form an ortho-mosaic, the
photographs should be taken with a constant sun angle, and at the same season
of the year. Otherwise, the tone and texture variation between the individual
ortho-photos will be quite pronounced and objectionable.
If your purpose is photogrammetric triangulation, the flight plan is governed by
the topographic mapping considerations. Flight lines are planned to give 60%
overlap in both directions, so that any internal pass point or tie point lying in the
shaded area will appear on nine photographs, resulting in nine pair of
collinearity equations for the point. The alternate flight strips can then be used
for the topographic mapping. It does strengthen block triangulation, 60%
overlap in both directions is used also for the determination of ground points for
cadastral surveys and for establishing fill-in-ground control. Each internal point
in the block will appear on at least four photographs, thus strengthening its
ground position in the analytical intersection solution.
Photographic Scale
Photographic scale is determined by use of photographs. User should be able to
recognise his features of interest and able to resolve the smallest objects that
need to be identified. (Fig. 11)
According to scale other parameters like flying height, camera, orientation of
flight lines etc. are finalised.
-
Fig. 11 Scale of Photograph
Source-Self
Allowable Scale Variation
Scale variation in a photograph or between photographs is caused by variation
in the ground elevation, by a variation in flying height, or both. If the terrain is
undulating with large height differences, scale variation will be more. Higher
the elevation result into larger the scale and lower the elevation result into
smaller the scale.
Example:- Two photographs taken over terrain having an average elevation of
120 m above the datum and a range in elevation from 50 to 180m. In each case,
the average scale is to be 1:2500. With a 152-mm focal length (f), the required
flying height computed is 500m (H) above the datum. At an elevation of 50m,
the scale is
Scale on average elevation of 120m = f/H = 152 mm/380 (500-120)m = 1:2500
Scale on elevation of 50m = f/H = 152 mm/380 (450)m = 1: 2960.53
Scale on elevation of 180m = f/H = 152 mm/380 (320)m = 1: 2105
-
Scale variation also affects photographic coverage because rising or falling of
terrain with respect to the flying height alter the scale considerably, and is an
important factor to be considered when relatively low-altitude photography is
taken for mapping purpose. If terrain rises, the overlap between successive
photographs decreases if the photographs are taken with a constant time interval
between exposures. The width of terrain covered by the photographs becomes
narrower as the terrain elevation increases. In such situation side lap also
decreases and if it is not planned well, gaps between the flight strips may occur
in the high areas.
Relief Displacement
Mathematically it is the magnitude of displacement in image between the top
and bottom of elevated object i.e. the apparent leaning of elevated objects away
from the principal point (Fig.12) .Practically every point on the vertical aerial
photograph is displaced from its datum photograph position because of its
elevation above or below the datum. There is no relief displacement at Nadir.
Relief displacement on any pair of adjacent photograph always occur in
opposite directions because the relief displacement on each photograph radiates
outward from a point near the centre of photograph. relief displacement will
decrease as the flying height will increase. It is also evident that to maintain a
certain scale as the flying height is increased the focal length must be increased.
Relief displacement is very important in the way that it enables us to calculate
the height of objects. At the same time it relief displacement affects the
construction of mosaics. Since mosaicking consists of piecing adjacent
photographs together to form one composite picture, large relief displacement
on successive photographs will make it difficult or even impossible to form a
continuous uninterrupted picture. Relief displacements on any pair of adjacent
photographs always occur in opposing directions, because the relief
-
displacement on each photograph radiates outward from the principal point of
the photograph.
As seen in the diagram relied displacement will decrease as the flying height is
increased. It is also evident that to maintain a certain scale as the flying height is
increased, the focal length must be increased. These principles are taken into
account when the flight plan is designed.
Fig. 12 Relief Displacement
Source-Self
-
In general, relief displacement has no adverse effect upon map compilation in a
stereoscopic plotting instrument. In fact, as the relief displacement increase, the
more positively can elevations be measured in the instrument.
TILT OF THE PHOTOGRAPHS
The tilt of a photograph may be resolved into two components. One is the
amount in the direction of flight, and the other is the amount in the direction
normal to the flight line. The first is called y-tilt, or angle Ø. The second is
called x-tilt , or angle φ. When a photograph has undergone a y-tilt, the overlap
on one side will be greater than the desired amount of overlap, while the overlap
on the opposite side will be smaller than the desired amount. Two successively
exposed photographs with opposite y-till will cause the increase or decrease in
overlap to accumulate, whereas y-lilt in the same direction will, to a great
extent, cancel the sidelap to increase in overlap. An x-tilt of a photograph will
cause the sidelap to crease on one side of the flight line and to decrease on the
opposite side.
The effect of y-tilt on overlap can be taken into account by using the viewfinder
to control the overlap. If a fixed interval between exposures is held, as with the
use of an intervalometer, the effect of y-tilt on overlap must be allowed for by
decreasing the theoretically desired overlap.
The effect of x-tilt on sidelap must be allowed for by decreasing the computed
spacing between flight lines slightly to produce a slight increase in the desired
sidelap. This adjustment helps to ensure proper coverage, and at the same time
allows for certain abnormal relief displacements. As shown in Fig. 9(a).
-
CRAB AND DRIFT
Crab is the term given to designate the angle formed between the flight line and
the edges of the photograph in the direction of flight. It is caused by not having
the focal plane of the camera squared with the direction of flight at the instant of
exposure. The effect of crabbing is shown in (Fig. 13). Under normal flying
conditions, the camera can be corrected to allow for crabbing by a rotation of
the camera about the vertical axis of the camera mount. The consequence of
crab is to condense the effective breadth of exposure of the photography.
Fortunately the sidelap allowance will in most instances prevent gapping
between flight strips caused by crab.
Flight line
Fig 13: Effect of Crab
Source-Self
Drift is caused by the failure of the aircraft to stay on the predetermined flight
line. If the aircraft drifts to one side or the other of the flight line, loss of some
sidelap would be observed on the side opposite to the direction of drift. Drifting
from the predetermined flight line is the most common caused serious gapping
between adjacent flight lines. Gapping may be due to a poor flight-line map,
even though the pilot actually keeps the aircraft on the flight line as drawn on
the map. (Fig. 14)
-
Flight Line
Fig. 14 Effect of Drift
Source-Self
SELECTION OF FLYING HEIGHT
Once overlap requirements are finalised, other parameters of flight planning
decided out of them flying height is important. Several interrelated factors that
affect the selection of flying height, such as desired scale, focal length of
camera, relief displacement, permissible tilt, etc. need to be finalised. Others
factors to be considered are the precision of the photogrammetric equipment
used to compile topographic maps from the photography, physical limitations of
the stereoscopic plotting instrument to be used in the map compilation, and
factors peculiar to same forms of large-scale mapping.
Various types of photogrammetric equipment used in the process of map
compilation contain a certain inherent precision, but it is different for each type.
In general, the greater the precision in the system, the greater may be the flying
height. This relationship is advantageous, because when the flying height is
doubled it increases the ground coverage per photograph by four times, and by a
long way reduces the needed amount of ground control. Since vertical accuracy
in a photographic map is the limiting factor in the photogrammetric process, the
flying height is quite often related to the contour interval of the finished map.
The relationship is expressed as a precision factor, and is designated as the C-
factor of the photogrammetric equipment (including the operator). Thus,
-
Flying height = (contour interval)
(C-factor)
The C- factor is understood to be that value, used to calculate the flying height,
which will build photography satisfactory to obtain the desired vertical accuracy
in the map.