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Sight from a flight
Digital Methods
in Aerial Photography
Although the photographs provide an
excellent resolution, in today's digital world, the analogue data is
of little use until it is processed and converted into digital form.
Digital data has many advantages as the data can be processed with
ease, enhanced, analysed and presented in an easily understandable
form through the application of computers. In photogrammetry, aerial
photographs are utilised to form a three-dimensional model which
when further processed can yield mapping data. This process involves
the usage of computers and thus it is beneficial to store the raw
data in the a digital format so that each image point (pixel) can be
distinguished in terms of its position and brightness level. The
most important advantage of data in a digital format is that it can
be directly utilised for analysis in a GIS environment. There are
two different approaches to obtain aerial photographs in digital
format (a) scanning of existing aerial photograph and (b) direct
digital acquisition.
Existing aerial photographs can be
scanned using scanner. In this process the aerial photograph is
scanned by either moving the charge coupled device (CCD) sensor head
over the photo or moving the photo carriage over a static sensor.
Once the image is scanned, the digital form of the image can be
easily imported to a software where the data can be processed for
GIS or remote sensing application. The recent trend in digital
aerial photography is the use of digital cameras. Although still in
its early stages, this technology will prove to be a major
breakthrough in the field of aerial photography. Digital cameras use
the CCD technology in which the patterns of blacks, greys and whites
are electronically recorded as digital values, each representing the
brightness of a specific point within the image. The main advantage
of this technology includes its size, weight, dynamic range, optical
sensitivity, stability, linearity and durability. Currently, the
major disadvantages of this technology are its exorbitant price and
the limited size of the CCD sensor array which can be used only for
small format applications.
Using GPS to enhance Aerial
Photography
Knowledge of exact locations of several features
on the surface of the earth helps in registration of the photograph.
Registration of a photograph means that a coordinate system can be
overlaid on the photograph such that location of each feature on the
photograph can be represented by a set of co-ordinate points. Prior
to registration, the exact coordinates of the features that are seen
on a photograph are determined by surveying techniques. Enough
targets must be placed to ensure that a sufficient number of them
(usually three or more) appear in each photograph. The role of GPS
comes in the exact location of features on the ground. Recent
advances in handheld Global Positioning System (GPS) receivers have
made the task of precisely locating features fast, easy, and
inexpensive.
Earlier the use of GPS receivers for
registration of features on photograph was not advised as it would
provide inaccurate results, correct to only 100 meters, which could
be corrected upto 10 meters using correction algorithms. However,
recently GPS receivers have been prepared which are accurate within
a few centimetres. The data provided by the GPS requires
post-processing on a computer before it can be used, but this
once-difficult procedure has become much simpler now. So to obtain
adequate ground control, we only have to select a number of
naturally occurring features on the ground and acquire GPS positions
for them. Once the photographs are registered then they "fit" within
some standard coordinate system such as latitude/longitude or
Universal Transverse Mercator (UTM).
The above method works
very well when the photographed area is easily accessible. However,
large areas like dense forests, steep hills etc. are not accessible.
If these areas have to be covered by an aerial photo then the
process of photo registration is not very accurate. It is only
possible to approximately register such a photograph in the absence
of traditional ground control. This is possible if the camera's
position and orientation (latitude, longitude, and direction of
travel) are known at the time the photograph is taken. Modern GPS
receivers can help in recording such information so that data having
accuracy within 100 meters is saved and is transferred to a computer
for correction such that the corrected data accurate to within 2 to
5 meters is obtained.
In order to register such photographs,
the flight path along with the exact time the photograph is taken is
recorded. The record of the exact time each photo is taken helps to
find the corresponding inflight position. A system is developed such
that each time a photo is taken, a GPS records position and time
data.
After the flight, the record of the complete flight
path along with the times each photo was taken is exported to a
computer and subsequent processing and differential correction of
the flight path data gives out data having an accuracy of 2-5
meters. Then the two datasets are merged to provide the position
data for each photograph.
GIS Database with Aerial
Photographs
Each point in a digital aerial photograph
represents a geographic location with a characteristic value
associated with it. This information forms the basis of the
geographic data related with the point. A collection of such data
for all the associated points in the aerial photograph results in a
database that can be used for analysis of the photograph in a GIS
environment. Registered photographs can be incorporated into a
geographic information system (GIS) and used to create or update
maps. Aerial Photographs which are not digital, require manual
scanning of the photograph. Once the photograph is scanned, it
results in a digital image. But this image does not represent any
data as it is just a collection of pixels of varying values. To
define the data present in the digital image, either photogrammetric
operations are carried out or the photograph undergoes digitisation
process. Both these operations provide the image a well-defined data
structure wherein each point provides attribute as well as spatial
data associated with the point. Once such a database forms, GIS
operations can be undertaken for analysis work related to the area
represented by the image.
Recent Trends in Aerial
Photography
- Small Format Aerial Photography Except a very few
areas, aerial photography of most of the parts of the earth have
been carried. With the advent of satellite remote sensing, even
these areas have been covered. With rapid urbanisation and
industrialisation the need of the hour is very high resolution
maps i.e. large scale maps. In this respect small format aerial
photography is playing a key role. In this type of photography a
small format (35mm) automatic camera is used. This type of camera
allows for hands-free operation and is light enough to be lifted
by a large kite. Critical camera settings like focus, shutter
speed etc. are controlled by a microchip and light meter within
the camera body. Simple acquisition of airphotos can be attained
by enabling the camera's timer function to take photograph after a
few seconds.
- Low Altitude Remotely Piloted Vehicle (RPV) Aerial
Photography The latest improvement in aerial photography is
the utilisation of a remotely piloted vehicle (RPV) for aerial
photography. Utilizing an unmanned, remotely piloted vehicle,
offers cost effective alternatives to traditional aerial
photography. Since RPV's are not limited by the same restrictions
as full-size aircraft, they can be operated at altitudes of less
than 500 feet, even in residential areas where full-size aircraft
are restricted to a minimum altitude of 1000 feet. As a result of
the freedom associated with this method of aerial photography
unique, low cost aerial photographs from previously impossible
perspectives for a multitude of applications can be obtained.
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