13 Aerial Photography
Dr. Puneeta Pandey
CONTENTS
1. Aim of the Module
2. Introduction
3. History of Aerial Photography
4. Commercial Aerial Photography
5. Basic concepts of Aerial Photography
6. Types of Aerial Photography
7. Types of Film
8. Applications of Aerial Photography
9. Advantages of Aerial Photography over ground-based observation
10. Conclusion
11. References
1. Aim of the Module
- To understand the basic concept of Aerial Photography To study the history of Aerial Photography
- To study the applications and advantages of Aerial Photography
2. Introduction
Aerial photography is the process of capturing of photographs of features of the earth’s surface from a certain height or an elevated position using high resolution camera mounted on platforms like helicopters and other light aircrafts which include fixed-wing aircraft, balloons, drones, blimps and dirigibles, pigeons, rockets, kites, vehicle-mounted poles and parachutes. The mounted cameras may be triggered automatically or remotely; hand-held photographs may be taken by a photographer. There are certain things taken into consideration while dealing with fundamental understanding of aerial photography which include scale, film, focal length, overlap, stereoscopic coverage, fiducial marks, roll and frame numbers, and flight lines and index maps.
Figure 1: Antique postcard using kite photo technique (circa 1911) (http://www.wikiwand.com).
Figure 2: Giza pyramid complex, photographed from Eduard Spelterini’s balloon on Nov. 21, 1904 (http://www.commons.wikimedia.org).
Figure 3: A German observation plane, the Rumpler Taube (http://www.en.wikipedia.org).
Figure 4: A drone carrying a camera for aerial photography (http://www.en.wikipedia.org).
Figure 5: A camera is attached to pigeon for capturing of aerial photos
(http://www.northstargallery.com).
3. History of Aerial Photography
The first known aerial photograph was first practiced in 1858 over Paris, France by the French photographer and balloonist G. F. Tournachon, known as “Nadar”. The landscape captured by the photograph was of the French village of Petit-Becetre taken at 80m from ground using a tethered hot-air balloon. However, the photographs did not survive for long and therefore the oldest existing aerial photograph is titled “Boston, as the Eagle and the Wild Goose See It”, depicts Boston from an altitude of 630m, captured by J. W. Black and S. A. King on October 13, 1860 (Figure 6). In addition hot air balloons, pigeons, kites and rockets are also used to carry the cameras in the air to capture the photographs. Aerial photograph from a rocket mounted camera was taken for the first time by the Swedish inventor, Alfred Nobel in 1897. From an aeroplane, the first successful aerial photography was taken in 1909, by Wilbur Wright.
Figure 6: Balloon view of Boston captured by J. W. Black and S. A. King on October 13, 1860 (https:// www.smithsonianmag.com/…/this-picture-of-boston-circa-1860)
The famous english meteorologist E.D. Archibald initiated Kite aerial photography in 1882 with the help of an explosive charge on a timer to capture successful photographs from the air. On April 24, 1909, the first ever use of a motion picture camera was mounted on an aircraft over Rome in the 3:28 silent film short, Wilbur Wright und seine Flugmaschine.
The use of aerial photography rapidly developed during the war, as aircrafts were equipped with cameras to verify and record enemy movements and defense systems. In 1913, Germany adopted the first aerial camera, a Gorz. The French army began the war with numerous squadrons of Bleriot observation aircraft laden with cameras for survey and also developed procedures for acquiring print out in record time into the hands of field commanders.
In 1915, C. J. Moore-Brabazon invented the first purpose-built and practical aerial camera aided by the Thornton-Pickard company, greatly enhancing the efficiency of aerial photography. Moore-Brabazon also introduced the inclusion of stereoscopic techniques into aerial photography, permitting the height of objects to be differentiated on the landscape by comparing photographs taken at different angles.
4. Commercial aerial photography
Francis Wills and Claude Graham White the World War I veterans, founded the first commercial aerial photography company in the UK was Aerofilms Ltd., in 1919. Later on, the Aircraft Manufacturing Company (the De Havilland Aircraft Company), hired an Airco DH.9 along with pilot entrepreneur Alan Cobham. Another successful breakthrough in commercial aerial photography was given by the American Sherman Fairchild who started his own aircraft firm Fairchild Aircraft in 1935, especially for the purpose of aerial survey missions. Later on, Fairchild developed a high altitude camera with nine-lens in one unit that could capture a photo of 600 sq. miles with each exposure from 30,000 feet. Fairchild also designed and develop airplanes with high-wings and enclosed cabins as a more stable and protected platform for photography that lead to his strong commitment in the business of aerial photography. This aerial map (overlapping photographs) of Manhattan Island became a commercial success of Fairchild which was later on used by several New York City agencies and businesses.
5. Basic Concepts of Aerial Photography
5.1 Camera: In early days the cameras used for photography were often no more than a light-tight box with a pinhole at one end and the exposed light sensitive material fitted against the opposite end. Later on the pinhole camera was replaced by simple lens camera. The Cameras and their applicability for aerial photography are the simplest and oldest of sensors exploited for remote sensing of the earth’s surface. The camera is a framing system which takes a near-instantaneous “snapshot” of an area of the surface. The camera systems are passive optical sensors that use a lens to form an image at the focal plane, the plane at which an image is precisely defined.
5.2 Film: Black and white film are most common films used in aerial photos, however infrared, colour, and false-colour infrared film are also used for certain special projects. The photographic films are sensitive to light from 0.3 μm to 0.9 μm in wavelength which include the ultraviolet (UV), visible, and near-infrared (NIR). Panchromatic films are sensitive to the UV and the visible portions of the spectrum. The panchromatic film is the most common type of film used in aerial photography and produces black and white images.
5.3 Focal length: It is the distance from the middle of the camera lens to the focal plane (i.e. the film) and is calculated after calibration of the camera. With the increase in focal length, image distortion decreases. The relationship between the focal length (f), object distance (o) and image distance (i) is given below
1/f=1/o+1/i ——-(Eq. 1)
5.4 Scale: It is the ratio of 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).
Another method used to verify and evaluate the scale of a photo is to determine the ratio between the camera’s focal length and the plane’s altitude above the ground being filmed.
Unit Equivalent, Representative Fraction and Ratio are some of the ways that can be used to express the scale. For example
A photographic scale of 1 mm on the image 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
Scale can be large or small
Large Scale- Larger-scale based photos (e.g. 1: 25,000) cover small regions in greater detail. A large scale photograph means that ground features are expressed in more detailed size at large. The area of ground coverage on the photograph is less than at smaller scales.
Small Scale- the photos that are captured on small scale (e.g. 1: 50,000) cover ground features of large regions/areas in less detail. The area of ground coverage captured on the photo is greater than at larger scales. An example for 1: 50,000 scale is given below-
A variety of photographic scales, such as 1: 3,000 (large scale) of selected areas, and 1: 50,000 (small scale) are available at The National Air Photo Library.
The scale at a point on a truly vertical photograph is given by
S = f/ H- h ——-(Eq. 2)
Where,
S = photographic scale at a point
f = camera focal length
H= flying height above datum
h = elevation above datum of the point
5.5 Exposure: At any point in the image, the exposure is determined by the irradiance at that point multiplied by the exposure time, expressed as
E= sd2t/4f2 ———— (Eq. 3)
Where
E= exposure, J mm-2
s= scene brightness, J mm-2sec-1
d= diameter of lens opening, mm
t= exposure time, sec
f= lens focal length, mm
From the Eq. 2, it can be seen that the exposure varies with respect to shutter speed t and/or the diameter of the lens opening d. The various combinations of d and t will produce equivalent exposures for a given camera.
5.6 Fiducial marks: these are small registration marks (figure 7) that are exposed on the edges of a photograph. The measurement of distances between fiducial marks are precisely is done when a camera is calibrated, and such information is being used during compilation of topographic map by cartographers.
Figure 7: The image showing fiducial marks (http://www.hosting.soonet.ca)
5.7 Overlap: it is the amount by which one photograph comprises the area covered by another photograph, and is expressed as a percentage (%). The photo survey is designed to get 30% lateral overlap (between photos on adjacent flight lines) and 60% forward overlap (between photos along the same flight line)
5.8 Stereoscopic Coverage: It is the representation of the three-dimensional (3D) view which results when two overlapping photos (known as stereo-pair), are viewed with the help of a stereoscope. Each photograph of the stereo-pair offers a slightly different view of the same area, which the brain merges and interprets as a 3-D view.
5.9 Roll and Photo Numbers: It is a type of identification number which allows searching of the photo in NAPL’s archive, besides metadata information, the plane’s altitude, the focal length of the camera, and the weather conditions. Each photograph is assigned a unique index number according to the photo’s roll and frame.
5.10 Flight Lines and Index Maps: the contractor of aerial survey 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, and straight lines are drawn linking the circles to show photos on the same flight line. This graphical representation is known as air photo index map, which allows the photos to relate with their corresponding geographical location. On NTS map sheets, small-scale photographs are indexed on 1: 250,000 scale, and larger-scale photographs are indexed on 1: 50,000 scale NTS maps.
5.11 Color-Mixing Process: The color mixing is very important for an observer to see the real true colored picture of the landscape. The trichromatic theory of color vision explains that when blue, green and red elements are stimulated by different amount of light, we perceive color just like the phenomenon behind the rod cells and cone cells of the human eye. The red, green and blue are termed as additive primaries whereas yellow, magneta and cyan are known as complementary colors of blue, green and red light. There is a concept called hue cancellation method which states that when the certain colors are mixed together, the resulting colors are not what would be intuitively expected. For example, when red and green are mixed together, the resultant color is yellow not reddish green.
There are two types of color mixing: Additive and Subtractive.
5.11.1 ADDITIVE Mixing: The superimposition of two beams of light corresponds to additive mixing.
The additive mixing of colors is unintuitive as it does not related to the mixing of physical substances which would correspond to subtractive mixing. Regardless of being unintuitive, it is theoretically simpler than subtractive mixing.
In the absence of color, the result is black. If all three primary colors (red, blue and green) are showing, the result is white. When red and green colors are combined, the result is yellow and with that of red and blue, the result is magenta. For blue and green combination, the result is cyan.
Additive mixing is used in television and computer monitors to produce a wide range of colors using only three primary colors. A pixel is a combination of the three primary colors. Projection televisions usually have three projectors, one for each primary color.
5.11.2 SUBTRACTIVE Mixing: The mixing of physical substances corresponds to subtractive color mixing; therefore it corresponds to our perception about mixing colors. To explain the mechanism, let us mix red paint with yellow paint. The red paint is red because when the ambient light strikes it, the composition of the material is such that it absorbs all other colors in the visible spectrum except for red. The red light, not being absorbed, reflects off the paint. The similar mechanism describes the color of all material objects — note that light is not a material object — and so applies to the yellow paint as well.
In subtractive mixing of color, the absence of color is white and the presence of all three primary colors is black. By convention, the three primary colors in subtractive mixing are yellow, magenta and cyan. The secondary colors are the same as the primary colors from additive mixing, and vice versa. Subtractive mixing is used to create a range of colors when printing on paper by combining a small number of ink colors, and also when painting. Upon mixing yellow and blue, Green color is produced and with that of red and yellow, orange is produced.
The color film photography rely on the principle of subtractive color mixture using superimposed yellow, magenta and cyan dyes (Subtractive primaries) to control the proportionate amount of green, blue, and red light that reaches the eye. Therefore, the subtractive mixture of yellow, magenta and cyan dyes on a photograph is used to control the additive mixture of blue, green and red light reaching the observer. To achieve this, color film is manufactured with three emulsion layers that are sensitive to blue, green and red light but contain yellow, magenta and cyan dye after processing.
5.12 Filters: The use of filters or color screens has been very frequent in ordinary landscape photography. With the help of filters a selective wavelength of light reflected from the scene is allowed to reach the image plane (film). They are placed in front of the lens in the optical path of a camera. Filters are often used for the purpose of correct brightness rendering, calculated for a given color sensitive plate so that the resultant feedback to the light of the spectrum copies the sensitiveness of the eye, which is greatest in the yellow-green. Such filters for use with the common orthochromatic plates are of a general yellow color. The filters used in aerial cameras consist of organic dyes suspended in glass or in a dried gelatin film. The most commonly used filters are absorption filters which allows differentiation of objects with nearly similar spectral response patterns in major portions of the photographic spectrum.
ND Filter or neutral density filter is possibly the most useful filter in the aerial photography. This filter is a sunglass lens for the camera, decreasing light levels without altering color or having any other side effects.
The Camera filters are of two types-
Screw-on filters come in different sizes, related to different lenses of the camera. A lens with a 52 mm mount can only work with 55 mm filters.
6. Types of Aerial Photography
6.1 Oblique Aerial Photographs
Oblique photographs are those photographs that are taken at certain angle with respect to the ground surface (ie not directly overhead as in ‘Vertical’). The photographs taken at low angle are known as low oblique photographs whereas those taken at high or steep angle are known as high or steep oblique photographs that include the horizon. Both low and high oblique can be taken as ‘wide shots’ or ‘close-ups’.
Figure 8: Abalone point, Irvine Cove, Laguna Beach an example of low-altitude aerial photography (http://www.sky-photo.net).
Figure 9: Oblique Aerial Photo (http://www.wikiwand.com).
6.2 Vertical Aerial Photographs
Vertical photographs are taken straight down with the camera tipping directly down at 90˚ (or with <3˚ tilt) to its centre point and cover relatively a smaller area. In the viewfinder, all the four corners of the ground framed must be more or less at equidistant from the film/sensor plane. This helps in producing a map-like perspective and permits the consequential photograph to be ‘scaled’ and dimensions taken from it. True Verticals are proven to be very useful in mapping and should be considered as a series of overlapping images or mosaics. The vertical photographs are chiefly used in image interpretation and photogrammetry.
Figure 10: Vertical Orientation Aerial Photo (http://www.quora.com).
6.3 Ortho-Rectified Vertical
In this type of aerial photograph, all the geographical and topographical distortions are removed from a true vertical image and have been corrected optically. The Ortho-rectified photo is a simulation of a photograph captured from an infinite distance, looking directly down to nadir. While capturing the photograph of a landscape, distortions occur as a result of defective optical lenses and digital sensors, the tilt of the camera/sensor (relative to the ground), and other aspects. The images cannot be used for mapping and scale its measurement which are not ortho-rectified. Ortho-rectified photos are commonly used in geographic information systems (GIS) cartography to create maps. These images can be broadly deployed once after their alignment or registration with known coordinates. These orthophotos are extensively used in online mapping systems such as Google Maps. The ‘Google Earth’ overlays satellite imagery or orthophotos over a digital elevation model (DEM) to simulate 3D landscapes.
6.4 Trimetrogon
This type of photography is a combination of three photographs captured at the same time, one vertical and two high obliques, in a direction at 90˚ to the line of flight. The obliques are captured at an angle of 60˚ from the vertical, sidelap the vertical photography, and resulting composites from horizon to horizon.
6.5 Mapping
It is a type of aerial photography in which two or more vertical photos are joined or mosaic to produce a larger area.
6.5.1 Combinations
Depending on their purpose, the combination can be done in several ways, a few are listed below.
6.5.1.1 Panoramas are often produced by stitching several photographs overlapping and adjacent images captured (coupled with computer software) with one hand held camera. A loose term, usually referring to an exceptionally ‘wide’ shot, which includes a large area of the horizontal view.
6.5.1.2 In pictometry one vertical and four low oblique photos are produced by five rigidly mounted cameras and such images can be used together.
6.6 Aerial videography
Aerial video is becoming more popular with innovative approaches and advancements in video technology. With the help of GPS, video may be incorporated with meta-data and afterward synced with a video mapping program. The aerial videos are emerging ‘Spatial Multimedia’ which can be used for understanding of scenes and tracking of object. Such spatial multimedia is the appropriate and timely bound combination of digital media including still photography, stereo, motion video, panoramic imagery sets, audio, immersive media constructs, and other data with date-time and location information from the GPS. The combination of digital video, global positioning systems (GPS) and automated image processing will help in improving the accuracy and cost-effectiveness of data collection and reduction.
7. Types of Film
a. Infrared: This is a black-and-white film that is sensitive to infrared (IR) waves that can be used to detect artificial camouflage materials and to capture photographs at night if there is infrared radiation. b. Panchromatic: This type of film is used in the average hand-held small camera. This film is commonly used film in aerial photography which records the amount of reflected light from objects in tones of gray running from white to black.
c. Color: This film is the same as that used in the average handheld camera and is timely dependent. It is limited in its use as it requires time to process and its need for clear, sunny weather.
d. Camouflage Detection: This type of special film helps in recording natural vegetation in a reddish color. When artificial camouflage materials are photographed, they appear purple or bluish.
8. Applications of Aerial Photography
Aerial photography can be very useful in infinite number of ways. Some important applications of aerial photography include defense and security system, environmental studies, inspection of power lines and grid system, cartography, land-use planning, archaeology, surveillance, commercial advertising and business activities, movie production, etc. Some of these are described below as-
8.1 Environmental Protection- Nature and Wildlife
Aerial photography provides a great deal of benefits related to the environment is the documentation of a activities/situation and monitoring its change over time with the help of aerial photos in order to raise awareness through campaigning. Through such efforts public opinion can influence/guide the willingness of government to integrate environmental data into existing plans, policies and regulations in order to guard the environment. Destruction of wildlife habitat frequently happening due to unorganized, uncoordinated and defectively planned land development and has been a major cause of threat to the environment and the wildlife.
Figure11: Aerial photograph showing destruction of forest habitat (http://www.sierraclub.org)
8.2 Geography & Cartography
Aerial photography has been considered as an essential part of cartography (mapmaking process) in the modern era. The aerial photos offer a straightforward/easy depiction of the physical and cultural landscape of a region at a given time. These aerial photos after interpretation provide a symbolic and pictorial basis frequently critical for the studies by ecologists, historians, geographers, geologists, archaeologists, and other professionals.
8.3 Engineering & Urban Planning
The concept of smart cities, high profile development of urban sectors is the need of modern age. Therefore, the aerial photos proved to be very important and have gained esteem among developers, planners and engineers for mapping at small scale for the purpose of urban and land development. Integration of information obtained from aerial photos with Geographic Information System (GIS) mapping is very useful for the analysis, strategic planning and evaluation in planning and engineering of urban sectors.
Figure 12: A photograph of urban sector showing the pattern of settlements(http://www.pinterest.com)
8.4 Land cover and land use classification
The aerial photographs are being frequently used for land use land cover (LULC) information for regional scale planning and development. These aerial photos after registration and digitization can be used to classify the ground features and evaluates the change in area (change detection analysis).
8.5 Agriculture & Precision Agriculture
Aerial photographs facilitate more precise interventions and techniques in the field of agriculture. Being a farming management notion, precision agriculture is based on observing and reacting to intra-field variations. The precise agriculture depends on modern technologies like Aerial Imagery and information technology Global Positioning System (GPS).
8.6 Journalism
In past few decades, aerial photos have been used more frequently to carry out the practice of investigation and reporting of incidents/events. They provide precise information about a geographical idea of a particular location and explore the information given to the public.
Figure 13: An aerial view of property damage due to fire incident (http://www.pressgazette.co.uk).
8.7 Surveillance and Monitoring System
For the purpose of monitoring of earth on a large scale, the aerial photos are used to integrate and complement satellite images in order to validate the interpretation of data. And in situations of rapid change aerial photos on a small scale can directly provide with constant information to the existing Surveillance System. They can also feed surveillance functions of vast properties, frontiers and maritime domains. With the help of systems such as APDER, the aerial photos could provide key information in surveillance in special situations such as regional violence or civil unrest where observatory mechanism and other related resources are limited.
8.8 Tourism
For some countries the tourism is a back bone of their economy and therefore aerial photography can play a significant role in tourism promotion. The tourism composed of eco-tourism and leisure industries presently uses aerial photography for fresh presentation and proposal strategies. The aerial photos are used for promotion of tourism and their main resources/assets, which consists of a unique landscape and attractive scenery, remarkable cultural heritage, sports activities/facilities and other services. All the organizations and agencies who deal with tourism use aerial photography as one of the main tools to nourish their business. Hotels, resorts, tour guides and travel agents use aerial photos for their advertisement and tourism activities to gain information related to business spots or desired location.
8.9 Climate Change
Climate change is one of the most threatening factors for the survival of human, flora and fauna, glaciers, forests, and other natural resources. In order to combat with drawbacks of climate change, it is very important to monitor the climate changes on regular intervals. The aerial photographs play such a role in monitoring the changes that could occur in natural landscape such as melting of glaciers, shrinking of forests and water bodies, drought, expansion of settlements and other commercial set ups. Therefore, it is very essential for the researchers to collect information and maintain records in changes over different seasons and years to follow the effects of climate change and associated risks to ecosystems.
Figure 14: Aerial photographs depicting melting of Grinnell glacier in Montana, USA
(http://www.demilked.com).
8.10 In other earth sciences
The aerial photographs can also be very useful to investigate the mechanism of natural changes, such as variations in land resources, precipitation and geology over time that may lead to disasters such as droughts and landslides.
9. Advantages of Aerial Photography over ground-based observation
-Aerial photography provides an advanced vantage point.
-It offers a permanent recording facility.
-It has broader spectral sensitivity than the human eye.
-It provides a better spatial resolution and geometric fidelity than several ground-based sensing methods.
10. Conclusions
At the end of the module, the student would have understood the basic concept of aerial photography and gained an insight into the history and types of films used in aerial photography. Besides, the applications of aerial photography and its advantages over ground-based methods have been discussed in this module.
11. References
- http://professionalaerialphotographers.com/content.aspx?page_id=22&club_id…id.
- http://www.colorado.edu/geography/gcraft/notes/remote/remote_f.html
- http://www.commons.wikimedia.org
- http://www.demilked.com
- http://www.en.wikipedia.org
- http://www.environmentalscience.org/principles-applications-aerial-photography
- http://www.hosting.soonet.ca
- http://www.northstargallery.com
- http://www.nrcan.gc.ca › … › National Air Photo Library › About Aerial Photography
- http://www.pinterest.com
- http://www.pressgazette.co.uk
- http://www.quora.com
- http://www.sierraclub.org
- http://www.sky-photo.net
- http://www.wikiwand.com
- https://www.smithsonianmag.com/…/this-picture-of-boston-circa-1860