19 AERIAL PHOTOGRAPHY

Dr. M P Punia

Objectives

• Student will get to know how aerial photography is done.
• Student will acquire skill how to study data and apply aerial photography data

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.

     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,

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/ArtMID/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.

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)

    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,

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.