Flying for Aerial Photography: Drifting, Crabbing, Atmospheric Effects, Scattering, Auxiliary Instruments (Especially for GATE-Geospatial 2022)

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Apart from the combined effects of the lens, the film and the development process on the quality of the resulting photographs, there are still other considerations, the major one being the way the aerial photographs were taken. The task of flying for aerial photography is very demanding. It is necessary to satisfy the following conditions:

  • The aeroplane must fly in a straight line, and at a constant speed so that a forward 60 per cent overlap of the photographs is maintained
  • It must turn around at a predetermined distance and to maintain the second flight line parallel to the first and to have a side-lap of 20 per cent.
  • The flying height is predetermined and should be constant throughout.
  • The aerial camera should not be tilted at the time of exposure.
Flying for Air Coverage

To achieve all these conditions, very skilful navigation of the aeroplane is required. Modern airborne navigational equipment, the Doppler and Inertial systems must be used.

Drifting

Owing to the presence of air currents, drifting of the aeroplane will occur, causing it to fly off course, and gaps in aerial coverage will occur (Figure a and b) . This defect can be made up only by turning the aeroplane round to compensate for the drift.

Drifting

Crabbing

To prevent crabbing from occurring, the aerial camera should also be rotated on its mount to allow for this angular change of the aeroplane (Figure c) . It is important to note that the exposure is made as the aeroplane is moving, and that image movement will occur. This may or may not be detected according to the photographic scale since the human eye cannot see any displacements less than 0.05mm. In general, for large-scale photography, image movement will become more prominent.

Crabbing
Correction by Rotating the Camera Mount

Overall, for the most efficient execution of survey flights; Heimes suggests that the calibration of the aircraft in flight under the anticipated operating conditions should be carried out beforehand. ″

Atmospheric Effects

Another major consideration is the atmospheric effects as the atmosphere is an extremely complex medium through which light rays pass. One of these effects is photogrammetric refraction caused by the refraction of light rays away from the vertical when passing through the air of decreasing density before reaching the aerial camera. This results in a small angle between the incoming refracted ray and the straight line connecting the ground point and the camera station. This angle is thus a function of the refractive index of the air in all points along the path of a ray. This can be eliminated only by a scale change, introduced, for example, by changing the focal length of the lens. This error can be avoided if photography is carried out only in relatively stable atmospheric conditions.

Scattering

Another effect is scattering which involves deflection or absorption and re-emission of the light by gas molecules and dust particles in the atmosphere. As these particles and molecules are small with a size less than 0.1 λ (λ is the wavelength of light) , Rayleigh scattering occurs which indicative of clear atmospheric conditions is characterised by a blue sky as a result of the blue light being scattered more. As the particles get larger in size, the scattering is more pronounced. Another type of scattering called Mie scattering is produced by particles with a diameter between 0.1 λ and 25 λ, i.e. approaching or exceeding the wavelength of the light, which includes atmospheric aerosols, dust, haze, smoke, etc. A sky that appears white to red results. Mie scattering generally occurs in the lower atmosphere (below 4,560m) whilst Rayleigh scattering is generally found in the higher atmosphere (about 9,120m) . These different types of scattering can adversely affect the quality of the photographic image obtained with an aerial camera by reducing the contrast. This undesirable effect can be eliminated by placing a minus-blue filter in front of the lens. This minus-blue filter is, in fact, a yellow filter made of high-quality coloured glass which can absorb the short-wavelength violet and blue lights. There are also other kinds of filter such as the ultra-violet filter to take out lights of short wavelength and the red filter which cuts out all lights except the red. For colour aerial photography, colour filters are used to raise or lower the colour temperature of the light coming into the lens to that of the standard colour temperature. As an example, a filter of the blue colour system will raise the temperature, whilst a filter of the amber system will lower the temperature. Thus, in the US Coast and Geodetic Survey, a filter which can cut out all lights below about 0.42µm is used for about 90 per cent of aerial colour photography, and a ‘peach-shaded’ filter which can cut out light below 0.38µm is employed for early-morning and late-afternoon photography.

Auxiliary Instruments

Today there are auxiliary instruments used in conjunction with the aerial camera when photographs are being taken. The purpose is to provide supplementary data on the orientation of the aeroplane at the moment of each exposure. These instruments are the Statoscope, the Airborne Profile Recorder (APR) , the Horizon Camera and the Gyroscope. The Statoscope is a more sensitive device than the altimeter and is used to measure the variations of pressure relative to the first exposure. In this way, the variation in flying height of the aeroplane from Mean Sea Level in its course of flight is known. It is possible to determine the flying height up to an accuracy of ±1 to 2 metres. The Airborne Profile Recorder makes use of radar which is transmitted from the aeroplane to the ground and is then received back by the aeroplane. The time taken by the radar signals in return is measured. Since the velocity of the radar pulse is known, the distance of the aeroplane from the ground surface can be computed. If a continuous recording of these height readings is made during the flight, a profile of the terrain can be drawn. By combining this with the statoscope record which indicates the variations in the flying height of the aeroplane, an accurate determination of the profile of the ground from Mean Sea Level is possible. The Horizon Camera is used in conjunction with the aerial camera to take photographs of the horizon, from which the amount of tilt of the aeroplane can be determined accurately. Finally, the gyroscope is used to stabilise the aerial camera and to ensure its verticality. Also, the deviation of the camera from the vertical can be measured. All these auxiliary instruments can help in the absolute orientation of the pair of aerial photographs when it is used in plotting. But they add considerably to the cost of aerial photography. For a more detailed discussion of these auxiliary instruments, one is referred to the papers by Schermerhorn, Kennedy, Eaten, and Trott.

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