high-oblique-aerial-photograph

Aerial photographs are those which are taken from airborne platform. The camera for aerial photography generally locates onboard in an aircraft and captures multiple overlapping images while passing through its flying path. In remote sensing, this is called the Aerial Photogrammetry. The camera is focused towards the Earth surface, and captures the images of surface features, topography, atmosphere, and others.

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Photogrammetry is highly related with this aerial photography. One of the major kind of photogrammetry is Aerial Photogrammetry, in which the overlapping imagery of earth surface, taken from an airborne platform, are processed through traditional stereo-plotter or modern computerized systems, to study the Earth.

It is important to mention that aerial photograph was first captured in 1858 by French photographer and balloonist Gaspard-FĂ©lix Tournachon, famous known as Nadar.

Basic Concepts of Aerial Photography

Focal Length : Focal length is the distance between the center of the camera lens and the center of the photo-plate or focal plane, as it is generally called. Distortion of image decreases along with the increase of the focal length.

Scale : A scale of photograph denotes the ratio of a certain distance between two points on the photograph, as compared to the proper distance between that two points on the ground or earth surface. For instance, if a 2 kilometres long road covers 2 centimetres in an aerial photograph, then the scale of the photograph will be calculated as following.

Distance on the photograph/Actual distance on the ground
= 2 cm./2 km.
= 2 cm./2,00,000 cm.
= 1/1,00,000
So, the scale is 1:100000

This scale can be expressed in three ways, e.g., statement scale (1 cm.=1 km.), representative fraction (1/100000), and ratio scale (1:100000).

Film : Film is a sensitive substrate that is used in the camera to capture photographs. Various films are  used in aerial photographs, such as panchromatic, infrared, etc.

Overlapping : Overlapping refers the percentage of extend to which one photograph covers a certain amount of area of another photograph. This includes 60% forward overlapping which is along the flight line, and 40% lateral overlapping that is area between adjoining flight lines.

Aerial_Photograph_overlapping
Diagrammatic presentation of the overlapping of aerial photographs

Platforms of Aerial Photography

As the camera for aerial photography is located onboard an aircraft, hence the aircraft acts as a platform for aerial photography. Similarly there are many other platforms for aerial photography, such as UAV or Unmanned Aerial Vehicles or commonly known as Drone(s), blimps, balloons, and many others.

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Classification of Aerial Photograph

Aerial photograph can be classified on various basis, such as the position of camera axis, R.F. scale, coverage angle, combination of film and filter, lens system, etc.

1. According to camera axis position

On the basis of the camera axis position, it is classified into three groups. A camera axis denotes the imaginary line joining the centre of the photo-plane and the focus point on the ground through the centre of the camera lens. These three types are as follows.

Types_of_Aerial_Photograph
Diagrammatic presentation of aerial photographs base on the position of camera axis

  • Vertical Aerial Photograph : When the camera axis generates a perfect 90° angle with the ground or the earth surface, then the obtained photograph is called the vertical photograph. But it is quite hard to obtain a perfect vertical aerial photograph because of the curvature of the earth surface. Thenceforth a deviation of ∓3° is considered.
  • Low-Oblique Aerial Photograph : Similarly, when the camera axis are inclined between 15° and 30°, then the aerial photograph so obtained is called the low-oblique aerial photograph.
  • High-Oblique Aerial Photograph : When the camera axis are inclined to 60° or more, then the aerial photograph so obtained is called the high-oblique aerial photograph.
The high-oblique and low-oblique aerial photographs have a special significance in the fields of reconnaissance survey.

2. Based on the scale of photograph

If the R.F. or Representative Fraction scale of the imagery are considered, then aerial photograph is classified into another three types. The fact is that a small scale represents a larger geographic area at a relative size of the photograph. Whereas a large scale covers relatively small geographic area at the considered size of the photograph, and provides a better details than that of the small scale photograph. Thus on this basis, the three types of aerial photographs are as follows.
  • Small Scale Aerial Photograph : When a scale of 1:30,000 or smaller is taken into account to capture an aerial photograph, then that is called small scale aerial photograph. These photographs cover a large area in less detail.
  • Medium Scale Aerial Photograph : If the scale of areal photograph ranges between 1:15,000 and 1:30,000, then the obtained photograph is referred as a medium scale photograph.
  • Large Scale Aerial Photograph : The areal photographs with a scale of 1:15,000 or larger are considered as a large scale aerial photograph. It covers a small area with great detail than that of the small scale photographs.

3. On the basis of coverage angle

Now taking the coverage angle into the account, there are four types of aerial photographs, viz.,
  • Narrow angle areal photograph in which less than 60° is covered by the camera onboard.
  • Standard angle aerial photograph. It provides a coverage of to the order of 60°.
  • Wide angle aerial photograph covers the angle of around 90°.
  • Lastly, the ultra wide angle aerial photograph is that when it covers almost 120° angle.

4. Considering the film and filter of camera

On the basis of the variation of film and filter combination of the camera, there can be seven types of photograph obtained. These are,
  • Panchromatic : Panchromatic refers to the single band sensor, used in the camera for aerial photography. This produces black and white or grayscale images.
  • Colour : The film used in colored aerial photography captures the visible wavelengths separately and produces colorful images. The obtained photographs are very much helpful to observe surface features and others.
  • Infrared : In this case, the film or the bands of camera sensor can detect only the infrared radiation and yields grayscale images as usual. These imagery are mainly used to study water bodies and vegetation cover.
  • Colour-Infrared : Colour-infrared aerial photograph denotes, the camera having the capability to detect both of the visible and infrared wavelengths.
  • Thermal Infrared : Thermal infrared aerial photographs are produced by the camera with the film to capture thermal energy, that emits from the object under investigation in the form of heat. These photographs are useful for studying thermal gradient maps and others related.
  • Radar : These are the photographs obtained by the aerial photography cameras that can detect only radar or microwaves. The imagery contain noise and radiometric techniques can be applied to reduce this.
  • Spectra-zonal : Spectra-zonal images are obtained, when some specific portions of the electromagnetic spectrum are captured by the camera sensor.

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5. Based on Lens System

Lastly, based on the number of lens used for capturing photographs, or simply following the lens system, aerial photographs can be classified into  many types. Such as,
  • Single Lens System : It is the most common as well as general lens system for aerial photography, where, as the name suggests, the only lens is attached with the camera.
  • Multiple Lens System : This includes different lens combination. The notable one is three lens combination, in which three camera lenses are arranged together for aerial photography. This system is also known as the Trimetrogon Lens System. Besides this, there are two lens, four lens, nine lens systems also.



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