Aerial photographs: Type, Characteristics, Advantages and Limitations

Aerial photography is the science of taking a photograph from a point in the air, usually from a fixed wing aircraft, for the purpose of making some kind of study of the ground surface. The use of aerial photography (which is part of the remote sensing family of investigative techniques) in road location and geotechnical surveying is a well-accepted road engineering practice.

Aerial photographs can be studied using 

(a) simple optical magnifiers and stereoscopes,
(b) sophisticated optical/mechanical stereoplotters that can accurately measure distances or objects in the photographs,
(c) optical devices that allow the simultaneous viewing of an aerial photograph and a printed map,
(d) electrooptical density-slicing systems that allow a photographic image to be separated into
a number of grey levels (which can be presented in different colours on a television monitor), or
(e) optical viewers that enable simultaneous viewing of up to four photographic multispectral images.

Types of Aerial photographs

A) Types of aerial photographs on the basis of orientation of camera axis:

i. Vertical photograph: An aerial photograph taken with the camera axis/optical axis coinciding with the vertical to the ground is known as vertical photograph.
ii. Tilted photograph: An aerial photograph in which the camera axis is unintentionally tilted by small amount (less than 3°) from vertical/perpendicular to the ground is known as tilted photograph.
iii. Oblique photograph: An aerial photograph taken keeping the camera axis intentionally tilted from the vertical is known as oblique aerial photograph. The amount of tilt ranges from 30° to 60 °. The advantage of oblique photographs is that they cover large area and they also provide side view of features.

Oblique photographs are of two types
- Low oblique: Oblique photographs in which horizon is not seen. Tilt is 30°.

- High oblique: Oblique photographs in which horizon is seen. Tilt is 60°.

B) Types of aerial photographs on the basis of angle of coverage:

i. Narrow angle photograph: Angle of coverage is less than 60°.
ii. Standard or Normal angle photography: Angle of coverage is of the order of 60°.
iii. Wide angle photograph: Angle of coverage is of the order 90°.
iv. Superwide of Ultrawide photograph: Angle of coverage is of the order of 120°.

C) Types of aerial photographs on the basis of special properties of films, filters and photographic equipment:

i. Panchromatic: Records only single band data (visible region of spectrum).
ii. IR: Records only red and IR portions of EMS.
iii. Color: Records all reflections of visible portion in color or natural colors.
iv. Color IR: Records spectral colors and IR in combination resulting in false colors.
v. Thermal imagery: Records only thermal IR emissions of objects.
vi. Radar imagery: Records reflection of radar waves.
vii. Spectrazonal: Records only selected part of the spectrum.

D) Types of aerial photographs on the basis of lens system:

i. Single lens photograph.
ii. Multiple lens photographs.
- Three lens photograph (Trimetrogon photograph).
- Four lens photograph.
- Nine lens photograph.

Advantages of aerial photography

In general, aerial photographs greatly facilitate the location and design of a roadway.

1. On a large scheme, the time and costs required to locate a major new road and to

prepare plans for tender purposes are normally much reduced as compared with

the conventional ground survey approach. The less well-mapped the terrain in

which the road is being located the more the savings.

2. The ability to survey large areas of land ensures that the most suitable location is

less likely to be overlooked.

3. A complete inventory of all ground features at a point in time is available for

reference purposes, and subsequent analysis.

4. Topographic maps produced from aerial surveys can be more accurate than

those prepared from ground surveys.

5. Road profiles and cross-sections can very often be developed without physically

encroaching on private lands. Thus, landowners are not upset and land values

are not affected during the road location investigation.

6. Skilled technical personnel can be released from repetitive work and used more

profitably on more demanding problems.

7. Aerial photographs are easily intelligible to the general public, and are particularly

useful as visual aids at public hearings (as well as at technical meetings).

Characteristics of aerial photographs

The scales of vertical aerial photographs currently taken from fixed-wing aircraft typically vary from 1:500 to 1:120 000. The choice of scale depends upon:

(1) the intended usage of the photography; 

(2) the normal cloud base and the extent to which it affects flying height; and 

(3) problems associated with scale distortion and acceptable limits for the intended usage. 

Measurements are taken from a vertical photograph of flat land can be as accurate as those from a printed map, whereas a similar photograph of rugged terrain can have a variety of scales depending upon the elevations of the ground area at the locations considered, and the shapes of objects can be altered and displaced about the centre of the photograph.

When planning aerial photography an average scale is used for rough calculation purposes. This average scale, Save, is given by the equation 

 Save = f (H – have).............................. (1.1)

where f = focal length of the camera, H= flying height, and have = average elevation of the terrain being photographed.

Practical scale recommendations11 for route surveys are 

(a) 1:16 000 to 1:30 000 for reconnaissance photography, extended to 1:120 000 for very large projects or to 1:8000 for small projects, 

(b) 1:8000 to 1:16 000 for preliminary investigations, extended to 1:25 000 for large projects and 1:5000 for small projects, and 

(c) 1:10 000 to 1:500 for detailed investigation or monitoring, with the exact scale depending upon the type of information being sought.

If horizontal and vertical measurements are to be taken from air photographs it is first necessary to establish the positions and heights of some air-visible ground control points on each photograph. The horizontal points are used to determine the scale and true orientation of each photograph, to provide a basis for correcting cumulative errors that arise when strips of photographs are assembled, and to link the photographs to the standard geographic (i.e. National Grid) coordinates. The vertical control points are needed before point elevations or contour lines can be derived.

Limitations on usage of aerial photography

Whilst aerial photography generally facilitates the location and design of roads there are certain practical limitations on its usage.

First and possibly most important, the taking of quality airphotos cannot be guaranteed in inclement conditions. Good photography needs a clear atmosphere, and if it is taken on wet or misty days, in a haze, or under smoky conditions, the photography may be of limited value.

Also, a topographic map cannot be accurately developed if the ground is covered by snow or obscured by the leaves of trees. Thus, for example, in temperate areas where the ground has a deciduous cover, the taking of aerial photography must await the time of leaf-fall, whilst its usage to survey heavily-wooded tropical areas may not be appropriate at all.

If the area to be studied is relatively small and well-mapped it may be more appropriate

to use conventional ground survey methods rather than aerial procedures.