Introduction to Aerial Photographs - Solutions

 CBSE Class 11 Geography

NCERT Solution
Chapter 29
Introduction to Aerial Photographs

1. Multiple choice questions.
(i) In which of the following aerial photographs the horizon appears?

(a) Vertical
(b) Near-vertical
(c) Low-oblique
(d) High-oblique.

Ans. (c) Low-oblique. It is a photograph taken with a camera inclined about 30 degree from the vertical. 

(ii) In which of the following aerial photographs the Nadir and the principle points coincide?

(a) Vertical
(b) Near-vertical
(c) Low-oblique
(d) High-oblique.

Ans. (a) Vertical. A vertical photograph is taken with the camera pointed as straight down as possible. 

(iii) Which type of the following projections is used in aerial photographs?

(a) Parallel
(b) Orthogonal
(c) Central
(d) None of the above.

Ans. (c) Central. Central projections are projections from one plane to another where the first plane's point and the image of the second plane, lie on a straight line from a fixed point, not on either plane. 

2. Answer the following questions in about 60 words.
(i) State any three advantages that an aerial photograph offers over ground based observations.

Ans. The photographs taken from ground provide us with a view of the object similar to the way we see them with our own eyes. In other words, we get a horizontal perspective of the objects photographed. The basic advantages that aerial photographs offer over ground based observation are:

(a) Improved vantage point and Time freezing ability: A vantage point is a position or standpoint from which something is viewed it considered. Aerial photography provides a bird's eye view of large areas, enabling us to see features of the earth surface in their spatial context. An aerial photograph is a record of the surface features at an instance of exposure. Therefore, it can be used as a historical record.

(b) Broadened Sensitivity: The sensitivity of the film used in taking aerial photographs is relatively more than the sensitivity of the human eyes. Our eyes perceive only in the visible region of the electromagnetic spectrum, i.e. 0.4 to 0. 7 µm whereas the sensitivity of the film ranges from 0.3 to 0.9 µm.

(c) Three Dimensional Perspective: Perspective means an image as it is seen by the eye. Aerial photographs are generally taken with uniform exposure interval. It enables us in obtaining stereo pair of photographs. Such a pair of photographs helps us in getting a three-dimensional view of the surface photographed.

(ii) How is an aerial photograph taken?

Ans. An aerial photograph, in broad terms,  is any photograph taken from the air. Normally, air photographs are taken vertically from an aircraft using a highly-accurate camera. Aerial photographs are taken from the camera kept in an aeroplane or helicopter. These are taken from aerial cameras which is a precision camera specifically designed for use in aircrafts.
It makes use of Aerial Film which is a roll film with high sensitivity, high intrinsic resolution power and dimensionally stable emulsion support. Therefore we can say that aerial photography is an art, science and technology of taking aerial photographs from an air-borne platform.

(iii) Present a concise account of aerial photography in India.

Ans. Aerial photography in India was taken for the first time in1920 when large-scale aerial photographs of Agra city were obtained.
Subsequently, Air Survey Party of the Survey of India took up aerial survey of Irrawaddy Delta forests, which was completed during 1923-24. Subsequently, several similar surveys were carried out and advanced methods of mapping from aerial photographs were used.
Today, aerial photography in India is carried out for the entire country under the overall supervision of the Directorate of Air Survey (Survey of India) New Delhi. Three flying agencies, i.e. Indian Air Force ; Air Survey Company, Kolkata and National Remote Sensing Agency, Hyderabad have been officially authorised to take aerial photographs in India.

3. Answer the following questions in about 125 words.
(i) What are the two major uses of an aerial photograph? Elaborate.

Ans. Aerial photographs are used in topographical mapping and interpretation. These two different uses have led to the development of photogrammetry and photo/image interpretation as two independent but related sciences.
(a) Photogrammetry refers to the science and technology of making reliable measurements from aerial photographs. The principles used in photogrammetry facilitate precise measurements related to the length, breadth and height from such photographs. Hence, they are used as the data source for creating and updating topographic maps.
(b) Image Interpretation is an art of identifying images of objects and judging their relative significance. The principles of image interpretation are applied to obtain qualitative information from the aerial photographs such as land use/land cover, topographical forms, soil types, etc. A trained interpreter can thus utilise aerial photographs to analyse the land¬use changes.

(ii) What are the different methods of scale determination?

Ans. Scale is the ratio of a distance on an aerial photograph the distance between the same two places on the ground in the real world. It can be expressed in unit equivalents like 1 cm= 1,000 km( or 12,000 inches) or as a representative fraction (1:100,000). Scale determines what objects would be visible, the accuracy of estimates and how certain features will appear.
There are three methods to compute the scale of an aerial photograph using different sets of information.
Method 1:
By Establishing Relationship Between Photo Distance and Ground Distance:
 If additional information like ground distances of two identifiable points in an aerial photograph is available, it is fairly simple to work out the scale of a vertical photograph. The corresponding ground distances is expressed by Dg and for which the distances on an aerial photograph is expressed as Dp. Both are measured. In such cases, the scale of an aerial photograph will be measured as a ratio of the two, i.e. DP/ Dg.

Example : The distance between two points as measured in ground is 200 mts. and the corresponding distance between the images of the same two points on an aerial photograph is 1.0 cm, then the scale of the photograph would be:
SP = DPDG = 102×250,000  = 150,000  

Method 2:
By Establishing Relationship Between Photo Distance and Map Distance:
 The distances between different points on the ground are not always known. However, if a reliable map is available for the area shown on an aerial photograph, it can be used to determine the photo scale. In other words, the distances between two points identifiable both on a map and the aerial photograph enable us to compute the
"scale of the aerial photograph (Sp).
The relationship between the two distances may be expressed as under: (Photo scale : Map scale) = (Photo distance : Map distance) We can derive Photo scale (Sp) = Photo distance (Dp): Map distance (Dm) x Map scale factor (msf)

Method 3:

By Establishing Relationship Between Focal Length (f) and Flying Height (H) of the Aircraft: If no additional information is available about the relative distances on photograph and ground/map, we can determine the photo-scale provided the information about the focal length of the camera (f) and the flying height of the aircraft (H) are known. The photo scale so determined could be more reliable if the given aerial photograph is truly vertical or near vertical and the terrain photographed is fiat. The focal length of the camera (f) and the flying height of the aircraft (H) are provided as marginal information on most of the vertical photographs. Aerial photograph may be used to derive the photo-scale formula in the following way:
Focal Length (f): Flying Height (H) = Photo distance (Dp):
Ground distance (Dg).