CHAPTER 6 : Introduction To Aerial Photographs NCERT SOLUTION CLASS 11TH PRACTICAL WORK IN GEOGRAPHY | EDUGROWN NOTES

Short Answer Type Questions:

Q1.What are the advantages of aerial photographs?
Answer:
Given below are the advantages of aerial photographs:

• 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. 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.
• 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 pm whereas the sensitivity of the film ranges from 0.3 to 0.9 pm.
• Aerial photographs are generally taken with uniform exposure interval. It enables us in obtaining stereo pair of photographs.
• Such a pair of photographs also helps us in getting a three-dimensional view of the surface photographed.

Q2.The distance between two points on an aerial photograph is measured as 2 centimetres. The known distance between the same two points on the ground is 1 km. Compute the scale of the aerial photograph (Sp).
Answer:
Sp = Dp : Dg
= 2 cm :1 km
= 2cm : 1 x 100,000 cm
= 1 : 100,000/2 = 50,000 cm
= 1 unit represents 50,000 units
Therefore, Sp = 1: 50,000.

Q3.The distance measured between two points on a map is 2 cm. The corresponding distance on an aerial photograph is 10 cm. Calculate the scale of the photograph when the scale of the map is 1: 50,000.
Answer:
Photo scale (Sp) = Photo distance (Dp) : Map distance (Dm) x Map scale factor (msf)
= 10 cm : 2 cm x 50,000
= 10 cm : 100,000 cm
= 1 : 100,000/10 = 10,000 cm
= 1 unit represents 10,000 units
Therefore, Sp = 1: 10,000

Q4.Compute the scale of an aerial photograph when the flying height of the aircraft is 7500 m and the focal length of the camera is 15 cm.
Answer:
Focal Length (f) : Flying Height (H) = Photo distance (Dp) : Ground distance (Dg)
Sp = f: H
Or Sp = 15 cm : 7,500 x 100 cm
Or Sp = 1 : 750,000/15
Therefore, Sp = 1 : 50,000

Q5.Differentiate between:
(i) Aerial photograph and map.
(it) Aerial photographs and satellite images.
(iii) High oblique photograph and low oblique photograph.
(iv) Vertical photograph and high oblique photograph.
(v) Vertical photograph and low oblique photograph.
Answer:
(i)

Basis	Aerial Photograph	Map
Projection	It is a central Projection.	It is an orthogonal Projection.
Accuracy	An aerial photograph is geometrically incorrect. The distortion in the geometry is minimum at the centre and increases towards the edges of the photographs.	A map is geometrically correct representation of the part of the earth projected.
Scale	The scale of the photograph is not uniform.	The scale of the map is uniform throughout the map extent.
Impact of enlargement or reduction	Enlargement/reduction does not change the contents of the photographs and can easily be carried out.	Enlargement/reduction of the maps involves redrawing it afresh.
Utility	Aerial photography holds good for inaccessible and inhospitable areas.	The mapping of inaccessible and inhospitable areas is very difficult and sometimes it becomes impossible.

(ii)

Basis	Aerial Photographs	Satellite Images
Meaning	These are taken from an aeroplane or helicopter.	These are taken from man-made satellite launched in space.
Accuracy	These are less accurate as compared to satellite images.	These are more accurate as compared to aerial images.
Suitability	These are suitable for taking photographs of the earth.	These are used for taking photographs of other planets and celestial bodies.
Utility	Aerial photography holds good for inaccessible and inhospitable areas of the earth	It is useful in knowing climatic condition and weather forecast.
Disastermanagement	It is more useful in disaster management for providing relief.	It is more useful in disaster management by predicting weather and taking preventive actions.

(iii)

Basis	High Oblique Photograph	Low Oblique Photograph
Optical Axis	Deviation is more than 300 degree from the vertical axis.	Deviation is more than 30 degree from the vertical axis.
Coverage	It covers largest area.	It covers larger area.
Difference in comparison to map	It is greatest with high oblique photographs.	It is greater with low oblique photographs.
Utility	It is useful in illustrative comparison.	It is useful in recoqnaissance survey.

(iv)

Basis	Vertical Photograph	High Oblique Photograph
Optical Axis	Tilt< 3 degree i.e. exactly or nearly coincides with the vertical axis.	Deviation is more than 300 degree from the vertical axis.
Horizon	Horizon does not appear	Horizon does appear
Coverage	It covers small area.	It covers largest area.
Scale	Scale is uniform if the terrain is flat.	Scale keeps decreasing from foreground to background.
Difference in comparison to map	It is least with vertical photographs.	It is greatest with high oblique photographs.
Utility	It is useful in topographical and thematic mapping.	It is useful in illustrative comparison.

(v)

Basis	Vertical Photograph	Low Oblique Photograph
Optical Axis	Tilt< 3 degree i.e. exactly or nearly coincides with the vertical axis.	Deviation is more than 30 degree from the vertical axis.
Horizon	Horizon does not appear	Horizon does appear
Coverage	It covers small area.	It covers comparatively larger area.
Scale	Scale is uniform if the terrain is flat.	Scale keeps decreasing from foreground to background.
Difference in comparison to map	It is least with vertical photographs.	It is relatively greater with low oblique photographs.
Utility	It is useful in topographical and thematic mapping.	It is useful in reconnaissance survey.

Long Answer Type Questions:

Q1.Explain about different types of aerial photographs on the basis of position of camera axis.
Answer:
Types of Aerial Photographs Based on
the Position of the Cameral Axis:
1. Vertical Photographs: While taking aerial photographs, two distinct axes are formed from the camera lens centre, one towards the ground plane and the other towards the photo plane. The perpendicular dropped from the camera lens centre to the ground plane is termed as the vertical axis, whereas the plumb line drawn from the lens centre to the photo plane is known as the photographic/optical axis. When the photo plane is kept parallel to the ground plane, the two axes also coincide with each other. The photograph so obtained is known as vertical aerial photograph

2. Low Oblique: An aerial photograph
taken with an intentional deviation of 15° to 30° in the camera axis from the vertical axis is referred to as the low oblique photograph. This kind of photograph is often used in reconnaissance surveys. It is shown with the help of following figure.

3. High Oblique: The high oblique are photographs obtained when the camera axis is intentionally inclined about 60° from the vertical axis. Such photography is useful in illustrative surveys. It is shown with the help of following figure.

Q2.How can we classify aerial photographs on the basis of scale?
Answer:
There are three types of Aerial Photographs Based on Scale.

1. Large Scale Photographs: When the scale of an aerial photograph is 1:15,000 and larger, the photography is classified as large-scale photograph. It is shown with the help of following figure.
   
2. Medium Scale Photographs: The aerial photographs with a scale ranging between 1 : 15,000 and 1 : 30,000 are usually treated as medium scale photographs as shown in the figure given below:
   
3. Small Scale Photographs: The
   photographs with the scale being smaller than 1 : 30,000, are referred to as small scale photographs as shown in the figure.
   

Q3.Explain in detail about aerial photographs and satellite images.
Answer:
These days, it has become very easy to draw’ maps and interpret them through aerial photographs and satellite images. It requires a special type of training to understand and interpret aerial photographs. Many countries are making use of aerial photographs not only for defence purposes but also for planning land use, development of cities and towns, development of multipurpose projects etc.

Satellite images are used to predict climatic conditions. It becomes easy to predict weather by using satellite images, wre can find mineral availability, land use plan by making use of satellite images.

Q4.Explain different types of projection.
Answer:
There are three types of projection:
1. Parallel Projection: In this projection, the projecting rays are parallel but not necessarily perpendicular. The triangle ABC is projected on LL1 as triangle abc.

2. Orthogonal Projection: This is a special case of parallel projections. Maps are orthogonal projections of the ground. The advantage of this projection is that the distances, angles or areas on the plane are independent of the elevation differences of the objects. In the figure given below an orthogonal projection is where the projecting rays are perpendicular to the line LL1

3. Central Projection: Figure given below shows an example of Central Projection. In this figure the projecting rays Aa, Bb and Cc pass through a common point O, which is called the perspective Centre. The image projected by a lens is treated like a central projection. An aerial photograph, as discussed earlier is a central projection. In an absolutely vertical flat terrain the aerial photograph will be geometrically the same as the corresponding map of the area. However, because of the tilt of the photograph and relief variations of the ground photographed, an aerial photograph differs geometrically from the map of the corresponding area.