Chapter 6 Introduction To Aerial Photographs | Class 11th Practical Work in Geography revision notes

Aerial Photographs Revision notes

• When we look to an object directly – horizontal perspective
• When we look below – birds eye view – aerial perspective
• The photographs taken from an aircraft or helicopter using a precision camera are termed aerial photographs.
• Aerial Camera: A precision camera specifically designed for use in aircrafts.
• Aerial Film: A roll film with high sensitivity, high intrinsic resolution power and dimensionally stable emulsion support.
• Aerial Photography: Art, science and technology of taking aerial photographs from an air-borne platform.
• Aerial Photograph: A photograph taken from an air-borne platform using a precision camera.
• Fiducial Marks: Index marks, rigidly connected at the central or corner edges of the camera body. When the film is exposed, these marks appear on the film negative.
• Forward Overlap: The common area on two successive photographs in the flight direction. It is usually expressed in per cent.
• Image Interpretation: An act of identifying the images of the objects and judging their relative significance.
• Nadir Point: The foot of the perpendicular drawn from the camera lens center on the ground plane.
• Principal Point: The foot of the perpendicular drawn from the camera lens center on the photo plane.
• Principal Distance: The perpendicular distance from the perspective center to the plane of the photograph.
• Perspective Centre: The point of origin (perspective center) of the bundle of light rays.
• Photogrammetry: The science and technology of taking reliable measurements from aerial photographs.

Uses of Aerial Photography

• development of photogrammetry and photo/image interpretation
• Photogrammetry: It refers to the science and technology of making reliable measurements from aerial photographs – precise measurement of length, breadth and height – creating and updating topographic maps
• Image Interpretation: It is an art of identifying images of objects and judging their relative significance – get qualitative information on land use and soil and analyses land use data
• India – started in 1920s for Agra; Irrawaddy Delta forests in 1923 – 24,
• 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 as A, B and C respectively
• The procedure for indenting aerial photographs for educational purposes could be made with APFPS Party No. 73, Directorate of Air Survey, Survey of India, West Block IV, R. K. Puram, New Delhi.

Advantages of Aerial Photography

• Improved vantage point: Aerial photography provides a bird՚s eye view of large areas
• Time freezing ability: An aerial photograph is a record of the surface features at an instance of exposure – provide historical record
• Broadened Sensitivity: 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.
• Three Dimensional Perspective: Aerial photographs are normally taken with uniform exposure interval that enables us in obtaining stereo pair of photographs

Types of Aerial Photographs Based on the Position of the Cameral Axis

• Vertical Photographs: While taking aerial photographs, two distinct axes are formed from the camera lens center, one towards the ground plane and the other towards the photo plane.
• If such a deviation is within the range of plus or minus 3o, the near-vertical aerial photographs are obtained. Any photography with an unintentional deviation of more than 3o in the optical axis from the vertical axis is known as a tilted photograph.
• 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. In reconnaissance surveys.
• High Oblique: The high oblique are photographs obtained when the camera axis is intentionally inclined about 60° from the vertical axis

Types of Aerial Photographs Based on Scale

• Large Scale Photographs: When the scale of an aerial photograph is 1: 15,000 and larger
• Medium Scale Photographs: The aerial photographs with a scale ranging between 1: 15,000 and 1: 30,000
• Small Scale Photographs: The photographs with the scale being smaller than 1: 30,000
• Small area and try to zoom it – large scale map covers smaller area with greater detail and then from a colony to a city

Geometry of Aerial Photographs

• To understand the geometry of an aerial photograph, it is important to appreciate the orientation of the photograph with respect to the ground
• Parallel Projection: In this projection, the projecting rays are parallel but not necessarily perpendicular. The triangle ABC is projected on LL1 as triangle abc
• Orthogonal Projection: This is a special case of parallel projections with light source at infinity. Maps are orthogonal projections of the ground – here distances, angles or areas on the plane are independent of the elevation differences of the objects. Orthogonal projection where the projecting rays are perpendicular to the line LL1.
• Central Projection: 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 as in aerial photographs. 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.
• vertical line (plumb line as indicated by the direction of gravity)
• For an oblique photograph, the angle between the camera axis and the plumb line is the tilt angle.
• The geometry of the positive and the negative planes are identical.

SP = distance b/w camera lens and negative plane = focal length

SP_G = distance b/w camera lens and ground = flying height

Aerial Photograph

• It is a central Projection.
• An aerial photograph is geometrically incorrect. The distortion in the geometry is minimum at the center and increases towards the edges of the photographs.
• The scale of the photograph is not uniform.
• Enlargement/reduction does not change the contents of the photographs and can easily be carried out.
• Aerial photography holds good for inaccessible and inhospitable areas.

Map

• It is an orthogonal Projection.
• A map is a geometrically correct representation of the part of the earth projected.
• The scale of the map is uniform throughout the map extent.
• Enlargement/reduction of the maps involves redrawing it afresh.
• The mapping of inaccessible and inhospitable areas is very difficult and sometimes it becomes impossible.
• Even vertical aerial photographs do not have a consistent scale unless they have been taken of a flat terrain. Aerial photographs need to be transformed from perspective view to the planimetric view before they can be used as map substitute. Such transformed photographs are known as orthophotos.
• In a perspective view, all light rays reflected from the Earth՚s surface pass through a single point at the center of the camera lens. A planimetric (plan) view looks as though every position on the ground is being viewed from directly above.
• A map cannot be directly traced out of an aerial photograph. The reason is that there is a basic difference in the planimetry (projection) and perspective of a map and an aerial photograph.
• Method 1: By Establishing Relationship Between Photo Distance and Ground Distance
• Method 2: By Establishing Relationship Between Photo Distance and Map Distance
• Method 3: By Establishing Relationship Between Focal Length (f) and Flying Height (H) of the Aircraft
• 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 expressed as RF. Scale determines what objects would be visible, the accuracy of estimates and how certain features will appear
• Method 1: By Establishing Relationship Between Photo Distance and Ground Distance scale of an aerial photograph will be measured as a ratio of the two, i.e.. Dp/Dg

Question: The distance between two points on an aerial photograph is measured as 2 cm. The known distance between the same two points on the ground is 1 km. Compute the scale of the aerial photograph (Sp) .

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

Scale of Aerial Photograph

• Method 2: By Establishing Relationship Between Photo Distance and Map Distance, 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)

Question: 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.

Sp = Dp: Dm x msf

Or = 10 cm: 2 cm x 50,000

Or = 10 cm: 100,000 cm

Or = 1: 100,000/10 = 10,000 cm

Or = 1 unit represents 10,000 units

Therefore, Sp = 1: 10,000

• Method 3: By Establishing Relationship Between Focal Length (f) and Flying Height (H) of the Aircraft
• Focal Length (f) : Flying Height (H) = Photo distance (Dp) : Ground distance (Dg)

Question: Compute the scale of an aerial photograph when the flying height of the aircraft is 7500m and the focal length of the camera is 15cm.

Sp = f: H

Or Sp = 15 cm: 7,500 x 100 cm

Or Sp = 1: 750,000/15

Therefore, Sp = 1: 50,000

Marginal Information Given on Vertical Aerial Photographs

⚹ 793 is a Photo Specification number maintained by the 73 APFPS Party of the Survey of India. B is the Flying Agency that carried out the present photography (In India three flying agencies are officially permitted to carry out aerial photography. They are the Indian Air Force, the Air Survey Company, Kolkata and the National Remote Sensing Agency, Hyderabad, identified on the aerial photographs as A, B and C respectively) , 5 is the strip number and 23 is the photo number in strip 5.