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# Geometry and types of aerial photographs

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### Geometry and types of aerial photographs

1. 1. The most important geometric properties of an aerial photograph are those of an angle and scale. Angle of Arial Photographs  The angle at which aerial photograph is taken is used to classify the photograph into one of three types viz. vertical, high oblique and low oblique.  Vertical photograph taken with a single lens is the most common type of aerial photography used in remote sensing applications.
2. 2.  The vertical photography is taken with the camera axis pointing vertically downwards.  Oblique photography is taken with the camera axis pointing obliquely downwards (intentional inclination of the camera axis).  High oblique photography incorporates an image of the horizon into the photographs while low oblique photographs do not.
3. 3.  A ‘truly 'vertical aerial photograph is rarely obtainable because of unavoidable angular rotations or tilts, caused by the angular attitude of the aircraft at the instant of exposure.  These unavoidable tilts cause slight (1 to 3 degrees ) unintentional inclination of the camera optical axis, resulting in the acquisition of tilted photographs .  Vertical photographs have properties similar to those of a map with a approximately constant scale over the whole photograph, and therefore can be used for mapping and measurements.
4. 4. Most vertical aerial photographs are taken with frame cameras along flight line ,or flight strips. The line traced on the ground directly beneath the aircraft during acquisition of photography is called Nadir line. This line connects the image centers of the vertical photographs. Successive photographs are generally taken with some degree of Endlap. Not only dose this lapping ensure total coverage along a flight line, but an endlap of at least 50% is essential for total stereoscopic coverage of a project area.
5. 5. a: conditions during exposure b: resulting photograph
6. 6. Light rays from terrain objects are imaged in the plane of the film negative after intersecting at the camera lens exposure station, L. The negative is located behind the lens at a distance equal to the lens focal length, f. Assuming the size of a paper print positive (or film positive) is equal to that of the negative, positive image positions can be depicted diagrammatically in front of the lens in a plane located at a distance f. This rendition is appropriate in that most photo positives used for measurement purposes are contact printed, resulting in the geometric relationships shown.
7. 7. The x and y coordinate positions of image points are referenced with respect to axes formed by straight lines joining the opposite fiducial marks recorded on the positive. The x axis is arbitrarily assigned to the fiducial axis most nearly coincident with the line of flight and is taken as positive in the forward direction of flight. The positive y axis is located 90 counter- clockwise from the positive x axis. Because of the precision with the fiducial marks and the lens are placed in a metric camera, the photo coordinate origin, o, can be assumed to coincide exactly with the principal point, the intersection of the lens optical axis and the film plane.
8. 8. Images for terrain points A, B, C, D, and E appear geometrically reversed on the negative at a’, b’, c’, d’, and e’ and in proper geometric relationship on the positive at a, b, c, d, and e. Scale of Arial Photographs (Photographic Scale)  The scale of a photograph expresses the mathematical relationship between a distance measured on the photo and the corresponding distance measured on the ground.  A photograph scale is an expression that states one unit of distance on a photograph represents a specific number of units of actual ground distance.
9. 9.  Scales may be expressed as unit equivalents (1 mm = 25 m), representative fractions (1/25,000) or ratios (1: 25,000).  Unlike maps, which have a constant scale throughout, the aerial photographs have a range of scales that vary in proportion to the elevation of the terrain involved.  The most straight forward method for determining photo scale is to measure the corresponding photo and ground distances between any two points. The scale S is then computed as the ratio of the photo distance d to the ground distance D.  S = photo scale = photo distance/ground distance = d/D.
10. 10. The scale of a photograph is determined by the focal length of the camera and the vertical height of the lens above the ground. The focal length (f) of the camera is the distance measured from the centre of the camera lens to the film. The vertical height of the lens above the ground (H-h) is the height of the lens above sea level (H), minus the height of the ground above see level (h), when the optical axis is vertical and the ground is flat. These parameters are related by formula S = f / (H h )
11. 11. The aerial photographs can be divided into :-  On the basis of the direction/ position of the axis of the camera  On the basis of the angles of coverage and focal length  On the basis of the films used in the cameras.
12. 12.  On the basis of the direction/ position of the axis of the camera : 1) Vertical 2) Horizontal 3) Oblique 4) Convergent 5) Trimetrogon
13. 13. The axis of the camera is vertically adjusted to take the photographs. The area covered through vertical air photos are often square in shape at the uniform plane. In simple words, These photographs are taken with an air borne camera aimed vertically downward from the plane.
14. 14. The horizontal air photos are also known as Terrestrial air photos. In the production of such air photos, the axis of the camera is horizontal.
15. 15. In the oblique air photos, the adjustment of the axis of the camera ranges from the vertical to angular position. The areas covered by oblique air photos assumed the shape of a trapezium. An oblique photograph is divided into two types:- 1. Low Oblique Photographs 2. High Oblique Photographs
16. 16. One which does not have the horizon showing is called a Low Oblique Photographs and the axis of the camera is 0 .
17. 17. An oblique photographs showing the horizon is called a High Oblique photographs and the axis of the camera is tilted to 30 to 60 .
18. 18. The convergent air photos are also oblique, but an area is simultaneously Photographed by two cameras.
19. 19. In Trimetrogen air photos, three cameras are used simultaneously amongst which the central camera is vertical, and the other two are adjusted to oblique position. The cameras are so fixed that the entire area from right horizon to the left horizon is photographed.
20. 20.  On the basis of the angles of coverage and focal length The lenses used in the camera, are of the following types according to the angles of coverage and the focal length : 1. Narrow Angle < 60 - More Focal Length 2. Normal Angle 60 - 75 3. Wide Angle 75 - 100 4. Super wide Angle >100 - Low Focal Length.
21. 21.  On the basis of the films used in the cameras. On the basis of these category the air photos are divided into three types such as : 1. Black and white. 2. Infra red (IR) 3. Colored
22. 22. This is also known as PANCHROMATIC . This is most widely used type of aerial photograph. This is mainly used for study of geological mapping, glacial deposit, coastal formation, relief features etc. this films are cheapest and easily available.
23. 23. It is similar like a black and white or panchromatic film but the differences between panchromatic film and infra red is only spectral sensitivity. It is used in applications for mapping vegetation, types of crops.
24. 24. Colored photography is mainly used for interpretation purpose. There are three colors Yellow , Magenta (Blue+ Red) and Blue Green when these three colors mixed together they form different colors. This types of photographs are mainly used for mapping cultivated land, environment/vegetation cover, geology, geomorphology etc.
25. 25. o Rampal . K.K , Handbook of aerial photography and interpretation. o Lillesand Kiefer Chipman, Remote Sensing and Image Interpretation