Aerial photography- Concept and Terminologies

Uttiya Chattopadhyay, A Scholar Of Visva Bharati, Geography Department

1. AERIAL PHOTOGRAPHY Presented by Uttiya Chattopadhyay [email protected] VISVA BHARATI , GEOGRAPHY DEPT.

2. Definition of Aerial Photography When photographs of ground features are taken from a hill peak or high pillar, or helicopter or aero plane with a high quality of camera, is known as aerial photography. The outputs or products are said aerial photographs. It is a remote sensing technique, by which accurate photographs can be taken both in micro and macro scale.

3. SOME TERMINOLOGIES OF AERIAL PHOTOGRAPHS PRINCIPLE POINT FIDUCIAL MARKS PLUMB POINT TILT & TILT DISTORTION ISO CENTRE CONJUGATE PHOTO CENTRE MEAN PHOTO BASE LENGTH OVERLAPPING FLIGHT AXIS FLYING ALTITUDE CRAB AIR BASE RADIAL DISPLACEMENT FLIGHT MAP SHORAN

4. PRINCIPLE POINT The imaginary line which comes through the centre of the lens of camera , touches on a point of the photographic film or plate at the 90 degree angle, that point is principle point. 90 degree Photo plate Principle point

5. FIDUCIAL MARKS In aerial photographic cameras, there are some signs like < , > , +, - etc, in the four sides or in four diagonal angles, which is automatically printed in photo plates, these marks are called fiducial marks. When these four marks are joined together, the point created is the principle point. Principle point

6. PLUMB POINT The imaginary line from the lens touching the film, perpendicular to the earth surface is called plumb point. When photography is accurately perpendicular then principle point & plumb point become the same. Principle point plumb point

7. TILT & TILT DISTORTION The inclination which is created by the perpendicular & angular situation/placement of the camera is called tilt . And when photography is taken with tilt or inclined situation, the distortion remained in the shape of the object, is termed tilt distortion . TILT TILT DISTORTION

8. ISO CENTRE If photography is not perpendicular, then the point where perpendicular line & camera axis coincide, is called iso centre. Iso centre is that point which goes through lens and meets the film at (90 – θ /2) angle. Iso centre

9. CONJUGATE PHOTO CENTRE When two aerial photo are overlapped then two immediate side by side principle points are called conjugate photo centre or cpc . It helps to draw the flight line. CPC FLIGHT LINE

10. MEAN PHOTO BASE LENGTH The average distance between the principle points and the conjugate photo centers. It is expressed by ‘b’.

11. When aerial photographs are taken then there is a common portion between two immediate photos, this technique of taking overlapped photo is called overlapping. It helps to get 3d image and analysis. OVERLAPPING Overlapped portion

12. FUSED PHOTOGRAPHS OR STEREO PAIR

13. FLIGHT AXIS The imaginary line at least between two principle points is called flight axis.

14. FLYING ALTITUDE & FLYING HEIGHT The height of the plane from the datum level is called flying altitude and the height of the plane from the land surface is called flying height. Flying height Flying altitude

15. CRAB During flight sometimes disorder between flight line and camera situation, happen. Then flight axis and flight line do not remain parallel, it is called crab.

16. AIR BASE In the flight route, the distance between two exposure station is called air base. Air base

17. RADIAL DISPLACEMENT In earth surface an object remains in horizontally or perpendicularly. But it aerial photographs it is horizontally represented. And according the height of the object it is situated according to radius. Thus this projected displacement of object in photographs is called radial displacement. r = Rh /H where R = distance between the apex point in photo and principle point, H = flight height, h = object height

18. FLIGHT MAP The map that a pilot has when he is in flight to take photograph. This map contains the necessary information of the area of survey.

19. SHORAN It is a short wave radiation radio machine to locate the accurate situation of the flight due taking photograph. It is received by the two transponder stations on the ground.

20. PHOTOMOSAIC To complete a map of a certain region or area, several maps are needed to be joined together. This joining of several map and creating a map of whole region is called photo mosaic. For this all maps should be brought in a same scale.

21. METHODS OF AERIAL PHOTOGRAPHY pin point photography – taking photography of a single object. Block photography – taking photography of an area with parallel strip.

22. TYPES OF AERIAL PHOTOGRAPHY on the basis of some aspects like survey area, angle of camera lens, exposure process, aerial photography is divided into two types, namely Vertical photography – 90 degree camera axis scale remains almost accurate does not need special correction used when map work is needed cannot cover extensive area comparatively difficult to understand the height or depth of object expensive Oblique photography - inclined camera axis extensive area can be covered easy to interpret scale distortion difficult to prepare map

23. CONVERGENT PHOTOGRAPHY Oblique Photograph Oblique Photograph

24. Trimetrogon Photography Oblique Photograph Oblique Photograph Oblique Photograph

25. TYPES OF AERIAL PHOTOGRAPHS According to emulsion used , aerial photography is classified into 4 types namely Panchromatic (black & white) infra-red Color Color infra-red

26. SCALE OF AERIAL PHOTOGRAPHY The scale of the aerial photography is dependant upon the purpose of taking photographs. geological survey – 1:20000 to 1: 50000 Natural resource and forest resource – 1:10000 to 1:20000 micro study – 1:10000 to 1:50000 If the scale of the aerial photography is doubled, then 12 times larger print paper is required to print the photography. So scale is very important in terms of cost also. High altitude photography – 1: 80000 Medium altitude – 1:40000 Low altitude – 1: 15000 Small format aerial photography – 1:5000

27. Example: Map Scale is 1:50 000   1 Measure the direct distance between the same two points on the aerial photo e.g.. 10cm   2 Measure the direct distance between two points on the map e.g.. 6.5cm   3 Ratio of Scales = Ratio of Distances   Scale of Photo = Map Distance Scale of Map Photo Distance         Scale of Photo = 6.5cm 50 000 10 cm   Scale of Photo = 6.5 x 50 000 10 = 325 000 10   = 32 500 Scale of Photo = 1: 32 500   The aerial photo has a LARGER SCALE than the map LARGE SCALE photos or maps show a SMALLER AREA but in MORE DETAIL eg . 1:10 000   SMALL SCALE photos or maps show a LARGER AREA but in LESS DETAIL eg . 1:500 000

28. There are four types of resolution : spatial, spectral, temporal, and radiometric. spatial resolution is defined as the pixel size of an image representing the size of the surface area (i.e. m2) being measured on the ground, determined by the sensors' instantaneous field of view (IFOV); spectral resolution is defined by the wavelength interval size (discreet segment of the Electromagnetic Spectrum) and number intervals that the sensor is measuring; temporal resolution is defined by the amount of time (e.g. days) that passes between imagery collection periods for a given surface location; and radiometric resolution is defined as the ability of an imaging system to record many levels of brightness (contrast for example). Radiometric resolution refers to the effective bit-depth of the sensor (number of grayscale levels) and is typically expressed as 8-bit (0-255), 11-bit (0-2047), 12-bit (0-4095) or 16-bit (0-65,535). Geometric resolution refers to the satellite sensor's ability to effectively image a portion of the Earth's surface in a  single  pixel and is typically expressed in terms of  Ground Sample Distance , or GSD. GSD is a term containing the overall optical and systemic noise sources and is useful for comparing how well one sensor can "see" an object on the ground within a single pixel. For example, the GSD of Landsat is ~30m, which means the smallest unit that maps to a single pixel within an image is ~30m x 30m. The latest commercial satellite ( GeoEye 1) has a GSD of 0.41 m (effectively 0.5 m due to United States Government restrictions on civilian imaging). This compares to a 0.3 m resolution obtained by some early military film based  Spy satellite  such as  Corona . The  resolution  of satellite images varies depending on the instrument used and the altitude of the satellite's orbit. For example, the  Landsat archive offers repeated imagery at 30 meter resolution for the planet, but most of it has not been processed from the raw data.  Landsat 7  has an average return period of 16 days. For many smaller areas, images with resolution as high as 41 cm can be available. [5] Satellite imagery is sometimes supplemented with  aerial photography , which has higher resolution, but is  more  expensive per square meter. Satellite imagery can be combined with vector or raster data in a  GIS  provided that the imagery has been spatially rectified so that it will properly align with other data sets.

29. Radiometric Resolution The  radiometric resolution  of an imaging system describes its ability to discriminate very slight differences in energy The finer the radiometric resolution  of a sensor, the more sensitive it is to detecting small differences in reflected or emitted energy.

30. PROPERTIES OF AERIAL PHOTOGRAPHY Shape Tone/ Colour Pattern Texture (Roughness/Smoothness) Shadow Situation Interpretation template

31. USE OF AERIAL PHOTOGRAPHY IN GEOGRAPHY Landform and physiography Meteorology Natural vegetation Agriculture Environmental vigilance Transport and communication Human settlement Industries Counting objects Resource inventories land use mapping Forecast

32. The process of making maps or scale drawings from photographs, especially aerial photographs. The process of making precise measurements by means of photography. A more sophisticated technique, called  stereophotogrammetry , involves estimating the three-dimensional  coordinates  of points on an object. These are determined by measurements made in two or more photographic images taken from different positions (see  stereoscopy ). Common points are identified on each image. A line of sight (or ray) can be constructed from the camera location to the point on the object. It is the intersection of these rays ( triangulation ) that determines the three-dimensional location of the point. PHOTOGRAMMETRY

33. ACTIVITIES OF PHOTOGRAMETRY SCALE DISTANCE OF PHOTO & REAL GROUND DISTORTION ALTITUDE & HEIGHT ACCURACY SOFT & HARD COPY FLIGHT MAP etc

34. IF CAMERA’S FOCAL LENGTH (f) IS 5 INCH AND FLIGHT HEIGHT IS 15000 FOOT, THEN WHAT IS THE SCALE OF THE AIR PHOTO ? Photo scale – R.F. = f/H where f – focal length & H- flight height R.F = 5/(15000*12) = 1/36000 PHOTO SCALE = 1:36000

35. GEOMETRY OF AERIAL PHOTO PROJECTION TILT SWING

36. PROJECTION In the way the rays are projected in the photo film. Parallel projection orthogonal projection central projection

37. TILT Angle between the camera axis and plumb line. 1. Longitudinal or x tilt or fore & aft tilt, expressed by Φ (phi) 2. Lateral tilt or y tilt, denoted by w (omega)

38. SWING Angle between photo axis and actual flight line, denoted by x (kappa).

39. RELIEF DISPLACEMENT Any object when projected in aerial photograph, is displaced in some extent. This displacement of objects in aerial photographs is called relief displacement. It is expressed by letter ‘d’. Displacement increases from the photo centre of an aerial photo. From the value of relief displacement , we can easily calculate the height of an object. h = (d*H) /r where h is the height, d is the relief displacement, H is the flight height r is the distance of the displaced image from the photo centre.

40. PARALLAX Parallax  is a displacement or  difference  in the  apparent position  of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines

41. TYPES OF PARALLAX Absolute Parallax : the distance of the same object on parallax from the photo centre. It is expressed by P or b. Differential Parallax : the distance of absolute parallax between the apex & nadir of an object . It is expressed by Δ P.

42. AREA MEASUREMENT IN AERIAL PHOTGRAPH GROUND AREA = PHOTO AREA * 1/ S 2 WHERE ‘S’ IS THE SCALE OF THE PHOTO

43. Passive source Active source Only in day Day & night Photo not less than 6 mt No restriction Mostly used in survey purpose Can be used in all purpose Easy interpretation Difficult to interpret overlapping Not necessarily 3d available Not 3d Around 10 km exposure height 600 to 30000 km AERIAL PHOTO SATELLITE PHOTO

44. The wavelengths are approximate; exact values depend on the particular  satellite 's instruments: Blue , 450-515..520 nm, is used for atmospheric and deep water imaging, and can reach up to 150 feet (50 m) deep in clear water. Green , 515..520-590..600 nm, is used for imaging of vegetation and deep water structures, up to 90 feet (30 m) in clear water. Red , 600..630-680..690 nm, is used for imaging of man-made objects, in water up to 30 feet (9 m) deep, soil, and vegetation. Near infrared , 750-900 nm, is used primarily for imaging of vegetation. Mid-infrared , 1550-1750 nm, is used for imaging vegetation, soil moisture content, and some  forest fires . Mid-infrared , 2080-2350 nm, is used for imaging soil, moisture, geological features, silicates, clays, and fires. Thermal infrared , 10400-12500 nm, uses emitted radiation instead of reflected, for imaging of geological structures, thermal differences in water currents, fires, and for night studies. Radar  and related technologies are useful for mapping terrain and for detecting various objects.

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