Aerial photography abraham thomas

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  1. 1. AERIAL PHOTOGRAPHYABRAHAM THOMAS University of the Western Cape Bellville 7535, Cape Town, South Africa
  2. 2. OutlineDefinition of aerial photographyCharacteristics of aerial photographsAerial cameras and their typesGeometric properties of aerial photographsAerial Photo Interpretation ElementsAn Introduction to photogrammetryPhoto interpretation and photogrammetric equipmentTaking measurements from aerial photographsMapping with aerial photographs
  3. 3. Defining Aerial PhotographyThe term "photography" is derived from two Greek wordsmeaning "light" (phos) and "writing" (graphien). From Greekpht- , the stem of phs light, which is a unit of illumination.Photography means the art, hobby, or profession of takingphotographs, and developing and printing the film orprocessing the digitized array image.Photography is production of permanent images by means ofthe action of light on sensitized surfaces (film or array inside acamera), which finally giving rise to a new form of visual art.Aerial Photography means photography from the air.The word aerial originated in early 17th century. [Formedfrom Latin aerius , from Greek aerios , from ar air.]
  4. 4. Aerial Photography: An Overview Aerial Photography is one of the most common, versatile and economical forms of remote sensing. It is a means of fixing time within the framework of space (de Latil, 1961). Aerial photography was the first method of remote sensing and even used today in the era of satellite and electronic scanners. Aerial photographs will still remain the most widely used type of remote sensing data. Aerial photographs were taken from balloons and kites as early as the mid-1800s. 1858 - Gasper Felix Tournachon "Nadar" took the first aerial photograph from a captive balloon from an altitude of 1,200 feet over Paris.
  5. 5. Characteristics of Aerial PhotographySynoptic viewpoint: Aerial photographs give a birds eye view oflarge areas enabling us to see surface features in their spatialcontext. They enable the detection of small scale features andspatial relationships that would not be found on the ground.Time freezing ability: They are virtually permanent records of theexisting conditions on the Earths surface at one point in time, andused as an historical document.Capability to stop action: They provides a stop action view ofdynamic conditions and are useful in studying dynamic phenomenasuch as flooding, moving wildlife, traffic, oil spills, forest fires.Three dimensional perspective: It provides a stereoscopic viewof the Earths surface and make it possible to take measurementshorizontally and vertically - a characteristic that is lacking for themajority of remotely sensed data.
  6. 6. Characteristics of AerialPhotography (2)Spectral and spatial resolution: Aerial photographsare sensitive to radiation in wavelengths that are outsideof the spectral sensitivity of the human eye (0.3 m to0.9 m versus 0.4 m to 0.7 m).They are sensitive to objects outside the spatialresolving power of human eye.Availability: Aerial photographs are readily available ata range of scales for much of the world.Economy: They are much cheaper than field surveysand are often cheaper and more accurate than maps.
  7. 7. Aerial CamerasAerial photographs can be made with any type of camera (e.g. 35mm small amateur or 70 mm or special cameras that are purposebuilt meant for mapping).Many successful applications have employed aerial photographymade from light aircraft with handheld 35 mm cameras.For the aerial study of large areas, high geometric and radiometricaccuracy are required and these can only be obtained from by usingcameras that are purpose built.Aerial camera are precision built and specifically designed to exposea large number of films/photographs in rapid succession with theultimate in geometric fidelity and quality.These cameras usually have a medium to large format, a high qualitylens, a large film magazine, a mount to hold the camera in a verticalposition and a motor drive.
  8. 8. Aerial Cameras One of the smaller models of aerial camera, dated 1907, kept in Duetsches Museum, Germany.Source: Curran, (1988).
  9. 9. Types of Aerial CamerasThere are many types of aerial cameras:Aerial mapping camera (single lens),Reconnaissance camera,Strip camera,Panoramic camera,Multilens camera, the multi camera array(multiband aerial camera) andDigital camera.
  10. 10. Aerial Mapping (Single Lens) Camera Aerial mapping cameras (also called as metric or cartographic cameras)are single lens frame cameras designed to provide extremely highgeometric image quality. They employ a low distortion lens system held in a fixed positionrelative to the plane of the film. The film format size is commonly a square of 230 mm on a side. Thetotal width of the film used is 240 mm and the film magazine capacityranges up to film lengths of 120 metres. A frame of imagery is acquired with each opening of the camerashutter, which is tripped at a set frequency by an electronic devicecalled an intervalometer. They are exclusively used in obtaining aerial photos for remote sensingin general and photogrammetric mapping purposes in particular. Single lens frame cameras are the most common cameras in use today.
  11. 11. Aerial Mapping CameraAn aerial mapping camera (Carl Zeris RMK/A15/23) with automatic levelling andexposure control. It is mounted on a suspension mount, between the remotecontrol unit (left) and its navigation telescope (right). Source: Curran, 1988).
  12. 12. Single Lens Frame Camera A typical aerial mapping camera and its associated gyro-stabilised suspension mount.The principal components of a singlelens frame mapping camera. Source: Lillesand et al, 2005.
  13. 13. Panoramic Aerial CameraIn panoramic cameras the groundareas are covered by either rotatingthe camera lens or rotating a prism infront of the lens.The terrain is scanned from side toside, transverse to the flight direction.The film is exposed along a curvedsurface located at the focal distancefrom the rotating lens assembly, andthe angular coverage can extendfrom horizon to horizon.Camera with a rotating prism designcontain a fixed lens and a flat filmplane. Scanning is accomplished byrotating the prism in front of the lens.The operating principle of apanoramic camera
  14. 14. Panoramic PhotographPanoramic photograph with 180 degree scan angle. Note image detail, largearea of coverage and geometric distortion. Area near the two ends of thephotograph are compressed. Source: Lillesand et al, 2005.
  15. 15. Multiband Aerial CamerasMultilens camera systemMulticamera arraycomprising four 70mm camera Imaging digital cameracomprising eightsynchronously operatingCCD-based digital cameras
  16. 16. Multiband Aerial Photo ofWaterfront Area, Cape Town
  17. 17. Geometric Properties of AP The most important geometric properties of an aerial photograph are those ofan angle and scale.Angle of Arial PhotographsThe angle at which aerialGeometric Photo Typesphotograph is taken is used toclassify the photograph into one ofthree types viz. vertical, highoblique and low oblique. Vertical photograph taken with a single lens is the most common type of aerial photography used in remote sensing applications. The vertical photography is taken with the camera axis pointing vertically downwards. Oblique photography is taken with the camera axis pointing obliquelyHigh oblique photography incorporates an downwards (intentional inclinationimage of the horizon into the photographs of the camera axis).while low oblique photographs do not.
  18. 18. Geometric Properties: Camera AngleA truly vertical aerial photographis rarely obtainable because ofunavoidable angular rotations ortilts, caused by the angular attitudeof the aircraft at the instant ofexposure.These unavoidable tilts cause slight(1 to 3 degrees ) unintentionalinclination of the camera opticalaxis, resulting in the acquisition oftilted photographs.Vertical photographs haveproperties similar to those of a mapwith a approximately constantscale over the whole photograph,and therefore can be used for Geometric Photo Typesmapping and measurements.
  19. 19. Taking Vertical AP: Flying Pattern
  20. 20. Photographic Coverage Along A Flight Strip a: conditions during exposure b: resulting photograph
  21. 21. Flying Pattern
  22. 22. Basic Geometric Elements ofVertical Photograph
  23. 23. Vertical Aerial Photograph Image IDClock Fiducial markLevel bubble defining the frame ofAltimeterreference for spatial measurements Frame No. Vertical photo taken from with a 230 x 230-mm precision mapping film camera showing Langenburg, Germany
  24. 24. Geometric Characteristics: Photo ScaleScale of Arial Photographs (Photographic Scale)The scale of a photograph expresses the mathematical relationshipbetween a distance measured on the photo and the correspondingdistance measured on the ground.A photograph scale is an expression that states one unit of distanceon a photograph represents a specific number of units of actualground distance.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 aerialphotographs have a range of scales that vary in proportion to theelevation of the terrain involved.The most straight forward method for determining photo scale is tomeasure the corresponding photo and ground distances betweenany two points. The scale S is then computed as the ratio of thephoto distance d to the ground distance D.S = photo scale = photo distance/ground distance = d/D
  25. 25. Aerial Photo ScaleThe scale of a photograph isdetermined by the focal length of thecamera and the vertical height of thelens above the ground.The focal length (f) of the camera isthe distance measured from thecentre of the camera lens to the film.The vertical hei