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Chapter 6

Chapter 6. Photogrammetry Introduction to Remote Sensing Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 4 November 2004. Outline. Introduction Basic principles Aerial photographs Photographic scale Area measurement

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Chapter 6

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  1. Chapter 6 Photogrammetry Introduction to Remote Sensing Instructor: Dr. Cheng-Chien Liu Department of Earth Sciences National Cheng Kung University Last updated: 4 November 2004

  2. Outline • Introduction • Basic principles • Aerial photographs • Photographic scale • Area measurement • Relief displacement（高差位移） • Image parallax （影像視差） • Ground control • Mapping • Flight planning

  3. Introduction • Photogrammetry • Definition • The science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs • Measurement • Distance, area, elevations • Product • Digital elevation models • Orthophotos • Thematic GIS data • Other derived products • Approach of this book • Hardcopy  softcopy • Aerial photos  spaceborne images

  4. Introduction (cont.) • Subjects • Determining horizontal ground distances and angles from measurements made on a vertical photograph • Using area measurements made on a vertical photograph to determine the equivalent areas in a ground coordinate system • Quantifying the effects of relief displacement (高差移位) on vertical aerial photographs • Determination of object height from relief displacement measurement • Determination of object heights and terrain elevations by measurement of image parallax • Use of ground control points • Mapping with aerial photographs • Preparation of a flight plan to acquire aerial photography

  5. Aerial photographs • Photogrammetry  Vertical photographs • Unintentional tilts: <10 (<30) • Fig 3.6 • Basic geometric elements of a vertical photo • L: the camera lens exposure station • f: the lens focal length • X-axis: the forward direction of flight • Y-axix: 900 counterclockwise from the positive x-axis • O: the ground principal point • ABCDE  abcde  a’b’c’d’e’ • The x y photocoordinates

  6. Aerial photographs (cont.) • Measurement of photocoordinates • Hardcopy • Triangular engineer’s scale  rudimentary problem Metric scale • Glass scale built-in magnifying eyepieces • Coordinate digitizer • Comparator mono • Softcopy • Affine coordinate transformation • Source of error

  7. Photographic scale • Photographic scale = map scale • Large scale  small scale • Eq. 3.1: S = d / D • Ex 3.1 • Eq. 3.2: S = f / H' • Fig 3.7 • Eq. 3.4: S = f / (H – h) • Ex 3.2 • Eq. 3.5: Savg = f / (H – havg)

  8. Photographic scale (cont.) • Vertical photo  map • Perspective projection (透視投影)  orthographic projection • Fig 3.8 • Relief displacement

  9. Area measurement • Accuracy • Measuring device • Image scale variation due to relief • Tilt in the photography • Simple way • Ex 3.4 • Ex 3.5 • Ex 3.6

  10. Relief displacement • Characteristics • Lean away from the center of the photograph • Fig 3.12 • Correcting for relief displacement • Fig 3.14(a). Displacement of terrain points • Fig 3.14(b). Distortion of horizontal angles measured on photograph • Relief displacement • The datum plane: A΄B΄ a΄b΄ • Terrain points AB  ab • a΄b΄: the accurate scaled horizontal length and orientation of the ground line AB. • Angle distortion: b΄c a΄ bca. • b΄oa΄= boa (no distortion) • Ex 3.8

  11. Image parallax • Characteristics • Principle: moving train viewing window  relative movement  distance • Fig 3.15: Parallax displacements on overlapping vertical photographs. • Conjugate principal points the flight axis (Fig 3.16) • Parallax: pa= xa-xa΄

  12. Image parallax (cont.) • Object height and ground coordinate location • Fig 3.17 • Parallax relationships on overlapping vertical photos • Air base: B = L - L΄ • Parallax equation • Example 3.9 • Difference in elevation • Parallax measurement • In example 3.9 • Parallax 2 measurements required (cumbersome) • Fig 3.18: single measurement  parallax • Stereopair photographs fasten down with flight aligned p=x-x΄=D-d single measurement • a and a΄ are identifiable • Difficult to identify if the tone is uniform

  13. Image parallax (cont.) • Parallax measurement in hardcopy system • Fig 3.19: floating-mark principle • Demonstration of stereoscope • Fig 3.21: how to take readings • Ex 3.10 • Parallax measurement in softcopy system • Image correlation • Fig 3.22 • Reference window • Search window • Not constrained to the assumption of parallax equations • Collinearity equations • xyz  XYZ  (XL, YL, ZL)  (w, f, k) • Aerotriangulation

  14. Ground control • Ground control • Definition • Refers to physical points on the ground whose ground positions are known with respect to some horizontal coordinate system and/or vertical datum • Horizontal • Vertical • GPS promising • Accuracy is essential • Cultural features, e.g. road intersection • Ground survey  artificial target premarked

  15. Mapping • Stereoscopic plotting instruments • Photogrammetry  topographic maps • Stereoplotters • Concept: • Stereopair photo: terrain  ray  lens  image plane • Stereoplotter: photos  ray  terrain model  3D view • Three components • A projection system • A viewing system • A measuring and tracing system • Fig 3.23: a direct optical projection plotter • Image  tracing table  stereoview of terrain model • Relative orientation  absolute orientation • Anaglyphic viewing system. • Color filter red, cyan • Only for panchromatic photo • Polarized platen viewer (PPV) • Polarizing filter • Stereo image alternator (SIA) • Rapidly alternate the projection of the two photos

  16. Orthophotos • Orthophotos • No scale, tile relief distortions  Photomaps • Best of both worlds • Input to GIS • Digital format • Generation  analog orthophotos • Differential rectification • Orthophotoscopes • Orthophoto negative • Generation  digital orthophotos

  17. Coordinate transformations • 2D conformal coordinate transformation • Scale change • Rotation • Translation • Redundancy • Matrix method • 3D conformal coordinate transformation • 2D projective coordinate transformation • Collinearity equation

  18. Flight planning • Why need new photographs? • Outdated • Wrong season • Inappropriate scale • Unsuitable film type • Planning the flight • Weather clear weather  beyond control • Multi-task in a single clear day • Time  10am~2pm  illumination max shadow min.

  19. Flight planning (cont.) • Planning the flight (cont.) • Geometric aspects • f • Format size • S • Area size • havg • Overlap • Side lap • Ground speed • Example 3.11 • H΄ • Location, direction, number of flight lines • Time interval • Number of exposures • Total number of exposures

  20. Homework • Use your own digital camera to take a stereopair. Examine your stereopair using the stereoscope that we demonstrated in the classroom.

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