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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
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
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
introduction
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
introduction cont
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
aerial photographs
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
aerial photographs cont
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
photographic scale
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)
photographic scale cont
Photographic scale (cont.)
  • Vertical photo  map
    • Perspective projection (透視投影)  orthographic projection
    • Fig 3.8
  • Relief displacement
area measurement
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
relief displacement
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
image parallax
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΄
image parallax cont
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
image parallax cont1
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
ground control
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
mapping
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
orthophotos
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
coordinate transformations
Coordinate transformations
  • 2D conformal coordinate transformation
    • Scale change
    • Rotation
    • Translation
    • Redundancy
    • Matrix method
  • 3D conformal coordinate transformation
  • 2D projective coordinate transformation
    • Collinearity equation
flight planning
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.
flight planning cont
Flight planning (cont.)
  • Planning the flight (cont.)
    • Geometric aspects
      • f
      • Format size
      • S
      • Area size
      • havg
      • Overlap
      • Side lap
      • Ground speed
  • Example 3.11
  • Location, direction, number of flight lines
  • Time interval
  • Number of exposures
  • Total number of exposures
homework
Homework
  • Use your own digital camera to take a stereopair. Examine your stereopair using the stereoscope that we demonstrated in the classroom.