Photogrammetry

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Chapter 4

• Photogrammetry
• Introduction to Remote Sensing
• Instructor Dr. Cheng-Chien Liu
• Department of Earth Science
• National Cheng-Kung University
• Last updated 23 April 2004

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4.1 Introduction

• Photogrammetry
• Measurements
• Maps
• Digital elevation models
• Other derived products
• Photogrammetry ?
• Where
• What areal extent

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4.1 Introduction (cont.)

• Subjects
• Determining horizontal ground distances and
  angles from measurements made on a vertical
  photograph
• 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

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4.1 Introduction (cont.)

• Subjects (cont.)
• Generations of maps in stereoplotters
• Generation of orthophotographs and digital
  elevation models.
• Preparation of a flight plan to acquire aerial
  photography.
• Application of soft copy or digital
  photogrammetry.

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4.2 Geometric elements of a vertical photograph

• Photogrammetry ? Vertical photographs
• Unintentional tilts lt10 (lt30)
• Fig4.1
• 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 ? abcde
• The x y photocoordinates

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4.2 Geometric elements of a vertical photograph
(cont.)

• Measurement of photocoordinates
• Triangular engineers scale ? rudimentary problem
• Metric scale
• Glass scale ? built-in magnifying eyepieces (Fig
  4.2)
• Coordinate digitizer
• Comparator mono (Fig 4.3)
• stereo
• Precision 15 mm

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4.2 Geometric elements of a vertical photograph
(cont.)

• Sources of error
• Lens distortion
• Atmospheric refraction
• Earth curvature
• Failure of the fiducial axes to intersect at the
  principal pt.
• Shrinkage or expansion
• Usually, correct this error
• Slight tilt ? outweigh other sources
• Example 4.1 treat it as the problem of exchange
  rate

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4.3 Determining horizontal ground lengths,
directions, and angles from photo coordinates

• Fig 4.4(a). Displacement of terrain points
• Fig 4.4(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)

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4.3 Determining horizontal ground lengths,
directions, and angles from photo coordinates
(cont.)

• Fig 4.5
• determination of ground coordinates
• ??LOAA??LOAa? ?XA(H-hA)xa/fLikewise
  XB(H-hB)xb/f
• ??LA?A?La?a ?YA(H-hA)ya/fLikewise
  YB(H-hB)yb/f
• AB(XA-XB)2(YA-YB)21/2
• Example 4.2

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4.3 Determining horizontal ground lengths,
directions, and angles from photo coordinates
(cont.)

• Fig 4.6
• determination of line length and direction from
  ground coordinates
• Example 4.3

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4.4 Relief displacement of vertical features

• Fig 4.7 the radial nature of relief displacement
• Relief displacement ? radial distance
• Fig 4.8
• geometric components of relief displacement.
• ??AA?A???LOA?? ?D/h R/H, d/r D/R?hdH/r
• Example 4.4
• relief displacement ? height

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4.4 Relief displacement of vertical features
(cont.)

• Premise
• Truly vertical photo
• Accurate H
• Clearly visible objects
• Precise location of the principal point
• Accurate measurement technique
• Correcting the image positions of terrain points
  appearing in a photograph
• Example 4.5

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4.5 Image Parallax

• Parallax
• Principle moving train ? viewing window ?
  relative movement ? distance
• Fig 4.9 Parallax displacements on overlapping
  vertical photographs.
• Conjugate principal points ? the flight axis (Fig
  4.10)
• Parallax pa xa-xa?

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4.5 Image Parallax (cont.)

• Fig 4.11
• parallax relationships on overlapping vertical
  photos.
• Air base
• Parallax equation
• Example 4.6
• Difference in elevation

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4.6 Parallax measurement

• In example 4.6
• parallax ? 2 measurements required (cumbersome)
• Fig 4.12 single measurement ? parallax
• Stereopair ? photographs fasten down with flight
  aligned px-x?D-d ? single measurement
• a and a? are identifiable
• Difficult to identify if the tone is uniform

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4.6 Parallax measurement (cont.)

• Employment
• Fig 4.13 Floating mark principle
• Half marks
• Left one fixed and right one moves along the
  fight direction ? fuse together ? one mark
  floating
• Parallax bar
• prC
• where r the parallax bar reading
• Cconstant
• Determination of c
• given p, measure r ? C p - r
• C S Ci
• Usually use the two principal points
• Example 4.7

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4.6 Parallax measurement (cont.)

• Parallax Wedge (Fig 4.16)
• Constitution 2 converging lines on a transparent
  sleet
• Can be thought of as a series of parallax bar
  reading
• Fig 4.17 determination of the height of a tree
  using a parallax wedge
• Example 4.8
• Measure absolute parallax

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4.7 Ground control for aerial photography

• Ground control
• Horizontal
• Vertical
• GPS ? promising
• Accuracy is essential
• Cultural features, e.g. road intersection
• Ground survey ? artificial target premarked

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4.8 Use of ground control in determining the
flying height and air base of aerial photographs

• Flying height determination
• Altimeter ? approximate H.
• S f /(H-h)
• Example 4.9
• Ground control ? H
• Given ground length AB
• elevations hA, hB
• focal length f.
• photocoordinates (xa, ya).(xb, yb) eg.
  (4,1) ? (4,4) ? H
• Iteration H2AB (H1-hAB) /AB1 hAB
• where hAB the average elevation of the two
  end points of AB
• Example 4.10

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4.8 Use of ground control in determining the
flying height and air base of aerial photographs
(cont.)

• Air Base determination
• Ground control ? B
• Given H one vertical control point eq(4.10) ? B
• Example 4.11
• Given two control points ? B
• Example 4.12

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4.9 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 4.18 a direct optical projection plotter
• Image ? tracing table ? stereoview of terrain
  model
• Relative orientation ? absolute orientation

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4.9 Stereoscopic plotting instruments (cont.)

• Stereoplotters (cont.)
• Fig 4.19 three projectors ? 2 adjacement
  stereopairs to be oriented at once
• 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.

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4.9 Stereoscopic plotting instruments (cont.)

• Tracing table platen
• Floating mark ? raise and low
• Platen table height ? terrain elevations
• Mapping features ? pencil
• Compile contours

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4.9 Stereoscopic plotting instruments (cont.)

• Viewing the photographs in stereo through a
  binocular system
• Mechanical or optical-mechanical projection
  plotters.
• Fig 4.20
• Coordinatiograph
• Electronic image correlator
• Fig 4.21 analytical stereoplotter

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4.10 Orthophotos

• Orthophotos
• No scale, tile relief distortions ? Photomaps
• Best of both worlds
• Input to GIS
• Digital format
• Generation ? analog orthophotos
• Differential rectification (Fig 4.22)
• Orthophotoscopes
• Orthophoto negative

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4.10 Orthophotos (cont.)

• Fig 4.23
• an early version of a direct optical projection
  orthophotoscope
• Principle of operation

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4.10 Orthophotos (cont.)

• Topographic orthophotomap
• Fig 4.24 operating principle of direct optical
  projection
• Fig 4.25contour line overlay orthophoto
  orthophotoscope
• Fig 4.26a contour map
• Fig 4.26b 3-D perspective view of the terrain
• Stereomates
• Fig 4.27 an orthophoto and a corresponding
  stereomate that may be viewed stereoscopically.

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4.11 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 ? 10am2pm ? illumination max shadow min.

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4.11 Flight planning (cont.)

• Planning the flight (cont.)
• Geometric aspects
• f
• Format size
• S
• Area size
• havg
• Overlap
• Side lap
• Ground speed
• Example 4.13

• H?
• Location, direction, number of flight lines
• Time interval
• Number of exposures
• Total number of exposures

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4.12 Soft copy photogrammetry

• Distinctions between traditional analog systems
  and digital systems
• Photographs ? digital raster images
• Mathematical modeling (computer-based
  environment)
• Sources digitized photos, digital cameras,
  electro-optical scanners,
• Trend from now to future