Transcript Chapter 3

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
4.1 Introduction
 Photogrammetry:
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Measurements
Maps
Digital elevation models
Other derived products
 Photogrammetry 
• Where
• What areal extent
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
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.
4.2 Geometric elements of
a vertical photograph
 Photogrammetry  Vertical photographs
• Unintentional tilts: <10 (<30)
 Fig4.1
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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
4.2 Geometric elements of
a vertical photograph (cont.)
 Measurement of photocoordinates
• Triangular engineer’s scale  rudimentary problem
Metric scale
• Glass scale  built-in magnifying eyepieces (Fig 4.2)
• Coordinate digitizer
• Comparator mono (Fig 4.3)
stereo
Precision: 1~5 mm
4.2 Geometric elements of
a vertical photograph (cont.)
 Sources of error
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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
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)
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/f
Likewise: XB=(H-hB)xb/f
• ∵△LA΄A~△La΄a ∴YA=(H-hA)ya/f
Likewise: YB=(H-hB)yb/f
• AB=[(XA-XB)2+(YA-YB)2]1/2
 Example 4.2
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
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
∴h=dH/r
 Example 4.4
• relief displacement  height
4.4 Relief displacement of
vertical features (cont.)
 Premise:
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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
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΄
4.5 Image Parallax (cont.)
 Fig 4.11
• parallax relationships on overlapping vertical
photos.
• Air base
• Parallax equation
• Example 4.6
• Difference in elevation
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 p=x-x΄=D-d  single
measurement
• a and a΄ are identifiable
Difficult to identify if the tone is uniform
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:
p=r+C
where r= the parallax bar reading
C=constant
Determination of c:
given p, measure r  C = p - r
C = S Ci
 Usually use the two principal points
• Example 4.7
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
4.7 Ground control for aerial
photography
 Ground control:
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Horizontal
Vertical
GPS  promising
Accuracy is essential
Cultural features, e.g. road intersection
Ground survey  artificial target premarked
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: H2=AB (H1-hAB) /AB1 +hAB
where hAB: the average elevation of the two end points of AB
Example 4.10
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
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
1. A projection system
2. A viewing system
3. A measuring and tracing system
• Fig 4.18: a direct optical projection plotter
Image  tracing table  stereoview of terrain model
Relative orientation  absolute orientation
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.
4.9 Stereoscopic plotting instruments
(cont.)
 Tracing table platen
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Floating mark  raise and low
Platen table height  terrain elevations
Mapping features  pencil
Compile contours
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
4.10 Orthophotos
 Orthophotos
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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
4.10 Orthophotos (cont.)
 Fig 4.23
• an early version of a direct optical projection
orthophotoscope
• Principle of operation
4.10 Orthophotos (cont.)
 Topographic orthophotomap
• Fig 4.24: operating principle of direct optical
projection
• Fig 4.25:contour 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.
4.11 Flight planning
 Why need new photographs?
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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.
4.11 Flight planning (cont.)
 Planning the flight (cont.)
• Geometric aspects
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f
Format size
S
Area size
havg
Overlap
Side lap
Ground speed
 Example 4.13
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H΄
Location, direction,
number of flight lines
Time interval
Number of exposures
Total number of
exposures
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