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, …
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