1. 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

2. 4.1 Introduction Photogrammetry: Photogrammetry  Measurements Maps
Digital elevation models
Other derived products
Photogrammetry 
Where
What areal extent

3. 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. 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.

5. 4.2 Geometric elements of a vertical photograph
Photogrammetry  Vertical photographs
Unintentional tilts: <10 (<30)
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  a’b’c’d’e’
The x y photocoordinates

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

13. 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΄

14. 4.5 Image Parallax (cont.) Fig 4.11
parallax relationships on overlapping vertical photos.
Air base
Parallax equation
Example 4.6
Difference in elevation

15. 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

16. 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

17. 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

18. 4.7 Ground control for aerial photography
Horizontal
Vertical
GPS  promising
Accuracy is essential
Cultural features, e.g. road intersection
Ground survey  artificial target premarked

19. 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

20. 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

21. 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

22. 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.

23. 4.9 Stereoscopic plotting instruments (cont.)
Tracing table platen
Floating mark  raise and low
Platen table height  terrain elevations
Mapping features  pencil
Compile contours

24. 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

25. 4.10 Orthophotos Orthophotos Generation  analog 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

26. 4.10 Orthophotos (cont.) Fig 4.23
an early version of a direct optical projection orthophotoscope
Principle of operation

27. 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.

28. 4.11 Flight planning Why need new photographs? Planning the flight
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.

29. 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

30. 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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