1. Photogrammetry Chapter 6 Introduction to Remote Sensing
Instructor: Dr. Cheng-Chien Liu
Department of Earth Sciences
National Cheng Kung University
Last updated: 4 November 2004

2. 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

3. Introduction Photogrammetry Approach of this book 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

4. 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

5. Aerial photographs Photogrammetry  Vertical photographs Fig 3.6
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

6. 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

7. Photographic scale Photographic scale = map scale Eq. 3.1: S = d / D
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)

8. Photographic scale (cont.)
Vertical photo  map
Perspective projection (透視投影)  orthographic projection
Fig 3.8
Relief displacement

9. Area measurement Accuracy Simple way Measuring device
Image scale variation due to relief
Tilt in the photography
Simple way
Ex 3.4
Ex 3.5
Ex 3.6

10. Relief displacement Characteristics Correcting for relief displacement
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

11. 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΄

12. 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

13. 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

14. Ground control Ground control Definition Horizontal Vertical
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

15. 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

16. 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
Orthophotoscopes
Orthophoto negative
Generation  digital orthophotos

17. Coordinate transformations
2D conformal coordinate transformation
Scale change
Rotation
Translation
Redundancy
Matrix method
3D conformal coordinate transformation
2D projective coordinate transformation
Collinearity equation

18. 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.

19. Flight planning (cont.)
Planning the flight (cont.)
Geometric aspects f
Format size S
Area size
havg
Overlap
Side lap
Ground speed
Example 3.11 H΄
Location, direction, number of flight lines
Time interval
Number of exposures
Total number of exposures

20. Homework
Use your own digital camera to take a stereopair. Examine your stereopair using the stereoscope that we demonstrated in the classroom.