1. Photogrammetry
Photogrammetry is the science and technology of taking spatial measurements from photographs and preparing geometrically reliable derivative products.
The techniques are based on the geometry of perspective scenes and on the principles of stereovision.
Two kinds of photographs used in photogrammetry: aerial and terrestrial.
Aerial photographs are usually acquired from aircraft but can also come from satellites, hot air balloons or even kites. Terrestrial photographs come from cameras based on the ground, and generally are used in different applications from aerial.
There are two main data extraction methods used for analysing these photographs:
a. Quantitative: that is size, length, shape, height, area, etc.
b. Qualitative: geology, vegetation, drainage, land use, etc.
Historically, the most common use of photogrammetry was to produce hardcopy topographic maps, now it is used to produce a range of GIS data products such as DEMs, accurate raster images backdrops for vector data

2. Aerial photographs Vertical Oblique
Vertical photographs most commonly used, but true vertical photographs are rare because of the angular attitude of the aircraft at the time of photography.
This results in slight (1-3º) unintended inclination of the optic axis of the camera, resulting in titled photographs.
However, for most practical applications, such photographs can be considered vertical photographs.

3. Aerial photographs

4. Aerial photographs: Some terminology
Fiducial marks
Edge of format
Fiducial axis
Fiducial marks - Index marks, usually 4, at the center point of each side of an air negative or photo. These are rigidly connected with the camera lens through the camera body—which forms images on the negative. Usually are a hairline, a cross, or a half-arrowhead.
Principal Point - optical or geometric center of the photograph - the intersection between the projection of the optical axis (i.e., the perpendicular to the center of the lens) and the ground. Can be located by the intersection of lines between opposite side/corner fiducial marks.
Nadir - The nadir, also called vertical point or plumb point, is the image of the intersection between the plumb line directly beneath the camera center at the time of exposure and the ground. The nadir is important because relief displacement is radial from this point and is a function of the distance of the displaced image from it. Unlike the principal point, there are no marks on the photograph that permit to locate the nadir.
Isocenter - The point on the photo that falls on a line half- way between the principal point and the Nadir point.
Lens
X axis
Plumb line
Principal point
Nadir
Isocenter
Y axis

5. Determination of nadir in oblique photos
Principal point
Nadir
Isocentre
Relief displacement is radial from nadir, and is a function of the distance of the displaced image from it.
Unlike the principal point, there are no marks on the photograph that permit to locate the nadir.
However, in areas where tall and perfectly vertical objects (e.g., towers, smokestacks, electric poles, tall buildings, etc.) are clearly located on the photograph, the nadir point may be determined by projecting lines along the displaced edges of these buildings

6. Flightline of Vertical Aerial Photography
Jensen, 2000

7. Block of Vertical Aerial Photography
Jensen, 2000

8. Block of Vertical Aerial Photography Compiled into Photomosaic
Columbia, SC
Original scale = 1:6,000
Focal length = 6” ( mm)
March 30, 1993
Jensen, 2000

9. Geometry of Overlapping Vertical Aerial Photographs
Conjugate principal point: The point in the overlapping photo that is equivalent to the principal point of adjacent photograph

10. Geometry of A Vertical Aerial Photograph Obtained Over
Exposure station
Camera lens
Positive print/ transparency
Focal length (f)
Altitude ASL (H)
Elevation ASL (h)
SEA LEVEL
Negative (Reversal of tone and geometry
Geometry of A Vertical Aerial Photograph Obtained Over
Flat Terrain
Ground points are denoted in capitals, the corresponding points on the image are denoted in small letters.
- X axis along the flight direction
Y axis perpendicular to X
Principal point - Origin

11. Geometry of A Vertical Aerial Photograph Collected Over Flat Terrain

12. Photographic scale Scale (S) = Photo distance/ground distance = d/D
56.0’
0.112” 6 ’
0.012”
Scale (S) = f/H′ Or
Scale (S) = f/(H – h)
Scale is dependent on the flying height ~ terrain clearance ~ terrain elevation

13. Geometry of A Vertical Aerial Photograph Collected Over Variable Relief Terrain:
Geometric distortion
Variation in the terrane elevation would result in scale variations across the photograph
Scale (S) = f/(H – h)
Generally the average scale based on average height is given
Higher h => larger scale (more distance on the photograph)
Smaller h => smaller scale (less separation on the photograph)

14. Map – Orthographic projection – No relief displacement
Photo – Perspective projection – Varied scale - Relief displacement
Map vs Photograph
On a map we see a top view of objects in their true horizontal (planimetric) positions (Orthographic projection)
On a photograph they are displaced from their true map positions due to geometric distortions
(perspective projection)
Objects at higher elevations (closer to the camera) appear larger than the corresponding objects at lower elevations
Tops of the objects are displaced from their bases (relief displacement), which causes any object standing above the terrain to “lean” away from the principal point.

15. Measurement of Object Height From A Single Aerial Photograph Based on Relief Displacement
Principal point r d
Line of flight
Given that the flying height is H

16. Measurement of Object Height From A Single Aerial Photograph Based on Relief Displacement
h = dH/r
d –relief displacement on the photograph
r – radial distance from the principal point to the displaced image point
h – height above datum of the object point
H – flying height above the same datum chosen to reference h B′ D H h r d O R
ΔLOB′ ~ ΔBAB′
D/R = h/H
or, on the scale of the photograph
d/r = h/H
d = rh/H

17. Measurement of Object Height From A Single Aerial Photograph Based on Shadow Length on Level Terrain
Sun’s rays

18. Object Height Determined by
0.119”
59.1’
Object Height Determined by
Shadow Length
0.241”
119.65’

19. Image parallax
Apparent change in relative positions of stationary objects caused by a change in viewing position
Objects closer to the viewing position appear to move with respect to the objects farther away
Eye base
Distance to the object
Parallax angle
Parallax

20. Parallax: Look at apparent motion of object against distant background from two vantage points; knowing baseline allows calculation of distance:
distance (in parsecs) =
1/parallax (in arc seconds)
1 parsec ~ 3.3 ly

21. Image parallax
Objects closer to the aircraft-mounted camera (that is, at higher elevation) would appear to move with respect to objects at lower elevation when the position of the air craft changes in successive exposures.
These relative displacements or parallax form the basis of stereo viewing (depth perception)
PP1
PP2
Parallax = a –b a b

22. Parallax on overlapping vertical photos
Parallax displacement occurs parallel to the line-of- flight
In theory the line-of-flight should be parallel to the fiducial x axis. In reality, there is a slight offset - the true flight direction is along the line joining the principal point and the conjugate principal point.
The line-of-flight for a stereopair defines the photocoordinate x axis. Line drawn through the principal point perpendicular to the flight line defines photocoordinate y axis.
Parallax of a point (Pa) = xa – x′a ,
where xa is the measured x coordinate of the image point a on the left photo of the stereopair
x′a is the measured x coordinate of the image point a on the right photo of the stereopair

23. + ==> ax o a ax o f a′ a′x o′ hA XA O AX a′x o′ ax o a′x H L L L′
Parallex = Pa = xa - x′a
DATUM

24. a a′ f a′x o o′ ax a′x ax o hA H XA AX O L
Parallex = Pa = a′xax = xa - x′a
Lo = Focal length = f
LL = Air base = B M xa ax o
a′x
x′a L
Δa′xLax is similar to the ΔLL′AX
Therefore, H
DATUM

25. a a′ f a′x o o′ ax hA H YA OA XA AX O L
Parallex = Pa = a′xax = xa - x′a
Lo = Focal length = f
LL = Air base = B M ya
Now ΔLoax is similar to the ΔLOAAX
Therefore, H YA OA
DATUM

26. Parallax Equations a a′ f a′x o o′ ax hA H XA OA AX O L M L′ A Where:ya H OA
Where:
Δh - Difference in the elevation of two points whose parallax difference is Δp
H′ - Flying height above the lower point
Pa – Parallax of the higher point
DATUM

27. The height of the Senate Condominium in Columbia
Flying height above the base of the building – 3000 feet
Principal point 4-5
Principal point 4-4
x axis
r=4.164”
Xb’=-3.606”
Xb=-0.267”
d=0.247”
Xa=-0.270”
Xa’=-3.82”
y axis

28. Parallax measurement Parallax = x – x′ = D - d x x′ PP1 PP2 + + + + d

29. Ground Control Points
A point on the surface of the earth of known horizontal and vertical location (i.e. fixed within an established co-ordinate system and datum) which is used to geo-reference image data sources, such as aerial photographs, remotely sensed images, and scanned maps.
When mutually identifiable on the ground and on a photograph, GCPs are used to establish the exact spatial position and orientation of a photograph relative to the ground
Historically GCPs have been established through ground survey techniques, now a days GPS are more frequently used.
Accurate ground control is essential to all photogrammetric operations because photogrammetrical measurements can only be as reliable as the ground control

30. FLIGHT PLANNING Parameters: Focal length of the camera to be used
The film format size
Photoscale desired
Size of the area to be photographed
Average elevation of the area to be photographed
The overlap desired
Side-lap desired
Ground speed of the aircraft
Based on the above parameters, mission planner decides:
The flying height above the datum
the location, direction, and the number of flight lines to be made
the time interval between exposures
the number of exposures on each flight line
the total number of exposures for the mission

31. Area to be photographed
FLIGHT PLANNING
North
10 km
Direction of flight lines?
N-S
Area to be photographed
16 km
Flying Height?
H=f/s + Mean elevation = 0.23/(1/25000) = 4110 m
Camera characteristics:
f = mm
Film format = 230 mm
Photoscale: 1:25000
End-lap – 60%; side-lap 30%
Average elevation: 300m
Beginning and ending flight lines should be along the boundaries
Aircraft speed – 160 km/hr
Ground coverage per photo?
= film format size/scale = 0.23 m/(1/25000) = 5750 m on a side
Ground separation between photos (in the flight direction)?
Advance per photo = 40% (60% overlap) = 0.40*5750 m = 2300 m (Between photocentres)
Time between exposures?
= 2300m/160 km/hr = s

32. Area to be photographed
FLIGHT PLANNING
North
Because time can set in seconds, the number is rounded off. Recalculate the distance between photos?
10 km
51 sec/phot * 160 km/hr = 2267 m
Area to be photographed
16 km
Number of photos?
= m per line/2267 m/photo =9.1 (use 10)
Flight line separation?
Camera characteristics:
f = mm
Film format = 230 mm
Photoscale: 1:25000
End-lap – 60%; side-lap 30%
Average elevation: 300m
Beginning and ending flight lines should be along the boundaries
30% sidelap = separation of 70% of the coverage = 0.70 * 5750 m = 4025 m between flight lines
Number of flight lines?
= 10000/ = 3.48 (use 4)
Adjusted flight line space?
= 10000/(4-1) spaces = 3333 m
Total number of photos?
= 10 photos per line + 4 lines = 40 photos

33. Principles of Remote Sensing : NR –603
Atmospheric windows and effects, corrections
Multispectral systems
Characteristics of important remote sensing systems: LANDSAT, IRS, ASTER, SPOT;
High resolution sensors
Hyperspectral sensors
Thermal systems
Microwave systems
Geostationary systems (?)
Interpretations and applications - agriculture, forestry, land-use mapping, geology, water resources etc etc.
History and development of remote sensing
Electromagnetic radiation - nature and sources, interaction with matter and atmosphere
… and Arial Photography/Photogrammetry.