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Principles of Photogrammetry Ayman F. Habib1
Chapters 1-4: Summary
• So far, we have been investigating the image acquisition process.– Chapter 1: General introduction– Chapter 2: Radiation source and properties– Chapter 3: Radiation interaction with the lens system
• Factors affecting the precision and the accuracy of the image coordinate measurements
– Chapter 4: Radiation interaction with the light sensitive material along the focal/image plane
• Analog & digital imaging systems
Principles of Photogrammetry Ayman F. Habib2
ENGO 431: Chapter 5
Vertical Photography
Principles of Photogrammetry Ayman F. Habib3
Overview
• Image versus map characteristics• Vertical photography: definitions and
characteristics• Image scale• Mathematical relationship between corresponding
image and ground coordinates• Relief displacement
Principles of Photogrammetry Ayman F. Habib4
An Image Versus a Map
Perspective center
Image Plane
A’ B’ C’ D’ E’
A BC
DE
e d c b a
Datum
Image
DatumA’ B’ C’ D’ E’
A BC
D E
a b c d e Map
Map
Principles of Photogrammetry Ayman F. Habib5
An Image Versus a Map
• Images have the following properties:– Perspective projection, and– Non-uniform scale.
• Maps, on the other hand, have the following characteristics:– Orthogonal (parallel) projection, and– Uniform scale.
Principles of Photogrammetry Ayman F. Habib6
Cone
CylinderBuilding
Orthogonal ProjectionPerspective Projection
Cone
Building Cylinder
Perspective Versus Orthogonal Projection
Principles of Photogrammetry Ayman F. Habib7
Perspective Projection Orthogonal Projectionhttp://www.e-topo.com/etoposite/pages/ortho_photography.aspx
Perspective Versus Orthogonal Projection
Principles of Photogrammetry Ayman F. Habib8
Perspective Projection
Orthogonal Projection
Perspective Versus Orthogonal Projection
Principles of Photogrammetry Ayman F. Habib9
• Perspective Projection • Orthogonal Projectionhttp://www.swisstopo.admin.ch/internet/swisstopo/en/home/products/images.html
Perspective Versus Orthogonal Projection
Principles of Photogrammetry Ayman F. Habib10
Vertical Photography
• Optical axis coincides with the plumb line → vertical image.
Principles of Photogrammetry Ayman F. Habib11
• Vertical images are taken with the camera optical axis coinciding with the plumb line (True Vertical Image).
• Nearly Vertical Image: There is a tilt angle between the camera optical axis and the plumb line of ±3˚.
Vertical Photography
Principles of Photogrammetry Ayman F. Habib12
Basic Elements of a Vertical Aerial Image
Flying heightabove datum (H)Perspective center
Principal distance Focal length
x-axis
y-axis
Format size
Flight direction
Fiducial markFiducial center (F.C.)
Principal point (p.p.)
Nadir point (n)
xo
yo
Principles of Photogrammetry Ayman F. Habib13
Fiducial Marks in Metric Cameras
Principles of Photogrammetry Ayman F. Habib14
Basic Definitions• Nadir Point (n): The intersection of the plumb
(gravity) line through the perspective center with the image plane.
• Principal Point (PP): The intersection of the normal to the image plane through the perspective center with the image plane.– The normal to the image plane is assumed to coincide
with the optical axis.• Principal Distance (c): The normal distance between
the perspective center and the image plane (compare with the focal length - refer to the lens equation).– Sometimes, it is denoted as the camera constant.
Principles of Photogrammetry Ayman F. Habib15
Basic Definitions
• Flying Height (H): The elevation of the perspective center above the stated datum.
• X-axis of the image coordinate system:– The line in the image plane through opposite fiducial
marks that are almost parallel to the flight direction.
• Y-axis of the image coordinate system:– The line in the image plane through opposite fiducial
marks that are almost normal to the flight direction.
Principles of Photogrammetry Ayman F. Habib16
Image Scale
Perspective center
Ground
Average terrain
Flying height H
Focal length f
h(average)h1
A B
b a
Principles of Photogrammetry Ayman F. Habib17
Image Scale
• Image scale: It is the ratio between a distance on the image and the corresponding distance on the ground.
• Since the image is a central projection, it does not have a uniform scale (only one exception exists).
• Image Scale = ab / AB = c / (H - h) – Assuming vertical photography
• Exception: Vertical image over a flat horizontal terrain has a uniform scale (i.e., it can be used as a map).
Principles of Photogrammetry Ayman F. Habib18
Tilt Effect on Image Scale
http://civisit.com/blog/?p=52
Principles of Photogrammetry Ayman F. Habib19
From Image to Ground Coordinates
Principles of Photogrammetry Ayman F. Habib20
From Image to Ground Coordinates
• Objective:– Derive the ground coordinates of object points from the
measured coordinates of the corresponding image points.
• Assumptions:– We are dealing with a vertical image.– We are dealing with an image captured by frame
camera:• Captured by analog or digital camera• Analog or digital format• Diapositive
Principles of Photogrammetry Ayman F. Habib21
Frame Camera
• The image footprint is captured through a single exposure.
Footprint
Focal Plane
Perspective Center
Principles of Photogrammetry Ayman F. Habib22
Negative Versus Positive
Negative
Diapositive
Footprint
Perspective Center
Principles of Photogrammetry Ayman F. Habib23
Flight Direction
xaya
a
Image Coordinate Systems
x
z
yPerspective Center
• Assumption: principal point coincides with the Fiducial center.
Principles of Photogrammetry Ayman F. Habib24
Analog Camera: RC30
http://www.kasurveys.com/Sensors.html
Principles of Photogrammetry Ayman F. Habib25
Fiducial Marks in Metric Cameras
Principles of Photogrammetry Ayman F. Habib26
Image Coordinate System: Digital Images
• Digital images can be acquired through either:– Scanning analog images– Direct use of digital cameras
• For scanned analog images, the image coordinate system is defined in the same way as analog imagery captured by metric cameras.
• For digital images captured by digital cameras, the image coordinate system is defined by the central row(s) and the central column(s).
Principles of Photogrammetry Ayman F. Habib27
Example of Photogrammetric Scanner
http://cmapspublic.ihmc.us/rid=1J5T5YMZV-15ZNLP5-1JMD/Balayeur%20optique.bmp
Principles of Photogrammetry Ayman F. Habib28
Digital Camera: DMCTM
http://cmapspublic.ihmc.us/rid=1235786299998_244221932_24870/Photogramm%C3%A9trie%20-%20cam%C3%A9ras%20num%C3%A9riques.jpg
Principles of Photogrammetry Ayman F. Habib29
Digital Images captured by Digital Cameras
x`
y`
Pixel Coordinates
y
x
Image Coordinates
Principles of Photogrammetry Ayman F. Habib30
Pixel to Image Coordinate Transformation
directioncolumnthealongsizePixelsizepixydirectionrowthealongsizePixelsizepixx
rowsofNumberncolumnsofNumbern
wheresizepixxxnysizepixynyx
r
c
r
c
____
:__)0.2/(__)0.2/(
Principles of Photogrammetry Ayman F. Habib31
Ground Coordinate System
X
Z
Y
A
XA
YA
ZA
Principles of Photogrammetry Ayman F. Habib32
Ground Coordinates from Image Coordinates
X
Z
Y
x
zy
PC
A
XA
YA
ZA
Principles of Photogrammetry Ayman F. Habib33
• Assumptions:– Vertical imagery,– The image and ground coordinate systems are parallel,– The origin of the ground coordinate system is vertically
below the perspective center, and– The principal point coincides with the Fiducial center
• From similar triangles, one gets:– XA = xa (H - hA) / c– YA = ya (H - hA) / c
Ground Coordinates from Image Coordinates
Principles of Photogrammetry Ayman F. Habib34
X
Positive diapositive contact print
Perspective center
Negative plane
f
f
Flying height (H)
Groundlevel
Y
xy
A
B
C
cb
a
a
bc
p
Ground Coordinates from Image Coordinates
Principles of Photogrammetry Ayman F. Habib35
Single Photo Positioning?
• Question:– Can we derive the three-dimensional coordinates of an
object point from a single photo?
• Answer:– No
• We assumed that we know the height of the object point under consideration.
Principles of Photogrammetry Ayman F. Habib36
(Single Light Ray)
Perspective Center
a (image point)
h
A
Single Photo Positioning?
Principles of Photogrammetry Ayman F. Habib37
Relief Displacement
Principles of Photogrammetry Ayman F. Habib38
Relief Displacement
f Focal length
Datum
R
H
H-hA
hA
A
A’ P
Ao
a a’ o
r'
rd (Relief displacement)
Principles of Photogrammetry Ayman F. Habib39
Relief Displacement
• The shift in the photographic position of an image point caused by the height of the corresponding object point above or below the datum.
• From similar triangles, one gets:– da = r hA / H
• Relief displacement occurs along the radial direction from the nadir point.
• For vertical imagery over flat horizontal terrain, the effect of relief displacement simulates a uniform change in the scale.
Principles of Photogrammetry Ayman F. Habib40
Relief Displacement
• Assumption: nadir point coincides with the Fiducial center.
Principles of Photogrammetry Ayman F. Habib41
Relief Displacement
Principles of Photogrammetry Ayman F. Habib42
Patch from the left image Patch from the right image
Relief Displacement
• Where are the principal points?
PP
PP
Principles of Photogrammetry Ayman F. Habib43
Relief Displacement
Light pole
Principles of Photogrammetry Ayman F. Habib44
Relief Displacement
• Relief Displacement increases with the radial distance.
Principles of Photogrammetry Ayman F. Habib45
Relief Displacement
• Relief Displacement is directly proportional to:– Radial distance from the nadir point, and– Object height above the datum.
• Relief Displacement is inversely proportional to:– Flying height above the datum.
• Relief displacement causes occlusion.
Principles of Photogrammetry Ayman F. Habib46
Relief Displacement: Characteristics
• Relief displacement is outward for points whose elevations are above the datum (diapositive).
• Relief displacement is inward for points whose elevations are below the datum (diapositive).
• Relief displacement occurs along radial direction from the nadir point of the image.– For vertical photographs: The nadir point (n), the
principal point (PP), and the Fiducial center (FC) are very close to each other.
Principles of Photogrammetry Ayman F. Habib47
Outward displacement
Relief Displacement: Characteristics
Inward displacement
Principles of Photogrammetry Ayman F. Habib48
Relief Displacement & Occlusion
Occluded Area
Principles of Photogrammetry Ayman F. Habib49
Relief Displacement & Occlusion
http://www.e-topo.com/etoposite/pages/ortho_photography.aspx
Principles of Photogrammetry Ayman F. Habib50
Occluded Area
Relief Displacement & Occlusion
http://www.e-topo.com/etoposite/pages/ortho_photography.aspx
Principles of Photogrammetry Ayman F. Habib51
Patch from the left image Patch from the right image
• Where are these patches relative to the original images?
Relief Displacement & Occlusion
Principles of Photogrammetry Ayman F. Habib52
Relief Displacement & Occlusion