Lecture 2: Data Acquisition
LSGI3242A – Digital Terrain Modelling
(Courtesy to Prof. Zhilin Li. Please note that this set of slides is for internal use only.
The citations are not appropriately done due to time constraints.)
Objectives
1.
2.
3.
To outline different types of data sources.
To describe different types of data acquisition
techniques.
To discuss the accuracy of data from different
sources.
2
1. Data Sources



Terrain surfaces (direct measurements)
Images (aerial/space/terrestrial platforms)
Existing maps
3
Different techniques for different sources

Field surveying by using total station theodolite and
GPS (global positioning systems) for direct
measurement from terrain surfaces.
 Cartographic digitization by using existing
topographic maps and digitisers.
 Photogrammetry by using stereo-pairs of aerial (or
space) images and photogrammetric instruments
 Laser scanning: actively providing its own illumination
in the form of lasers
 SAR: radargrammetry, interferometry and
radarclinometry, actively providing its own
illumination in the form of microwaves
4
Direct measurement on
the terrain surface


The continents occupy 29.2% of the earth’s surface.
Relief varies from place to place, and is covered by
natural and cultural features, apart from water
Different measurement techniques may be used
because some techniques may be less suitable for
some areas.
5
From aerial and space images
 Aerial images are the most effective way to produce
and update topographic maps
 the most valuable data source for large-scale
production of high-quality DTM.
 taken by analogue/digital cameras mounted on
aerial planes.
6
From aerial and space images
Aerial photographs can be
classified into different types
based on different criteria:
1. Based on colour:
hyperspectral, multispectral
and monochromatic (or
panchromatic) photographs
2. Based on the attitude of
photography: vertical (i.e.
main optical axis vertical),
tilted(3), and oblique (>3)
photographs. Commonly
used aerial photographs are
tilted photograph
Source: Geodetic Alignment of Aerial Video Frames
7
Types of aerial photographs
based on angular field of view
3. Based on angular field of view: normal, wide
angle and super wide angle photography
8
Aerial camera and aerial photography
Aerial photo (negative)
f
Perspective centre (lens)
Aerial photo (positive)
H
Main optical axis
(a) An aerial Camera
(b) Geometry of aerial
photography
the scale of the aerial photograph:
9
 Form
 in analogue form – recorded on films;and
 in digital form – scanned/CCD (chargecoupled device) camera
Acquisition
photogrammetry
airborne scanners
space images
radar
Source: William Emery, Adriano Camps, in Introduction to Satellite Remote Sensing, 2017
10
From existing topographic maps
Every
country
has
topographic maps and
these may be used as
another main data source
for digital terrain modelling
These form a rich source of
data for digital terrain
modelling But for some
developing countries, the
data sources maybe poor
A topographic map
11
From existing topographic maps
 The largest scale of topographic maps which cover
the whole country with contour lines is usually
referred to as the basic map scale. It indicates the
best quality of DTM that can be obtained from
existing contour maps.
Some basic map scales :
China
1:50 000
UK
1:10 000
USA
1:24 000
 topographic maps  the metric quality
 contour map density of contour lines and the
accuracy of the contour lines themselves
12
Topographic maps at different scales
Map scales and commonly used contour intervals
Source: Konecny, G., Bahr, H., Reil, W., and Schreiber, H. 1979. Use of Spaceborne Metric
Camera for Cartographic Applications. Report to the Ministry of Research and Technology of FRG.
13
Map scales and commonly used contour accuracy
In general, it is expected that the height accuracy of
any point interpolated from contour lines will be
about to 1/2 to 1/3 of the contour interval (CI)
14
2. Data Acquisition Techniques
2.1 Photogrammetry
 The word photogrammetry comes from Greek words photos
(meaning “light”), gramma (meaning that which is drawn or written)
and metron (meaning “to measure”).
It originally signified
measuring graphically by means of light (Whitmore and Thompson,
1966)
 Photogrammetry and Remote Sensing is the art, science, and
technology of obtaining reliable information from noncontact
imaging and other sensor systems about the Earth and its
environment, and other physical objects and processes
through
recording, measuring, analyzing and representation
http://www.isprs.org/isprs.html
15
A pair of aerial photographs
with 60% overlap
overlapping
fiducial mark
16
The development of Photogrammetry
In 1849, A. Laussedat, an officer in the Engineering Corps of the
French Army, is regarded by many as the “father of photogrammetry”
 Photogrammetry has undergone four stages of
development, each of these cycles are approximately 50
years long:
 Plane table photogrammetry 1850 ~ 1900
 Analog photogrammetry
1900 ~ 1960
 Analytical photogrammetry 1960 ~ present
 Digital Photogrammetry
1990 ~ present
17
(a) Optical plotter
(b) Optical-mechanical plotter
(c) Analytical plotter
(d) Digital photogrammetric
workstation
18
Photogrammetry

Forest inventory with photogrammetric point
cloud


https://www.youtube.com/watch?v=DfYJiYupjgA
Stereo plotter

https://www.youtube.com/watch?v=HD9iMTjtmY8
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The characteristics of the four stages of
photogrammetry
20
Basic principles
 The fundamental principle of photogrammetry is to
make use of a pair of stereo images (or simply stereo
pair) to reconstruct the original shape of 3D objects
Lateral overlap
forward overlap
21
A pair of aerial photographs
with 60% overlap
overlapping
fiducial mark
22
To measure the 3-D coordinates of the objects on
the stereo model
Two
overlapping
photographs
3D model
stereocomparator
23
Collinearity equation
S1
S2
a
a’
xf
a1 ( X A  X S )  b1 (Y A  YS )  c1 ( Z A  Z S )
a3 ( X A  X S )  b3 (Y A  YS )  c3 ( Z A  Z S )
y f
a 2 ( X A  X S )  b2 (Y A  YS )  c 2 ( Z A  Z S )
a3 ( X A  X S )  b3 (Y A  YS )  c3 ( Z A  Z S )
Z
Y
X
A
A stereo-model is formed by
projecting images points
from a stereo pair
The mathematical express
24
About the mathematical expression
 In the mathematical expression, XYZ is a geodesic
coordinate system; and
(i=1,2,3)
are the functions of the three angular orientation
elements (i.e. φ, ω, κ) as follows:
a1  cos  cos   sin  sin  sin 
b1  cos  sin   sin  sin  cos 
c1  sin  cos 
a 2   cos  sin 
b2  cos  cos 
c 2  sin 
a3  sin  cos   cos  sin  sin 
b3  sin  sin   cos  sin  cos 
c3  cos  cos 
25
Basic orientations
 Interior orientation
using fiducial marks’ image coordinates to unify the image
coordinate system
 Relative orientation
to restore the stereo model by removing the Y parallax using
at least 6 points’ observations
 Absolute orientation
to scale and orient the stereo model based on the GCP
26
Epipolar geometry
Each pair of bundle rays must be coplanar with the base
27
DPW: Digital Photogrammetric Workstation
VirtuoZo
Automatic image processing: matching
28
GPS/INS for georeferncing
Towards:
On-the-fly
processing
Real-time
photogrammetry
29
2.2 Radargrammetry and SAR Interferometry
 Radargrammetry acquires DTM data through the
measurement of parallax
 InSAR acquires DTM data through the determination of
phase shifts between two echoes;and
 Radarclinometry acquires DTM data through shape
from shading
Space Shuttle
Endeavour
30
Synthetic Aperture Radar (SAR)
Synthetic aperture imaging radar (SAR) is
a microwave imaging radar developed in
the 1960's to improve the resolution of
traditional (real aperture) radar
It receives and records echos reflected by
the target, and then maps the intensity of
the echo into a grey scale to form an image.
It is able to take clear pictures day and
night under all weather conditions
31
Synthetic Aperture Radar (SAR)
L
Flying track (Orbit)
w
Antenna
Antenna
Image Plane
e
f
Slant range
R
Projected Orbit
Flying
Height
Mid slant range Rm
Far slant range
Swath
Near slant range
H
Nadir
Y
X
Footprint
Y
Nadir
Cross track
Radar imaging geometry
E
F
Cross Track
Projection of radar image
The angular fields in the flying direction and the cross
track direction are related to the width (w) and the
length (L) of the radar antenna
32
The resolution of the radar image
 The minimum distance between
two distinguishable objects, the
most important measure of radar
image quality
 It is defined by the azimuth
resolution in the flying direction (△x)
and by the slant range resolution in
the slant rage direction (△R) or the
ground range resolution in the
cross track direction (△y)
 △y decreases near to the nadir, it
is the reason why SAR is always
side-looking
Resolution of radar images
33
The principle of SAR imaging
 The azimuth resolution (△x)
is dominantly determined by
the position and size of the
antenna
 If a C-band microwave
(5.66cm) real aperture radar
onboard the satellite is
employed to take images with
an azimuth resolution of 10m
from 785km away, the
required length of its aperture
is longer than 3km
Imaging geometry of SAR
34
The principle of SAR imaging
 The azimuth resolution (△x) of the synthetic aperture radar (SAR)
is much improved based on the principle of the Doppler
frequency shift caused by the relative movement between the
antenna and the target
 Indeed, it means that the azimuth resolution (△x) of a SAR is
only determined by the length of the real aperture of an antenna,
independent of the slant range R and the wavelength
 As a result, it is possible to acquire images with 5m azimuth
resolution by an SAR with a 10m real aperture length
onboard ERS-1/2
35
An example of the SAR image
Direction of slant range
Azimuth
Pixel
a  b  i  a 2  b2  e i
the plane coordinate system of
the SAR image and the complex
number expression of the pixel
It is the use of phase information that makes InSAR technology special
36
Interferometric SAR (InSAR)
 A pair of SAR images of the
same area taken at slightly
different positions can be used
to form an interferogram and
the phase differences recorded
in the interferogram can be
used to derive topographic map
of the earth’s surface. This
technology is called
Interferometric SAR (InSAR), or
SAR interferometry
USGS: Volcano Hazards Program
37
Principles of InSAR
 InSAR is a signal processing technique
rather than an instrument at the present
time
 It derives height information by using the
interferogram
Slant range
Phase component
Wavelength
38
Principles of InSAR
39
The process of DTM data acquisition by InSAR
40
An example of InSAR interferogram
Latitude
Longitude
41
Contour diagram of DTM of the same area
(produced from DTM generated by InSAR)
Coast line
Taiwan
Straits
Latitude
Longitude
42
Radargrammetry
Similar to photogrammetry,
radargrammetry is to form a
stereo
model
for
3D
measurement
In radargrammetry, two SAR
images collected with the
same-side or opposite-side
geometry are used to form the
stereo model
43
DEM based on RADARSAT stereopair
44
Principles of Radargrammetry
 3D reconstruction relies on
 determining the sensor-object stereo model
 searching for corresponding pixels from two
overlapping SAR images using imaging matching
techniques; and
 determination of 3D coordinates by solving the
intersection problem
45
Stereo configuration of radargrammetry
V1
V2
S2
S1
R2
S2
R1
S1
Z
P
P
O
Y
X
46
Factors affect accuracy of
DTM by radargrammetry
 terrain features such as
topographic slopes
 geographical conditions and
geometric
distortions
in
relation to radar look angles;
and
 intersection angles (oppositeside stereo configuration is
superior to the same-side
stereo)
47
22.6
Hong Kong ( ERS-1 SAR images )
800
22.5
700
600
500
22.4
400
300
200
22.3
100
(a) on 2 Mar, 1996
(b) on 18 Mar, 1996
113.8
0
113.9
114.0
DTM generated by (a) and (b)
114.1
48
2.3 Airborne laser scanning (Lidar)
ALS system: airborne LIDAR (LIght Detection And Ranging)
An example of 3D city model acquired by Lidar
49
Introduction

LiDAR stands for Light Detection And Ranging.
 Topographic airborne LiDAR is a laser profiling and
scanning system for topographic applications
emerged commercially in the mid-1990s.
 LiDAR does not only serve for topographic mapping
applications.
 It also serves for meteorology and atmospheric
environment applications.
 This course focuses on topographic
airborne LiDAR remote sensing.
50
Introduction
Mobile Mapping by Google
Satellite LiDAR (NASA CALIPSO)
Terrestrial LiDAR (Leica)
Police Laser Gun
Atmospheric LiDAR (SOR)
51
Airborne laser scanning system
 a laser range finder (LRF)
 a computer system to
control the on-line data
acquisition
 a storage medium
 a scanner, and
 a GPS/INS system for
determining the position and
orientation of the system
52
Airborne laser scanning system
It is the type of materials
hit by the pulses.The
wavelength of the laser
lies in, or just above, the
visual
range
of
the
electromagnetic spectrum,
i.e. in the range of 10401060 nm.
53
Laser Ranging



The laser ranging is the instrument that constructs
and emits the laser as aforementioned and records
the returned laser pulse in order to derive the
distance between the aircraft and the ground.
To determine the range measurement, the time pulse
method and the phase comparison method are
commonly used.
In the time pulse method, the distance between the
range unit and the point of the reflected ground
feature can be determined using the following
equation:R
t2
C
54
Laser Ranging


where t is the traveling time of the laser pulse from the
laser ranger to the reflected object and back from the
reflected object to the laser ringer, c is the speed of the
light and R is the range distance.
If the laser transmits as continuous waveform (sinusoidal
signal), the phase comparison method is used. The
mathematical expression of phase comparison method:
t
T  nT
2


where n is the number of full wavelengths, T is the period
of the signal, Ø is the phase difference between the
received and transmitted signal.
By obtaining the value t , the range distance can be
solved.
55
Laser Scanning

The laser scanning device is an optical scanning
mechanism with rotating mirror for cross track
scanning (perpendicular to the flight direction) and
makes a small footprint on the ground for each laser
emission.
56
Laser Intensity

Laser energy refers to the radiometric properties (or so
called LiDAR intensity) in laser scanning.
The laser energy is modelled by the laser range equation:-

PT GT  D 2
Pr 
 atm sys
2
2
4R 4R 4






where
Pr , PT = the received / transmitted laser power
D = the aperture diameter (m); R = range (m)
σ = effective target cross section
GT = antenna gain (= 4π / θT2), θT is the transmitter beam
width (θT = kλ / D), λ is the wavelength (m) and k = const.
ηatm ,ηsys = losses due to atmosphere / system inefficiency
57
From laser point cloud to DTM
 The collected LiDAR data has the following fields:
 x, y, z, I, number of returns, return number,
GPS time, etc (see LAS Specification).
 The process of acquiring ALS data:
 filtering--noise, outliers or gross errors
 classification--buildings or vegetation; and
 modelling
 Accuracy (depending on a number of factors):
 Vertical accuracy: 0.1 to 0.5 m
 Horizontal accuracy: < 0.3m
58
Obtained DTM from DSM using ground filtering
Remove the
above ground
features:
vegetation,
buildings.
59
2.4 Cartographic digitisation
Manual line
following
Automated
Linefollowing
Automated line
following
Manual scanning
Manual
Scanning
Automated
scanning
Cartographic digitization methods
60
Line-following digitalization
 a mechanically-based digitisation system
 a solid-state digitising tablet
An example of tablet digitizer
61
manual line-following digitisation
 operator doesn't need to do the line following
 data redundancy ; and
 the fidelity of the results to the original line
62
semi-automated line following devices
An operator is still required to
 supervise the system
 execute various operations such as the initial
positioning of the device on contours
 guide the device through areas of closely-packed
contours and cliffs
 insert contour elevation values, etc.
 the system is very expensive
63
Raster scanning
make fully automated digitisation possible
 Each line scan is divided into resolution units
 0 -- if nothing is present
 1 -- if there is a line
 Vectorisation follows can be manual (on-screen
digitization) or automated
Scan Head
Scan Head
X
Map
Y
Map
X
Y
drum scanners
flat-bed (right) scanners
64
Examples
Vector
contour
lines
Original raster map
65
2.5 GPS for direct data acquisition
The GPS satellite constellation
66
About GPS

Global Positioning System

Full name: NAVSTAR GPS (NAVigation Satellite
Timing And Ranging Global Positioning System)

Developed and owned by the US Department of
Defense (DoD)

Provides 24-hour world-wide positioning capability
67
GPS Segments
Space segment
 GPS satellites orbiting around
the earth and sending GPS
signals to the earth
 Control segment
 Stations on the earth that
monitor and control the
satellites
 User segment
 Any body who has the devices
(GPS receivers) to receive GPS
signals

68
The principles of GPS measurement
 Principle: range intersection
 the positions of satellites
 3 or more distances from the
receiver to the satellites
卫星
卫星
卫星
The orientation principle
of GPS
目标
69
To measure the distance
Suppose a satellite sends a signal to a GPS receiver
and it takes t seconds for GPS receiver to receive the
signal
D=c×( -
)
Where D is the distance,
represents the time when
the satellite transmits the signal ,
represents the
time when the signal reaches the GPS receiver, c is the
velocity of light, i.e., 299,792,458 metres per second
70
Error in clock
 cause significant error in the distance computed
 make GPS receivers very expensive
 less popular GPS application
 Solution
 Assume a constant error between the clock in the
GPS receiver and the clocks onboard the satellites
 A total of four or more satellites to be observed to
determine the position of a point (x,y,z,t)
 Differential GPS (DGPS): the satellite clock error
and atmospheric effects are diminished
71
Differential GPS and its service
constantly compare the real position with the position given by the GPS
system, use this error-information to improve GPS performance in a wide
range (up to hundreds of kilometers) around the base station
72
Principles of traditional surveying techniques
 through the measurement of distances and/or angles
 by theodolites and computerised total stations
P
P
D1
D
A

(a) From a known point A to
determine the position of P
A
1
D1
2
B
(b) From 2 known points A and B to
determine the position of P
73
3. A comparison between DTM data
from different sources
74
End
Q&A
75