Introduction to Airborne LiDAR and Physical Principles of - iitk.ac.in

129 Pages·2008·13.46 MB·English

Introduction to Airborne LiDAR and
Physical Principles of LiDAR Technology
(Lectures 1 and 5)
E. Baltsavias
Institute of Geodesy and Photogrammetry
ETH Zurich, CH-8093 Zurich, Switzerland
[email protected]
www.photogrammetry.ethz.ch
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Acknowledgements
For this presentation material has been used, without or with modifications, from various
ccoolllleeaagguueess, oorrggaanniissaattiioonnss aanndd ccoommppaanniieess, wwhhoo II wwaanntt ttoo tthhaannkk::
- C. Brenner, Leibniz University of Hannover
- D. Fritsch, J. Kilian, A. Wehr (Univ. of Stuttgart)
- G. Vosselman (ITC)
- U. Lohr ((at that time with firm Topposyys))
-A. Streilein (Swisstopo = Swiss Federal Office of Topography)
- N. Pfeifer (Technical University of Vienna)
- Firms Leica, Riegl, Optech, IGI, Toposys, Fugro, Swissphoto
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Contents
- HHiissttoorryy
- Basic components of ALS and functioning
- Range measurement principles
- Interaction of laser beam with targets and full waveform digitising
- Basic error sources
- PPrroocceessssiinngg oovveerrvviieeww,, ppooiinntt ccllaassssiiffiiccaattiioonn ((ffiilltteerriinngg)) aanndd ssttrriipp aaddjjuussttmmeenntt
- Quality control of data
- Overview of commercial systems
- Overview of applications and examples
- Bathymetric lidar
-AA sshhoorrtt ccoommppaarriissoonn ooff aaiirrbboorrnnee llaasseerr ssccaannnniinngg ttoo ootthheerr rreemmoottee sseennssiinngg tteecchhnnoollooggiieess
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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History
First optical laser developed in 1960 (Maiman, USA)
FFiirrsstt aaiirrbboorrnnee llaasseerr rraannggiinngg ttrriieedd oouutt iinn 11996600ss
Started to be developed from early and middle 1970s, espec. in N. America, particularly
for hydrographic and bathymetric applications
First in late 1980s, the use of GPS made accurate range measurements from airborne laser
profilers possible (Univ. of Stuttgart, Prof. Ackerman)
Beginning of 90s profilers replaced by scanners (ALS), and GPS combined with INS
1996 ISPRS Congress in Viennna: one ALS manufacturer, some reports and tests with ALS
1996-2000: work of several ISPRS Working Groups (WG) and one OEEPE WG on ALS.
Publication of a special issue of ISPRS Journal of Photo & RS gives a good overview.
Since 2000:ALS increasingglyy used in ppractice and in various apppplications;; increasingg
scientific investigations and tests and better methods; continuously improving ALS systems,
incl. waveform digitizing (espec. from 2004) and simultaneous double ALS (from 2006);
more and better software; more service providers and users
Often the term LiDAR is used: Light Detection And Ranging
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Basic components and functioning
- Active sensor: a very narrow, high energy ray is sent from a source (laser) to the scene,
reffllecttedd bbackk andd recorddedd. AActtiive means iitt workks dday andd niighhtt ((even bbetttter att niighhtt ddue
to no sun interference). Active also means it can measure in textureless areas including
shadows
- Here we treat only airborne Lidar. What is recorded is the Time of Flight (TOF) or rarely
the phase (called also continuous wave (CW) lasers), and almost always the intensity,
although intensity is rarely used. All commercial ALS systems use TOF. There is one
experimental CW ALS, called SCALARS (Univ. of Stuttgart).
- Basically: measurement of distance via polar technique, e.g. the direction of the ray and
the distance from the ray source to the scene are measured
- Most ALS work in near infrared (NIR), so are influenced by clouds, snow, rain etc. (no
weather independence, as radar)
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Basic components
- Laser transmitter and detector/receiver = range measuring unit
- Deflection mechanism of the laser ray, e.g. mirror, polygon
- GPS/INS (offset and misalignment angles between GPS/INS and laser unit must be known
-> for INS called boresight calibration). GPS is in differential modus with GPS reference
stations closeby. Dual frequency GPS is mostly used.
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devices (data size is huge), including precise timing device that synchronises all sensors
- Optionally: other film-based (e.g. RC30) or more often optoelectronic cameras (frame or
line) -> especially for generation of orthoimages, also video or standard CCDs for
attributation or annotation (see powerline example in applications).
- Platforms: airpplanes,, helicoppters ((esppec. for mappppingg of corridors or small areas)),, also
Unpiloted Airborne Vehicles (UAVs), including small ones (developments underway from
firm Riegl)
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Basic components of an ALS system
DGPS
Laser
transmitter
Deflection
INS
Control & Detector/ unit
Receiver
data recordingg
Ground
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Basic components of an ALS system
No fix rules exist for distance of
ggrroouunndd rreeffeerreennccee GGPPSS ssttaattiioonnss
from airplane. Often 10-50 km,
depending on topography (GPS
satellite visibility) and possible
GPS signal disturbances.
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Platforms
-AAeerriiaall
Mostly airplanes, then helicopters. Also Unpiloted Airborne Vehicles (with
small weight ALS, e.g. planned from Riegl), even balloons.
-Terrestrial
- Satellites
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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Some definitions
- PPuullssee rreeppeettiittiioonn ffrreeqquueennccyy ((PPRRFF)) oorr ppuullssee rraattee:: nnuummbbeerr ooff ppuullsseess sseenntt ppeerr sseeccoonndd
- Echoes (some call them also pulses): number of received pulse reflections recorded
for one sent pulse
- Minimum vertical object separation: minimum distance between 2 separable echoes
- Scan rate: number of scan patterns (e.g. scan lines) per second
- Field of View (FOV) or scan angle: across-flight angle that laser beam can cover
- Beam divergence: the angle showing the deviation of the laser beam from parallelity
International School on LiDAR Technology, IIT Kanpur, India, 31 March -4 April 2008
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