An Introduction to Laser Scanning - Part 2: Airborne LiDAR, the basics of trajectories
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Transcript of An Introduction to Laser Scanning - Part 2: Airborne LiDAR, the basics of trajectories
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AN INTRODUCTION TO LASER SCANNING
PART 2 – AIRBORNE LIDAR, THE BASICS OF TRAJECTORIES
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TRAJECTORIES• AIRBORNE LIDAR SYSTEMS USE 2D SCANNERS
• THE 3RD DIMENSION IS DERIVED FROM THE MOVEMENT OF THE AIRCRAFT
• THE PRECISE POSITION AND ORIENTATION OF THE LIDAR SYSTEM IN THE AIRCRAFT IS KNOWN AS A TRAJECTORY• TIME
• FROM GNSS• POSITION - LATITUDE, LONGITUDE, ELEVATION
• FROM GNSS• ORIENTATION (ATTITUDE) - HEADING, ROLL, PITCH
• FROM IMU
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GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS)• GNSS’ EMPLOY A CONSTELLATION OF ORBITING SATELLITES TO PROVIDE
GEOGRAPHICAL POSITION AND VELOCITY INFORMATION
• MANY SYSTEMS ARE IN USE AT THIS TIME, WITH THE MOST POPULAR INCLUDING:• UNITED STATES - GLOBAL POSITIONING SYSTEM (GPS)• RUSSIA – GLOBAL NAVIGATION SATELLITE SYSTEM (GLONASS)• CHINA – COMPASS / BEIDOU NAVIGATION SYSTEM (BDS) • EUROPEAN UNION – GALILEO
• RECEIVERS THAT CAN TRACK MULTIPLE GNSS CONSTELLATIONS WILL GENERALLY PROVIDE MORE ACCURATE POSITIONAL DATA, ESPECIALLY IN URBAN CANYONS WHERE SKY VISIBILITY IS LIMITED
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GLOBAL NAVIGATION SATELLITE SYSTEM (GNSS)• DUE TO ERROR SOURCES SUCH AS ATMOSPHERIC INTERFERENCE AND
SATELLITE CLOCK TIMING, GNSS RECEIVERS ARE OFTEN ONLY ACCURATE TO 1-2M
• ACCURACY CAN BE IMPROVED USING METHODS SUCH AS:• PRECISE POINT POSITIONING (PPP) - <30CM• POST-PROCESSING AGAINST A REFERENCE STATION – DIFFERENTIAL GNSS
(DGNSS) - < 2CM• REAL TIME KINEMATIC (RTK) CORRECTION - < 2CM
• GNSS IS USUALLY LOGGED AT 2-10HZ• ONE POSITION MEASURED EVERY 0.1-0.5 SECONDS
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INERTIAL NAVIGATION SYSTEM (INS)
• AN INS IS A COMBINATION OF AN INERTIAL MEASUREMENT UNIT (IMU) AND A CONTROL COMPUTER
• THE INS IS INITIALLY PROVIDED WITH ITS POSITION AND VELOCITY FROM AN EXTERNAL SOURCE, AND FOR AIRBORNE LIDAR THIS IS GNSS
• A KALMAN FILTER COMBINES THE DIFFERENT MEASUREMENTS TAKING INTO ACCOUNT ESTIMATED ERRORS TO PRODUCE A TRAJECTORY
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INERTIAL MEASUREMENT UNIT (IMU)
• MEASURES ROTATION AND ACCELERATION• COMBINED WITH POSITION AND VELOCITY FROM GNSS
RECEIVER TO PRODUCE A TRAJECTORY
• TWO MAIN TYPES OF IMU• FIBRE OPTIC GYRO (FOG)• MICRO ELECTRO MECHANICAL SYSTEM (MEMS)
• IMUS USUALLY LOG BETWEEN 100-500HZ
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TYPICAL AIRBORNE LIDAR TRAJECTORY
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TYPICAL AIRBORNE LIDAR POINTCLOUD
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