AN INTRODUCTION TO LASER SCANNING
PART 1
LIDAR• YOU WILL OFTEN HEAR THE TERM ‘LIDAR’
• LIGHT DETECTION AND RANGING• OR A COMBINATION OF THE WORDS LIGHT AND RADAR
• TECHNOLOGY TO MEASURE DISTANCES BY ILLUMINATING A TARGET WITH A LASER AND ANALYSING THE REFLECTED LIGHT TO DETERMINE DISTANCE
• NUMEROUS PLATFORMS, APPLICATIONS AND TECHNIQUES
HOW DOES IT WORK?
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TYPES OF LASER SCANNER• THERE ARE MANY DIFFERENT TYPES OF LASER SCANNER
• THE MOST COMMON TYPES FOR SURVEYING ARE:• TIME OF FLIGHT (PULSE-BASED)• PHASE SHIFT
• LATEST SCANNERS CAN MEASURE UP TO 1 MILLION POINTS PER SECOND
TIME OF FLIGHT SCANNERS• PULSE OF LIGHT EMITTED BY LASER SOURCE
• LIGHT IS REFLECTED OFF AN OBJECT BACK TO LASER SENSOR
• SPEED OF LIGHT IS KNOWN SO WE CAN CALCULATE DISTANCE
time
PHASE SHIFT SCANNERS• SIMILAR CONCEPT TO TIME OF FLIGHT SCANNERS
• CONSTANT BEAM OF LASER ENERGY IS EMITTED
• SCANNER MEASURES THE PHASE SHIFT OF THE RETURN LASER ENERGY TO CALCULATE DISTANCE
WHICH SCANNER?
Scanner Type
Pros Cons
Time of Flight
Good range(1m – 6km), multiple target capability
Expensive
Phase Shift Cheap, slightly more precise
Low maximum range (<500m), can have ambiguous measurements
WAVEFORM DATA - MULTIPLE TARGETS
WAVEFORM DATA - MULTIPLE TARGETS
WAVEFORM DATA - MULTIPLE TARGETS
Rock face with fine mesh- Jwaneng Diamond Mine, Botswana
WAVEFORM DATA - MULTIPLE TARGETS
WAVEFORM DATA – DIGITIZING AND ANALYSIS• THE ENTIRE WAVEFORM IS CAPTURED AND DIGITIZED• EXTRACTED ECHOES AT A CERTAIN TIME PROVIDE RANGE• ADDITIONAL INFORMATION ABOUT THE SHAPE OF THE PEAK INDICATES
QUALITY
WAVEFORM DATA – DIGITIZING AND ANALYSIS• MULTIPLE ATTRIBUTES CAN BE MEASURED FROM THE WAVEFORM DATA
LASER SCANNER INTENSITY• INTENSITY IS THE STRENGTH OR
AMPLITUDE OF THE RETURN SIGNAL
• THE INTENSITY DECREASES WITH RANGE AND ANGLE OF INCIDENCE
• SOME SCANNERS ARE CALIBRATED TO TAKE DISTANCE INTO ACCOUNT, THIS IS KNOWN AS REFLECTIVITY
AMPLITUDE & REFLECTIVITY
Amplitude Reflectivity
MEASURING IN 3D• LASER SCANNERS USE A MOVING MIRROR TO MEASURE ON A 2D PLANE
• ROTATING POLYGON MIRROR • OSCILLATING MIRROR
• TO MEASURE IN 3D THE SCANNER MUST EITHER:• ROTATE
• TRIPOD BASED SCANNER• MOVE
• PLATFORM MOVES PROVIDING A TRAJECTORY• EXAMPLES INCLUDE HELICOPTERS, FIXED WING, CARS, UAVS ETC.
DO YOU WANT TO KNOW MORE?
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