A New Generation of Handheld Laser Scanning (HLS) for Geotechnical - - PowerPoint PPT Presentation

A New Generation of Handheld Laser Scanning (HLS) for Geotechnical Studies

W K Leung Geotechnical Engineering Office 30 May 2019

Remote Sensing Techniques

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Unmanned Aerial Vehicles (UAV) Air-borne Light Detection and Ranging (LiDAR) Photogrammetry (Aerial Photographs, Satellite Imagery) Terrestrial Laser Scanning (TLS) Mobile Laser Scanning (MLS) Handheld Laser Scanning (HLS) InSAR

First return from tree top (non-ground point) Second return from branches (non-ground point) Last return from ground (ground point) 1st (and only) return from ground

Light Detection And Ranging (LiDAR)

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Handheld Laser Scanner (Previous Model)

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Laser Scanner Head Data-logger Spring

Handheld Laser Scanner (Current Model)

Cable Camera Laser Scanner Head Battery Data- logger

• Measurement range: 30 m or 100 m
• 300,000 measurement points/second
• Scan range noise: 10 - 30 mm

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Data Acquisition

SLAM – Simultaneous localisation and mapping:

• 2-D time of flight of laser pulses
• Inertia measurement unit at scanner head to give the
• rientation
• Mapping using the point cloud captured in the previous

5 seconds

• Use features on the environment to register data
• Output - Point cloud & trajectory
• Align the data without the need for external control to

produce a highly accurate 3-D point cloud of the area surveyed

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Control upper and lower cap

• f point cloud

Data Review

Real-time images through smartphone or tablet

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• 3-D models (DSM & DTM)
• Site records
• Plotting of cross-sections, dimension

measurement & volume estimation

Capabilities of HLS

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After urgent repair works

Applications: Emergency Landslide Mapping

Before urgent repair works

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HLS for mapping of natural terrain landslides: Supplement the data collected by UAV, especially at landslide trail where there are vegetation. For site measurements and record of channelization ratio

Rigid Barrier at Pokfulam Road (Feature No. 11SW-A/ND52)

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Applications: Mapping of As-built Condition Applications: Tree dimensions

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88°/160° 87°/329° 68°/236° Laser Scanning Point Cloud Joint Measurement Plane Fitting

Applications: Rock Joint Mapping

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• At least 3 reference points
• Only required if actual location is important

Method for Geo-referencing

No Rain Light Rain Medium Rain

Accuracy Checks

• Weather Check

(Feature No. 11NW-D/C 90)

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Daytime

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Night time

Accuracy Checks - Brightness Check (Feature No. 11SW-D/C465)

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Accuracy Checks - Dimension Check (t-test)

Site Setting

• No. of

Scans

Natural Terrain 3 Rigid Barrier 2 Flexible Barrier 2 Retaining Wall 2 Soil Slope (Hard Surface) 1 Soil Slope (Vegetated Surface) 2 Rock Slope 2 Garden 3 Seminar Room 1 Tunnel 2

TOTAL 20

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Dimension Checks (Daytime VS Night time)

The percentage error of the 20 scans in daytime (3 measurements per scan): 1.54, 1.28, 0.76, 0.74, 0.73, 0.89, 0.25, 0.70, 1.54, 0.31, 1.65, 1.10, 0.70, 1.20, 0.67, 0.23, 0.91, 1.11, 0.47, 0.94 The percentage error of the 20 scans in night time (3 measurements per scan): 2.71, 1.73, 0.87, 0.33, 1.09, 0.90, 0.56, 0.92, 1.45, 0.96, 0.47, 0.77, 0.78, 0.40, 0.53, 0.63, 1.21, 0.72, 0.49, 1.02

t-test

H0 (Null hypothesis): the % error for daytime and night time are NOT significantly different. H1 (Alternative hypothesis): the % error for daytime and night time are significantly different. Degree of freedom: (20-1) + (20-1) = 38 Critical value (5% significance level) = 2.024 t-value = 0.266 t-value < critical value

Null hypothesis is NOT rejected !

Similar statistical analysis carried

• ut for scanning under fine

weather and rainy weather. t-test suggested that there was no significant variation in the percentage errors.

Conclusion: HLS performs equally well under different brightness and weather conditions.

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Key Advantages of HLS

peedy Quick for data review and processing andy Lightweight and easy to carry ccurate Adequate for most geotechnical applications

• bust

Able to be used at night time and in raining conditions

• rtable

Applicable to remote sites/difficult terrain

• Emergency inspection and landslide mapping
• Projects requiring 3-D models and topographic data
• Unauthorised works (e.g. capturing of field situation)
• Site progress, amount of excavation/filling
• As-built construction records (e.g. completed LPMit

works)

• Rock joint measurement
• Coupled with other remote sensing techniques, for

example, photogrammetry, UAV etc.

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Potential Application Areas

Speedy Handy Accurate Robust P

• rtable

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Recent Increase in Measurement Range