Laser Scanning for Cave Surveying and Visualisation

Kevin Dixon CPC Geospatial 3D

Overview

       Why ?

Technology Issues Hardware Software Cave Surveying Summary

Why Laser Scan ?

 Expensive £15k-90k  Big, heavy (>10kgs) sensitive equipment  Lots of batteries required  Time consuming 1-4hr per detailed scan  So why do it ?

 3D realisation  Record the geological structure  Visualise black holes  Virtual cave exploration for non-cavers

Laser Principles



L

ight

A

mplification by

S

timulated

E

mission of

R

adiation  Gain medium is energy pumped to excite electrons to emit light which is amplified in optical cavity to produce a coherent light beam often at specific frequency  Most common uses – – DVD/CD Players and Burners, Fibre-optic Communication Bar-code Readers, Laser Printers, Laser Pointers, Mouse  Other uses – – – – Medical, dentistry, lightshow industrial cutting and marking Military target painting, ranging Directed energy weapon

Semiconductor Lasers 375 to 1800nm

 Most common wavelengths for ranging: – – – – – 532nm visible green, can measure through water 650/690nm visible red 785/870/905nm near infra-red 1064nm near infra-red, typically used for airborne LIDAR 1535nm inherently eye-safe, not absorbed by water

Laser Safety

 Class I/1 is safe  Class II/2 is safe during normal use < 1mW  Class IIIa/3R pose small risk of eye damage < 5mW  Class IIIb/3B severe eye damage < 500mW  Class IV/4 can burn skin > 500mW  Protection – – – Wavelength specific glasses Security Key Control Labels

Range Performance

Range requires some laser energy to be reflected back to the laser detector Reflectivity is a function of:

Surface Shape Incidence Angle Colour

Prism

Target Size

Flat

Texture Atmosphere

Convex

3D Lasers

Station Range Coordinates  Laser plus – – Angle Encoders Motors  Optional – – – – – Viewfinder Camera Accelerometers Gyro Compass Reference Azimuth Vertical Angle Horizontal Angle  Polar Coordinate System (Range, HA, VA) – – Add Station Coordinates Add Reference Azimuth  Get Cartesian Coords (Easting, Northing, Height)

Different Scanner Hardware

$ $$ $$$ $$$$$$

Large Tackle Bag plus Internal Protection 2kg

Scanner Gear for Caves

Normal Transit Case Scanner 10kg, Case 4kg Lightweight Case 2kg (Floats) Plastic Bags for Protection Small Battery 3kg / 3hrs Big Battery 5kg / 8hrs Mini 3kg or Normal 6kg Tripod

Laser Scan Integration

 Two Options  Coordinates – – – – Traverse Series of control points Requires more time Best for complex spaces  Overlap – Adjacent scans have sufficient common data – – – Software registration Quick Suitable for simple spaces

  Line of Sight System Creates ‘Shadow’

Shadow

 Line of Sight System  Scan From Another Direction  Reduces Shadow

Shadow

 Line of Sight System

Shadow

 Scan From Other Direction  Fills Shadow  Shadow Under Scanner ?

Scanning Issues

 High humidity limits range   Water buildup on scanner lenses Reflectivity ≡ Range – – Best: white flat perpendicular limestone Poor: Wet, Muddy, Mossy, Dark  Short ranges <0.5m can be difficult  Difficult Setups for Vertical Shafts  Curious cavers and tourists – ‘Tripod kick’ and ‘Laser Block’  Equipment Weight  Very Little Data from Narrow Rifts and Flat-out Passage ‘Cloud’ Waterfall

Stay Out of the Scan !

Software

 QuarrymanPro Scanner logs data on internal CF card  VoidScanner requires ruggedised laptop  MDL Model Software Creates Cartesian Space  MDL VoidWorks - Dedicated for Underground Mining – – Data Edit and Registration Void Modelling And Volumetrics  Data colour coded by area, height or reflectivity  Output options OBJ, DXF, CSV, VRML, Vulcan (Mine data format)

Cave Survey Grades



1

Low accuracy sketch with no measurements made  

2

Intermediate survey between Grades

1

&

3 3

Magnetic survey. Angles ± 2.5º; distances ± 50cm; station position error < 50cm.

 

4

Intermediate survey between Grades

3

&

5 5

Magnetic survey. Angles ± 1º; distances ± 1cm; station position < 10cm.



6

Magnetic survey that is more accurate than grade

5

.



X

Survey based on theodolite or total station.

Cave Survey Detail Grades



A

All passage details based on memory. 

B

Passage details estimated and recorded in the cave. 

C

Measurements of detail made at survey stations only. 

D

As per C and at significant changes in passage  Is another classification required ??



E

Measurements of detail made for majority of passage 

XE

Laser Scan Survey ????

Yordas Cave, Yorkshire

 First trial laser scan for caves  QuarrymanPro Laser Scanner used on Tripod  3 Setups, 5 hours scanning

Reads Cavern, Mendips

 Cave Electronics and Radio Group 19-20 April 2008  Demonstration of QuarrymanPro Laser Scanner  2 Setups, 2 hours scanning

OFD2, Wales

 Main Chamber and Top Entrance Series  QuarrymanPro Laser Scanner on Tripod  11 Setups, 8 hours scanning

OFD2 TBCNTE

St Michaels Cave, Gibraltar

 QuarrymanPro Laser Scanner  45 stations  5 days scanning  Determine cave volume for study  Laser scans identify geological features running through rift

Gaping Gill, Yorkshire

 QuarrymanPro and VoidScanner  Main Chamber, Entrance Shaft, Shakehole and River Bed  Also Mud Hall  20 setups including half way down winch  4 weekends scanning

GG Mud Hall

GG From West to East

GG Revolution

GG Heights for 2008

GG Jumbo x 2

Cave Scanning in the News

Yorkshire Post 27 Aug 2008 Yorkshire Dalesman Oct 2008

In-Work

 Titan, Derbyshire – – 8 scans in 1 weekend so far Humidity Issues Below Event Horizon – Yorks v. Derbys (GG v Titan)  York Minster – Is GG as big as Minster ?

 UK Big Cave/Pothole List – – Dimensions, Volume GG, GB, Titan, Time Machine, Smoo, Mud Hall...............

 Big Chambers Worldwide Scanning at Titan Breakthrough Pitch Head Picture Rob Eavis

Titan Side Elevation

Scans so far

Titan Revolution

Summary

 Current Laser Scanners for Cave Surveying are – – – Expensive, Hard Work Not Suitable for Very Small Passages Need to be Controlled by Traverse and/or Scan Overlap  3D Realisations – – – Aid Cave Studies Add Detail to Cave Mapping - Class XE Cave Survey ??

Provide a new means of Promoting Caves to non-Cavers  A smaller, low power, faster scanner with photogrammetry and ‘dead-reckoning’ would be useful – Unlikely to be within typical Caver budget !!

Thank You

 Yordas Cave - Roo Walters  Reads Cavern - BCRA CREG  OFD2 - Allan Richardson, SWCC, Meg Stark  St Michaels Cave - Dave Mattey, Gibraltar Caving Group  Gaping Gill - Meg Stark, Bradford PC, Craven PC  Titan - Dave Nixon, Rob Eavis, Katie Dent, Meg Stark

Future

 Set up new Company Geospatial 3D  Riegl Laser Scanner – – – 6mm Accuracy Camera Photo Overlay 100m Range  3D Laser Scanning – – – Heritage Underground Quarries and Mines  OFD2 Columns ??

Laser Range Techniques

Pulse (Time of Flight) Phase (Carrier Wave)  High Measurement Rate  Long Range  Better for Passive Targets  Good Outdoor  High Accuracy  Lower Cost  Can be Visible  Low Power

Phase (Carrier Wave)

Turn on the Laser Beam and modulate the intensity LASER Monitor the reflected signal LASER Compare the two signals Phase Angle Time Delay = Phase Angle 2 x π x Modulation Speed Range = Time Delay x Speed of Light 2 Range = Speed of Light 2 x Modulation Speed

Pulse (Time of Flight)

Send a short pulse of light out from the laser LASER REFLECTOR Monitor the reflected signal LASER REFLECTOR Compare the two signals Range = Time Delay x Speed of Light 2 Time Delay

Handheld Lasers

Make Model Stanley TLM 100 Bosch DLE50 Leica Hilti LTI MDL D3 PD38 TruePulse 360B IP54 LaserAce 150 Water Size (mm) Low IP54 IP54 IP54 IP63 100x58x32 125x45x24 120x65x28 120x50x90 Weight Range Accuracy Visible Tilt 30m 6mm Yes No 175g 110g 230g 50m 100m 70m 1.5mm

1mm 1.5mm

Yes Yes Yes No No Mag No No Yes No No 220g 110x100x50 400g 1000m 300mm 150m 50mm No No Mem Connect 20 Power 1x9V 4xAAA 2xAAA Price £103 £119 £229 £450 BlueTooth 2xAA Serial or Yes Yes BlueTooth 2xAA £1,325 Yes Soon 2000 BlueTooth Camcorder £1,145  None Are Waterproof  Only 2 have Tilt and Compass