Laser Scanning for Cave Surveying and Visualisation Kevin Dixon CPC Geospatial 3D

1. Laser Scanning forCave Surveyingand VisualisationKevin DixonCPCGeospatial 3D

2. Overview • Why ? • Technology • Issues • Hardware • Software • Cave Surveying • Summary

3. 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

4. Laser Principles • Light Amplification by Stimulated Emission of Radiation • 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

5. Semiconductor Lasers375 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

6. 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

7. Prism Flat Convex 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 Target Size Texture Atmosphere

8. 3D Lasers Station Coordinates Range • Laser plus • Angle Encoders • Motors • Optional • Viewfinder • Camera • Accelerometers • Gyro • Compass • Polar Coordinate System (Range, HA, VA) • Add Station Coordinates • Add Reference Azimuth • Get Cartesian Coords (Easting, Northing, Height) Vertical Angle Horizontal Angle Reference Azimuth

9. Different Scanner Hardware $ $$ $$$ $$$$$$

10. Scanner Gear for Caves Normal Transit Case Scanner 10kg, Case 4kg Large Tackle Bag plus Internal Protection 2kg Lightweight Case 2kg (Floats) Plastic Bags for Protection Small Battery 3kg / 3hrs Big Battery 5kg / 8hrs Mini 3kg or Normal 6kg Tripod

11. 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

12. Shadow • Line of Sight System • Creates ‘Shadow’

13. Shadow • Line of Sight System • Scan From Another Direction • Reduces Shadow

14. Shadow • Line of Sight System • Scan From Other Direction • Fills Shadow • Shadow Under Scanner ?

15. 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

16. Stay Out of the Scan !

17. 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)

18. 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.

19. 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 ????

20. Yordas Cave, Yorkshire • First trial laser scan for caves • QuarrymanPro Laser Scanner used on Tripod • 3 Setups, 5 hours scanning

21. Reads Cavern, Mendips • Cave Electronics and Radio Group 19-20 April 2008 • Demonstration of QuarrymanPro Laser Scanner • 2 Setups, 2 hours scanning

22. OFD2, Wales • Main Chamber and Top Entrance Series • QuarrymanPro Laser Scanner on Tripod • 11 Setups, 8 hours scanning

23. OFD2 TBCNTE

24. 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

25. 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

26. GG Mud Hall

27. GG From West to East

28. GG Revolution

29. GG Heights for 2008

30. GG Jumbo x 2

31. Cave Scanning in the News Yorkshire Post 27 Aug 2008 Yorkshire Dalesman Oct 2008

32. 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

33. Titan Side Elevation Scans so far

34. Titan Revolution

35. 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 !!

36. 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

37. 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 ??

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

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

40. 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 Time Delay x Speed of Light 2 Range = Time Delay

41. Handheld Lasers • None Are Waterproof • Only 2 have Tilt and Compass