Whilst my cave surveying efforts to date have been mostly focused around grovelling around in small passages trying to use a Disto and/ or Gopro and lights to capture data to create accurate models via photogrammetry my mind was slowly turning on how to survey bigger chambers, laser scanning is the obvious answer.
In 2016 I used a Geoslam Zebrevo to survey a few local show caves and other sites, this worked very well however the cost and chance of damage to the device in real caving situations is not something I would like to risk. I had heard that some people had attempted to create a DIY version as the SLAM code was apparently available open source, this triggered lots on internet searches on the subject. I wrote off the Cave-a-tron type system as it wouldn’t easily fit into a dry tube to be dived through sumps to survey passages beyond which is something I wanted to be able to do.
Sadly I didn’t find anything I felt I could build within my skills or cost means that could recreate a SLAM type scanner, however I did find some people had successfully built tripod scanners using fairly simple and off the shelf components. What was even better was that second hand laser units they had used were readily available on Ebay at quite a cheap price relative to the cost of a new unit.
The base laser unit itself was a Velodyne VLP16, a small compact unit that I had used professionally on a few occasions, a short time ago these retailed at around £5000 new but are now in the region of £2500. I snapped up two used ones on Ebay for: $400 and $250 (I’ve since damaged the $400 one).
At this stage I should give massive credit to the originators of this idea who’s designs and softwares have allowed me to realise my own version of their scanners. The first is Jason Bula’s:
https://github.com/jason-bula/velodyne_tls
His scanner and Matlab based code is quite rudimentary but paved the way for another person to further refine and inspire my device, Donny Mott’s:
https://github.com/Rotoslider/TLS_Pie
So to reiterate, I have simple re-arranged components they have used into a form factor which suits my needs and have used their software /code to process the data, nothing massively clever on my part. The result is a compact tripod based scanner that is far cheaper than any commercial offering, I can carry all components myself readily into dry passages or through sumps without external help as the scanner fits in a small pelicase or a small dry tube. I had recently signed up to CREG journal and partway through this process saw an article showing how a Cave-a-tron had been converted to be used on a tripod like the device I was building at the time, however the laser is very primitive compared to a Velodyne unit.
It took quite a few months of trial and error with various components to get data of an accuracy that I was happy with, this was mainly due to the use of a stepper motor with 50:1 ration planetary gearbox. It didn’t quite rotate at a consistent speed meaning that when overlaying two scans from the same location features were not in the same place as they should have been. I replaced it with a different stepper motor with a 30: 1 harmonic drive and finally was happy with the results. Many other things were tried or swapped in this time before the gearbox was identified as the cause such as power supplies, wiring looms and Arduinos so it wasn’t as straight forward as it sounds.
The main components are:
Velodyne VLP16: Laser scanner mounter vertically on a slowly rotating frame
Raspery Pi 4: Logging of Ethernet data from VLP16 using code written by Donny
Arduino: 3 x push button controls of the stepper motor and logging commands to the Pi4
Stepper motor and Harmonic drive: Provides the smooth slow rotation
Lipo battery: Power for all components
Various other DC-DC converters, switches, wires etc necessary to interface the components.
The scanner is mounted to a tribrach bolted onto a tripod and leveled, either button can be pressed to initiated a scan. The scanner then starts to slowly rotate and the Raspberry Pi4 logs the data to a file, no results are visible in realtime. Once the scan has finished the tripod is moved to the next location, leveled again and another scan initiated. In this manner progress through cave passage can be made, usually 7-10 m at a time between scans but this depends on the nature of the passage. Once back at home the raw scans are turned into point cloud files by Donny’s excellent piece of software and can be aligned together using cloud compare’s manual tools initially then its fine align tools. Below is a video taken by Duncan Price of the scanner in use in Wookey Hole.
Scans have so far been undertake in:
Badger Hole
Wookey Hole Chamber 20/ passages beyond
Wookey Hole Chamber 22
GB cavern (all of the large main passages and Great Chamber)
Numerous tests in my garden
Interested in how accurate my scanner and methods were I surveyed a close loop around a house, the loop was 76 m in length and contained 9 scanner locations, the misclosure was less than 20cm in XYZ between the same point visible in both the first and last scans.
This I believe to be far more accurate than what can be achieved with a Disto in a cave though it lacks alignment to either magnetic or true north so its accuracy is only in a relative sense. Alignment to a disto based centre survey could be used matching up common reference points in Cloudcompare to align the laser scanner data.
A commercial laser scanner would achieve accuracy far greater than this but the cost would be 15-100x more along with a large sense of paranoia with regard to damaging the unit in the cave environment. The Velodyne laser doesn’t log point colour but is does log intensity so this can be interesting to colour the resulting point clouds by.
The below video shows the data data I collected and aligned from GB cavern, Donny made the fly through after I sent him the data to show him what I had been doing with his ideas and softwares, it starts at the mud run in at the top of the major passage and goes down to the choke just below ladder dig.
At this point I feel I have succeeded in my goals, I have a relatively cheap laser scanner than I can take nearly anywhere (whether I want to is another matter, eg transporting through Daren Entrance crawl..) that produces results accurate to a few centimeters and can scan large chambers or passages with ease, now it is a case of working through suitable sites and producing laser scans of places that otherwise might never be scanned unless someone invested a lot of time or man power or money into carrying in commercially available scanner.
