Laser Scanning Mushpot Cave
Written by David McIntire, LSIT   
Sunday, 05 December 2010

A 3.380Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

The concept for this project was hatched in the brilliance-inducing state one achieves by moving rapidly between the dim, flickering ambiance of a GIS computer screen and the dim, flickering ambiance of a bat-filled lava tube cave, which were not dissimilar to the conditions in which I interned as a cartographic technician at the Lava Beds National Monument last summer. And while the brilliance-inducing conditions may have been suggested somewhat in jest, the opportunity to introduce the latest geospatial referencing technology to seemingly endless, relatively unmapped expanses of subterranean geographical and cultural richness was an opportunity not to be missed.

The formal purpose of the project was to acquire 3D digital imagery of Mushpot Cave, a lava tube cave and one of the most heavily visited and highly interpreted cultural and geologic features at the National Park Service's Lava Beds National Monument (LABE). The imagery would be collected using 3D laser scanning technology, made possible by Portland, Oregon-based David Evans and Associates, Inc. (DEA). The accuracy and detail of this imagery would allow LABE to provide a means by which those physically unable to access the caves would be able to virtually explore the cave, personnel to monitor the effect of visitation on the site, and compare subsequent imagery for structural movement from tectonic activity.

Additionally, the relationship with local universities is an important initiative for LABE, as partnering with students and faculty will develop community ownership and lifelong responsibility toward one our nation's greatest assets in general (our National Parks), and for Lava Beds National Monument locally, in particular. This is the first project to be implemented in conjunction with a newly created task agreement/memorandum of understanding (MOU) with the university. LABE's goal is to support further research conducted by the university and for the university students to become more involved with stewardship of local public lands. The collaboration between OIT, LABE and DEA will directly support these initiatives.

Ultimately, our goal was to capture and preserve the resources of this Monument in its present state for future generations. This involves implementing technology that would allow us to monitor and accurately replicate site features, as well as forging new relationships with the surrounding community, university and businesses that appreciate and share a sense of responsibility for America's National Parks.

A lava tube is an underground feature that was once a conduit for molten lava flowing out of a volcano. When the lava stopped flowing, the hollow tubes remained, garnished with a stunning array of geologic features such as lava balls, drip stalagmites, and my favorite, lava-cicles (like an icicle, but melted rock). Lava Beds National Monument is riddled with such features, and is even more impressive given the cultural history that took place within them, perhaps most notably of which were the events surrounding the Modoc War of 1872-1873 where Modoc Native America chief, Captain Jack, and approximately 50 warriors utilized these volcanic features to their advantage, holding hundreds of U.S. forces for five months, including the last battle where Native American forces defeated U.S. forces.

The single biggest challenge to commencing a project such as this, however, was the acquisition of a laser scanner. We had designed and established a local control network, traversed through the cave to set control points and their subsequent coordinates, but it was not until the Professional Land Surveyor's of Oregon (PLSO) annual conference that we met Jim Griffis, Senior Vice President at David Evans and Associates, Inc. who expressed interest in our proposed project and graciously offered to join us, providing both the technology and technical support necessary to carry out the scanning portion of the survey.

The scope of the project was estimated to be approximately 60 hours for the completion of this project. Due to academic schedules and instrument availability, field work would typically occur on Fridays and Saturdays and included reconnaissance of the proposed project area, meeting with all involved parties, setting control, performing the scans and post-processing data.

Prior to the big day, OIT seniors Robert Femling, LSIT, Garrett Jackson, LSIT and yours truly had used static GPS to create a local control network using Topcon HiPer Lite Plus GPS receivers to established coordinates for the survey of Mushpot Cave, as well as any future cave surveys within the vicinity. Establishing GPS control in the high desert landscape was fairly straightforward. The challenge came when attempting to use GPS to establish control points back in the cave, as it proved exceedingly difficult to maintain quality satellite signals when underground.

The even greater challenge was running a traverse through the cave where we set control points. Conceptually, the process is simple: backsight-foresight, move ahead, repeat. However, performing the traverse in approximately 500 feet of undulating, winding, subterranean environment with strict parameters as to where we could actually set physical control proved more difficult than originally anticipated. We walked through the cave numerous times, and it seemed that four or five control points could be set at major turns in the cave, keeping all points intervisible with at least two other points. When looking at a plan sketch of the cave, this indeed seemed plausible, but it is never that easy, right? One of the limiting factors in this process was the restriction on where we could set control. While Mushpot Cave varies significantly in width from shoulder-wide near the entrance and ballooning to a small theater near the center, we were restricted to using the 30-inch concrete footpath that snaked along one side of the cave, in an understandable attempt to restrict any further manmade changes to existing infrastructure within the cave. This severely limited our line of sight horizontally. Vertically, there were numerous instances where the roof of the cave dipped precipitously multistory heights to approximately three feet. The minimal lighting in Mushpot Cave, coupled with the use of reflectorless targets, made for difficulties in sighting the target in the inky darkness.

To overcome the challenges of running a traverse in this environment, we utilized many low instrument setups (less than three feet) and short distances that allowed us to get around or under obstructions. While it would have been possible to get electricity into the cave by utilizing a portable generator and several hundred feet of electrical cord, we were able to get around this by strategically placed caving headlamps that illuminated the prisms to insure quality sighting and accurate measurements. For control, we drilled 3/8" holes into the concrete footpath using cordless drills and masonry bits, then filled them with anchoring epoxy and inserted PK nails flush with the footpath, the purpose of the permanent control being for future use by LABE. Over time, these will develop a patina that will blend unnoticeably with the surroundings.

The big day arrived and the students met at a local diner for a belly-busting breakfast with DEA's Jim Griffis and Marcus Reedy, and OIT professor and faculty advisor, Mason Marker. Jim and Marcus were exceptional to work with throughout the day, providing expert support, while empowering students to carry out the scanning and gain invaluable experience. The process for using the Leica HDS 6000 scanner involved setting out targets that would be shot in with a total station set up on the aforementioned control points, thus establishing coordinates that could be utilized in postprocessing. The scanner was blazing fast, capable of gathering up to 500,000 points per second. The settings we used gathered approximately one million points over the course of a 7-minute, 360-degree scan. Thus, the most time-consuming part of the scan was moving from station to station and shooting in the targets. Use of the scanner was rather intuitive—­the most arduous decision-making process involved in its operation was determining where to set the scanner for each subsequent scan. This proved particularly challenging in the lava tube environment of Mushpot Cave, as ceilings and walls are continuously undulating, the floor is strewn with large boulders, and tributary tubes frequently intersect the main cave. Every setup was a challenge to determine where the previous and upcoming scans' shadows would occur and locating the best combination of scan coverage and setup efficiency.

In addition to the scanning, DEA has a camera setup whereby the apparatus sets into the scanner's tribrach and is calibrated to where the nodal point of the camera matches the nodal point of the scanner. The camera, with a fisheye lens, then shoots a 360-degree circle around the tripod as well as a shot upwards to capture the ceiling of the cave. This will later overlay the scan. The challenge in a dark cave where ceilings and walls are sometimes out of the effective range of the flash, is that photo quality is poor and does not capture the desired detail. To overcome this, everyone with an LED headlamp gathered around and aimed their lights at various places in the camera's view, thus providing sufficient light for a quality photo.

After 15 setups with the scanner, we had obtained quite good coverage of the cave's entire 500 feet of twisting passages. We waited for the moment of truth as Marcus Reedy downloaded the nearly 15 million points we had gathered into his laptop for us to view. In order to keep from overwhelming the laptop, we viewed the scans with only 1/25th of the total points collected.Even so, the result was quite breathtaking. Given that the current geospatial data that LABE currently has on file is width and height every few feet to produce a basic plan and profile, the park employees who viewed the basic results were ecstatic with the results. The possibilities that exist with the postprocessed data are seemingly limitless and we are excited to continue moving forward in those areas of opportunity!

Author Note: We are especially grateful to David Evans and Associates for affording us this opportunity and continuing their ongoing support of surveying education in general, and OIT Geomatics, in particular. We also thank the National Park Service at Lava Beds National Monument who are committed to working with the Geomatics program at OIT and we look forward to moving forward in this mutually beneficial relationship. We are also indebted to Professor Mason Marker at OIT, without whom these parties would have not been brought together, and without whose guidance this project would have been far less seamless if not altogether impossible. Finally, thanks to the OIT students who joined me on this project, namely Robert Femling, Garrett Jackson, Nick Yager and Ben Miller.

David McIntire is a 2010 graduate of OIT and performs hydrographic surveying on a NOAA Navigation Response Team. He enjoys surveying in caves where the weather is predictably comfortable, precipitation is nil and GPS remains elusive.

A 3.380Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE