An airplane with a lawn mower-sized sensor unit strapped to its belly flew back and forth over much of the tip of the lower peninsula a year ago. It was collecting lidar data. A research team that included UMBS Data Manager Jason Tallant and former UMBS instructor Meghan Howey ran that data through more refinement than corn syrup. As they reported in a recent paper published in the on-line journal PLOS ONE, the resulting images successfully identified 1000-year-old storage pits from the region’s Native inhabitants. And the entire process was faster and incorporated far more land than is possible with a ground survey team.
The presence of cache pits in Emmet and Cheboygan counties is not news. The first pits were identified on Burt Lake in 1987. Howey, now a professor at the University of New Hampshire, excavated some of the pits with her UMBS class, Field Methods in Archaeology, between 2005 and 2010. What is new is that a sensor three-quarters of a mile above the ground helped identify small bumps and hollows roughly the size of a gong.
Finding cache pits is intensive work. As noted in the article, “Communities dispersed food storage away from where they lived...and this means cache pits are not readily encountered during archaeological research on past habitation sites. Cache pit features are easily obscured by leaf litter and groundcover, making it hard to find them until standing almost right on top of them.” Moreover, the pits, which were 1-2 meters wide and deep when originally constructed, can look like the depressions left behind when a tree topples at its roots.
The traditional way to look for these artifacts is a walk-over archaeological survey. UMBS Resident Biologist Bob Vande Kopple, a co-author on the paper, has organized citizen scientists spring surveys. Late spring is best because the lack of groundcover and snow laying in surface depressions melts slower than on the ground, so cache pits are more obvious. Volunteers walked 50 meter transects along shore and nearshore landforms looking for possible features. To date, they have found 69 clusters of cache pits.
While a spring survey has its charms, it is neither fast nor comprehensive. Lidar works like radar, but with light. It shoots a beam of light at a target and measures how quickly and with how much intensity the light reflects back. The LiDAR scans Howey’s team used covered between a half and three-quarters of a mile in one pass, at around 140mph.
The team started with lidar data described in three dimensions, resulting in a “point cloud.” They fed these data into GIS (geographic information systems) software programs to generate a high-resolution, 3-D surface model of the lidar footprint. Using what is already known about cache pits -- their dimensions, their distance from the water table, their occurrence in clusters -- they narrowed the number of candidate pits from 2.5 million to 543.
Next it was time to see how good their filtering had been. The group compared the 543 sites from their most refined map with the sites previously identified from ground surveys. One hundred thirty-nine previously confirmed sites were among the 543 on the map. Moreover, Vande Kopple spot-checked an 8 new locations identified by the filtering process. For all of these, he confirmed the presence of cache pits.
These findings leave the group quite confident that the 543 sites they identified are cache pits. Of particular interest is the fact that many new locations are farther from inland lakes than almost all previously known sites. The authors concede, “We would never have examined such areas removed from the lakes without the aid of lidar and image processing routines.” This also points to how little is generally known about the region’s first human residents. “At best, we have a markedly incomplete record of the ways mobile hunter-gatherers of the upper Great Lakes regularly modified the natural world to ensure their own well-being.”