Careful measurement is the cardinal rule of archaeological science.
But seldom has the data collected been as exact and revealing as the point cloud generated by a 21st-century laser beam from the 19th-century surface of the USS Monitor gun turret.
Slicing up and down the giant artifact's encrusted iron walls, the high-definition surveying instrument records 1,800 measurements per second - each one part of a bright green trail of light that streaks across the ancient armor. Then it adds each sliver of numbers into an increasingly detailed and precise 3-dimensional matrix.
Only a few minutes pass before this relentless stream of data coalesces into a recognizable image on the screen of a laptop computer. And once it's transformed by a variety of software programs into drawings and models that can be manipulated at will, its potential for preserving the historic Union warship - and possibly penetrating some of its secrets - is virtually endless.
"In essence, we take a bunch of pictures. Then we stitch them together into digital images that are accurate to within a quarter inch," said Greg Conklin, vice-president of a California-based metrology - or measuring - firm called Optira.
"Ultimately, you'll be able to take the turret and turn it upside down, take it apart, move it around, even tunnel through it if that's what you want - and all without doing any harm to the original object."
Recovered from the waters off Cape Hatteras, N.C., in 2002, the 120-ton turret and its guns have been the subject of thousands of hours of intense archaeological work, beginning with the removal of a Civil War sailor's remains while the artifact was still on the bottom.
Yet since the team of conservators and archaeologists from and Monitor National Marine Sanctuary in Newport News successfully removed most of the historic features from the turret's interior, they've focused intently on finding the best way to measure the pioneering naval invention.
Beginning this past winter, nearly a half-dozen metrology firms have visited the colossal conservation tank inside the USS Monitor Center, where construction is expected to be completed sometime this spring. Employing a variety of 3-D scanning systems, they've recorded new, substantially more precise measurements of the Monitor's anchor, propeller and gun carriages, enabling conservators to see things that weren't previously apparent to the human eye.
"These scanning machines can capture minute details, including things that you can't see yourself or with a camera," said lead conservator Eric Schindelholz.
"We've been able to pick up faint hairline cracks - and even letters in inscriptions that had escaped us before."
Once converted into high-definition drawings and 3-D models, the data will provide an astonishingly precise record of the artifacts' geometry as well as their surfaces. So sensitive is the hand-held Leica scanner they employed this past November that metrologists from Pennsylvania-based MAGLEV Inc. were able to measure the Monitor's anchor with an accuracy of one ten-thousandth of a meter.
In addition to its value as historical information, the data will provide Schindelholz and his fellow conservators with invaluable benchmarks regarding future changes in the artifacts' condition. They'll also provide researchers with tools that could help answer some long-standing questions about the famous Civil War ship and the way it behaved in the water.
"One of the biggest questions we have has to do with the propeller fins. We don't know if they broke while the ship was sinking off Cape Hatteras or if it happened before then," the conservator said.
"But with this data, we hope to be able to find out how the Monitor might have moved and maneuvered with the fins partly broken off - and that could answer the question."
Like all of the previous scanning sessions, Optira's recent work inside of the turret's massive conservation tank was conducted pro bono. Conklin is a longtime member of the board of the National Marine Sanctuaries Foundation, which raises funds and donated services for numerous sanctuary projects around the country.
The turret required about two days of measurements, with scanning technician Travis Cray and Conklin moving the Leica laser scanner from location to location around, under and above the 22-foot-wide, 9-foot-tall turret. Guided by a dozen pre-determined reference points, the device recorded the amount of time it took each pulse of light to reach its target and return - then converted that data into distance measurements.
"There's virtually no error propagation with a scanner if you put it in the right place," Conklin explained. "It doesn't make mistakes."
Double-shooting the turret's surface, Conklin and Cray expect the laser to pick up every detail of the iron armor down to about a tenth of an inch.
Even before the data collection stage of their work ended, they discovered numerous features that previously eluded visual inspection.
Among the most important finds is evidence of what could be damage inflicted upon the Monitor's turret by the CSS Virginia's guns during the 1862 Battle of Hampton Roads.
"This thing perceives and records very subtle contours that are seldom apparent to the human eye," Conklin said.
"It would be very difficult to pick these impact areas up and measure them by hand."
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