This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Over at Ars Technica, we note an intriguing feature by Curt Hopkins on the use of physics-based technologies in archaeology -- in this case, LIDAR (LIght Detection And Ranging). It highlights the work of archaeologist Chris Fisher, who has been using a LIDAR system to map the terrain of Western Mexico:
Several years ago, Fisher started out with rugged handheld computers and a few GPS receivers to map the recently-discovered city Sacapu Angamuco in western Mexico, occupied from about 1,000 to 1,350 CE. The Purepechan or Tarascan people had proven more difficult to pinpoint archeologically than had their contemporaries and rivals, the Aztecs. But initial data gathering and geo-referencing allowed Fisher to identify the city at an important moment on the crux of empire, and to do so in a fraction of the time it would have taken with tape measures and grid-plotting....Last year, LiDAR enabled Fisher to create a full-fledged picture of the important Mesoamerican capital in greater detail. This included the discovery of several pyramids, ceremonial complexes and thousands of residences and other buildings that no one knew existed in the city.
Hopkins' piece inspired Jen-Luc Piquant to dust off an older blog post from 2007, suitably edited and updated for 2012. First things first: LIDAR is an optical remote sensing technology that exploits the same basic principle as radar and sonar -- sending out pulses that bounce off objects and analyzing the returning signals to determine an object's distance from the source -- except it uses light wave pulses instead of radio waves. (Yes, I know, radio waves are technically just another form of electromagnetic radiation; in this instance, "light" refers to the visible and near-infrared frequencies in the spectrum.) It's not so much a replacement technology, as a complementary one -- just one more tool in our growing arsenal for remote sensing and mapping.
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A LIDAR instrument transmits pulses of light to a target, and the parts of the spectra that are not absorbed by the target are reflected back (known as backscatter) to the system, which then are detected, stored and analyzed. It's the changes in the properties of the light when it scatters back that enable scientists to measure specific properties of the target.
Bats use ultrasonic pulses to hunt for their prey, emitting a series of pulses that become more frequent the closer it gets to its target, climaxing in a kind of "feeding buzz" as it locks in for the kill. Similarly, the more frequent the light pulses emitted in a LIDAR system, the more information is gathered, and the more accurately a target area can be mapped. For airborne topographical mapping, as many as 33,000 laser pulses can be transmitted every second.
LIDAR has been around for quite a long time, having been invented shortly after the first lasers appeared in 1958. But the technology was a bit ahead of its time, and languished for several decades until a whole bunch of other enabling technologies emerged. Early lasers were too expensive, frankly, and too heavy, too big, and required too much power, to make them practical for airborne applications.
When the solid-state diode pumped laser emerged, that changed: they were cheap, rugged, and compact, with comparably low power requirements. Computing technology also needed to advance to the point where it was fast enough, and cheap enough, to perform the kind of advanced data analysis required by a LIDAR system.
Most notably, early LIDAR systems could make accurate measurements in the centimeter range, but only for lasers fixed on the ground. This strictly limited its useful deployment, since once anyone placed a laser on a moving platform, all bets were off. Then came the Global Positioning System (GPS), and suddenly it became perfectly feasible to figure out exactly where a moving object might be in relation to a ground-based coordinate system. And LIDAR was finally dusted off and brought into the marketplace.
For remote sensing applications, the LIDAR system is mounted onto an aircraft equipped with a GPS receiver (de rigeur these days in just about any vehicle) to track its exact location and altitude. It also needs a high-accuracy inertial measurement unit (IMU) to track the pitch and roll of the airplane so that movement can be accounted for in the final analysis.
Basic physics tells us that objects spinning at a very high rate tend to maintain their relative position in space, so an IMU incorporates several spinning gyroscopes. By measuring the angle of tilt as each spins a spherical mass within a gimbal (or cage), and coupling that with an accelerometer to keep track of shifts in velocity, the system can tell us how far, how fast and in what direction a target is moving relative to a given starting point. Usually, all the data collected from the various instruments, when combined, can give an elevation that is accurate to within 6 inches.
Generally speaking, we can usually only image a feature or object roughly the same size as the wavelength of the EM radiation being used, or larger. Radio waves used in typical radar systems are great at detecting things like metallic objects -- which is why they're so useful for military and aviation applications -- but rocks or raindrops might not produce much in the way of detectable reflections at all, making them well nigh invisible to radar.
But because the wavelengths used are much shorter than radio waves, LIDAR systems are much better at detecting very small objects, like particles in the atmosphere. In fact, they are already used to study atmospheric conditions, notably the densities of various particles, not to mention all kinds of emerging applications in geology, seismology, and even archeology. Also, lasers use a very narrow beam, so LIDAR allows for mapping of physical features with much higher resolution than conventional radar. It can have a "footprint" of less than 1 meter, making it possible to map the floor underneath a forest canopy, or the urban canyons between tall buildings.
In England, Cambridge University is collaborating with the UK Environment Agency in the use of LIDAR imaging to produce terrain maps for large swathes of the countryside. It started out as a way of assessing flood risks, but then an organization called English Heritage contracted with the EA to conduct a LIDAR survey of Stonehenge -- one of the most studied landscapes in all of Europe, and a certified World Heritage Site. See the nifty video fly-through LIDAR of Stonehenge below, based on LIDAR data. How cool is that? It turns out that LIDAR is terrific for recording terrestrial features that have been leveled by many years of plowing: the WHS survey revealed several previously unrecorded banks in and around the Stonehenge site.
There's more than one kind of LIDAR system, each suited to a specific kind of application. If you just want to measure the distance from your instrument to a solid target, range-finder LIDAR should suit your purposes just fine. If it's a moving target, and you want to figure out how fast it's moving, you'll probably want to use Doppler LIDAR, which -- as its name clearly implies -- relies on the Doppler shift effect to determine an object's velocity.
If you're a meteorologist interested in measuring the specific concentrations of chemicals and such in the atmosphere -- ozone, water vapor, and pollutants -- or if you want to map a shallow river bed underwater, you're better off using differential absorption LIDAR. Underwater imaging in particular can be difficult using infrared and near-infrared preferred for terrestrial mapping, since water absorbs those wavelengths; only the blue-green end of the visible spectrum can penetrate water, for the most part.
Small wonder so many applications have emerged in the past decade for LIDAR systems. In geology and seismology, they're used to detect faults -- most famously, to locate the fault in Seattle, Washington -- and to measure the plumes of ash that Mount St. Helen's in Oregon occasionally burps out -- an indication of whether its internal distress is reaching a critical eruption point. Airborne LIDAR is used to monitor glacial melting and other coastal changes, while in forestry, LIDAR is used to study canopy heights and measure biomass, not to mention making the surveying process that much faster.
LIDAR has been used for search and rescue, too, most notably in the wake of the terrorist attacks of September 11, 2001. For several days after the World Trade Center fell, a small plane made several passes over Ground Zero in Manhattan (and also over the damaged Pentagon in Washington, DC) taking LIDAR readings of the debris -- courtesy of a company called EarthData. The company used the collected data to produce topographical images of the sites. This in turn helped rescue workers navigate the often-treacherous terrain by identifying unstable areas likely to shift or collapse.
The maps also enabled building and utility workers to locate foundation-support structures, elevator shafts, basement storage areas, and so forth. As workers moved deeper into the WTC's basement wreckage, LIDAR mapping showed where the integrity of the underground walls might have been compromised, thus making those areas more at risk of flooding. The maps were even able to measure the volume of the debris and how much reach the cranes would need to efficiently remove it.
LIDAR isn't just about mapping positions and elevations; it's also about integrating other aspects of feature recognition to make map production ever more automated. Several years ago, scientists from Sweden and Italy teamed up to use LIDAR to image the various types of stone used in the construction of Lund Cathedral. Located in Sweden, the cathedral is an impressive 12-century edifice that ranks as the largest Romanesque building in northern Europe. (Whether you're impressed probably depends on your fondness for the Romanesque period.) Not only could they "see" the differences between the stone used, but they could also tell, from a distance, which of the walls had moss and lichen growing on them.
As intensity recording is incorporated into LIDAR systems, scientists should be able to improve even further on this type of analysis. Intensity recording not only measures the distance between the LIDAR and the target, but it can determine the features of a landscape based on the strength of returning signals. That's because every reflective surface will absorb some wavelengths and reflect others. A concrete block, for instance, reflects almost every wavelength and absorbs very little, so the returning signal is very strong. Leafy vegetation, however, absorbs quite a bit more of the light, and hence returns a weaker signal. These data can also be turned into a visual image.
So light and lasers are an increasingly important tool in archaeological mapping. And the innovations just keep coming. A 2007 article in the San Francisco Chronicle profiled a retired civil engineer named Ben Kacyra, who invented "a camera-like device that uses lasers to scan three-dimensional objects -- such as archaeological ruins -- to create digital blueprints accurate to within a few millimeters."
At the heart of his device is a laser that emits light with sufficient power to bounce off a distant object and return to a sensor, capable of timing the intervals between signal and response. In this way, "the laser maps the surface of objects by taking millions of measurements at different angles."
Kacyra's system -- which he sold to Swiss company Leica Geosystems in 2001 -- has been used to study pre-Incan Peruvian ruins, the buried Roman city of Pompeii, and the cliff dwellings of the extinct Anasazi people in southwest Colorado. And once these digital blueprints are created and stored, it becomes so much easier to recreate portions of those sites and edifices in a virtual framework -- perhaps, one day, in Second Life.
Kacyra has already created a small reproduction of an ancient frieze that he scanned, using his mapping tool. As the Chronicle article put it, "Think of the archaeological equivalent of a reprint of a famous painting, a chance to hold a piece of history." Indeed. There lies the future of LIDAR.
Images:
(top) Three images illustrating three ways scientists can visualize LiDAR information. The top image is unfiltered LiDAR feedback, the second is filtered to show ground surface and prehistoric features, and the last is filtered even more to show ancient structures that remain. Credit: Chris Fisher. Source.
(middle) LIDAR images of ground zero rendered Sept. 27, 2001 from data collected by NOAA flights. Credit: NOAA/U.S. Army JPSD.
(bottom) Laser scan of a temple in the grand plaza of Tikal, an ancient Mayan city in Guatemala, courtesy of Kacyra Family Foundation. Source.