Aerial Mapping Fall '13 - Aerial Mapping Fall '13
Searching for Mayan Ruins with the Optech Orion ALTM
Studying history may be a lesser-known use of airborne lidar, but archaeologists are beginning to realize that a good point cloud can provide invaluable information that other methods cannot. In early 2012, Committee Films asked AeroMetric if they could film us for the History 2 channel as we surveyed Track Rock in Chattahoochee National Forest, Georgia. AeroMetric has applied its expertise to similar projects in the past, such as detecting wagon trails, burial grounds and native American sites, and our latest expedition illustrates several benefits of airborne lidar for this application.
The Track Rock site contains a large mound constructed over 1000 years ago. The filmmakers were intrigued by the idea that the mound was built by Mayans from what is now Mexico, and wanted to compare the features at Track Rock to a map of a typical Mayan mound.
The filmakers soon found that airborne lidar was the right choice. Aerial cameras are useful for surveying plains, but Track Rock is heavily forested. Land-based lidar systems are also impractical because the hilly forests offer few locations with a clear line of sight to the mound. Moreover, the Forest Service explicitly discourages visits on foot in order to preserve the site.
To fit the rushed filming schedule, all surveying was done on a single day, July 19, 2012. AeroMetric’s three lidar sensors were busy that day, but Optech generously provided an Orion C200 ALTM and quickly shipped it out in time for the survey.
The survey itself presented interesting challenges. First, the site has many features with vertical surfaces, such as walls and ditches, which high-altitude surveying would fail to capture. Furthermore, the Orion needed to penetrate foliage and undergrowth just when both were at their summer maximum density.
To resolve both issues, we flew several passes over the mound in different directions, operating at only 200 m with a wide scanner angle. This enabled us to capture the features from multiple sides and avoid data shadows. We also achieved a very high point density of 40-50 points/m2, which ensured that we would get enough ground returns.
Back on the ground the film crew wanted to visualize the data immediately. This was before the newest release of Optech FMS Nav, which creates a preliminary LAS file while you are still in the air, so we post-processed the data on a laptop in the back of our vehicle. As a shortcut, we skipped calibration and just used Applanix POSPac™ MMS and Optech LMS to create an SBET and point cloud. Incredibly, this took only an hour for the entire 4-hour data collection.
Even with such basic post-processing, we quickly showed the filmmakers a rough profile view of the features they sought, including a suspected terrace. Intrigued, the filmmakers asked us to develop the bare earth 3D model to compare its features to their map of a Mayan mound.
Back at our offices, we calibrated the sensor and fully processed the lidar data. As expected, it showed we collected many returns from the foliage and undergrowth. However, the Orion’s excellent signal-to-noise characteristics still captured the ground returns, and its 0.7-m target separation distance helped discriminate them from the low-lying undergrowth.
After two days of post-processing and point classification, we began matching features on the bare earth model to similar features on the filmmakers’ map. Although more than 1000 years of erosion made some features melt right into the terrain, something caught our eye: a unique “rock” on a terrace that resembled a similar feature on the Mayan map.
The Orion had scanned the rock from multiple angles through thick undergrowth, providing enough data to create a 3D model for identification and characterization. Once we located this feature, we knew there had to be other features nearby. By the time we delivered the model to the filmakers two weeks later, we had identified several features potentially matching their map, including the remains of a wall only 2 feet high.
AeroMetric’s experience with this fascinating project shows the utility of airborne lidar surveying for archaeological investigations: it penetrates dense forests to capture underlying features, and surveys where land-based sensors simply cannot go. The Optech Orion C ALTM has proven particularly useful for low-altitude operation with wide scanner angles to capture vertically aligned features. As for the Mayan investigation: excited by what our lidar data has revealed so far, the filmmakers have the data tools they need as they continue their pursuit of the possible trail of the Mayans in the American Southeast.
James Wilder Young is currently Senior Manager – LiDAR Solutions at Aerometric, Inc.
Optech is the world leader in the development and manufacture of advanced lidar and camera survey instruments. With operations and staff worldwide, Optech has refined technology for almost 40 years to empower surveyors with fast, accurate and cost-effective solutions.
Optech’s Orion and Pegasus ALTM airborne laser terrain mappers excel in the efficient acquisition of high-accuracy spatial data. They deliver unparalleled airborne data collection solutions, from high-altitude wide-area mapping to low-altitude corridor surveys. A complete line of fully integrated aerial imaging cameras, waveform digitizers, and custom mounting with gyro-stabilized solutions is also available.
Optech CZMIL is the most advanced airborne coastal zone mapping system available today, and produces simultaneous coastal topography, bathymetry in depths up to 50 meters, benthic classification, submerged object detection and water column characterization. It also maps objects on the surface of the water and features on land.
Optech CS-series aerial digital cameras—standalone or lidar-integrated—are rugged, high-precision, metric imaging systems with real-time image preview and camera control, INS integration and batch image processing. Optech delivers single- and multi-sensor solutions that combine visible, multispectral and infrared modules into lidar-integrated payloads.
Optech workflow tools—Optech FMS, LMS and HydroFusion—handle mission planning, survey operation and post-flight processing to deliver accurate, precise and high-quality data products.
Mobile and Terrestrial Surveying
The Optech Lynx SG1 collects engineering/survey-grade data while the Lynx MG1 collects mapping-grade. Both provide 3D lidar data rapidly and efficiently, integrating the latest innovations in lidar sensors with best-in-class imaging, navigation, product warranty and support.
The Optech ILRIS terrestrial laser scanner is a complete, fully portable 3D imaging and digitizing solution for commercial surveying, engineering, mining and industrial applications.
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