| GPS Survey: Recovering the Belle: Using GPS to Raise a 300
Year-Old Shipwreck
Archaeologists utilize the GeoTechnologies to raise a historic shipwreck from its watery grave.
By Stuart M. Dambrot It was the year 1684, and tensions between European rulers laying claim to the New World were mounting. Louis XIV of France was determined to quell Spain's growing control over the Gulf Coast and what is now the American Southwest. His solution: dispatch RenŽ Robert Cavelier, Sieur de La Salle, explorer extraordinaire, on an expedition to build forts along the mouth of the Mississippi and to invade and conquer Spanish provinces in Mexico.
La Salle, ambition blazing and hopes high, set out with four ships carrying 300 crewmen and colonists. Sadly, however, he overshot his mark by some 400 miles. In January 1686, La Salle's last ship, the Belle, was grounded during a storm and sank in Matagorda Bay off the Gulf Coast, not far from what is now Corpus Christi. Crew and passengers met an unpleasant end from disease, hostile Indians and poisonous snakes, and by the following year virtually all had perished. La Salle himself - who was never on good terms with most of his men - was murdered by a surviving member of his crew.
Beginning with La Salle's ill-fated journey, Matagorda Bay's strategic location led it to be frequented by numerous exploration efforts. After La Salle, the Spanish arrived and took over the deserted fort he had established, St. Louis., before moving inland. In the early 1800s, the pre-Texas revolution immigration wave saw a host of German colonists alight at Matagorda Bay; like the Spanish, they too relocated to an inland site. For marine archaeologist Barto Arnold, locating and excavating the Belle became an all-consuming goal. Arnold, heading an underwater archaeology project undertaken by the Texas Historical Commission in Austin, says GPS technology was key to the team's success. "In underwater archaeology, positioning is absolutely critical," notes Arnold. "GPS equipment was like manna from heaven."
A native of San Antonio with a master's degree in Archaeology from the University of Texas at Austin, Arnold joined the Texas Historical Commission in 1972 and has been the State Marine Archaeologist since 1975. "Marine archaeology is more than just a combination of archaeology and history," says Arnold. "It has a 'detective-like' quality that gives me a tremendous intellectual thrill every time I take part in a search and excavation."
At the same time, Arnold is quick to point out that marine archaeology and treasure hunting couldn't be more different. "Archaeology is a 'hard' social science that brings appreciation of history to everyone," explains Arnold, "but treasure hunting is completely unscientific, and in reality destroys archaeological data. Moreover," he continues, "the spoils of treasure hunting are sold for profit; archaeological artifacts are displayed in museums, in perpetuity, to promote education and tourism. We may not find gold and silver, but the artifacts we uncover and the stories they reveal are our reward."
Prior to discovering the Belle, Arnold directed a Texas Historical Commission magnetometer survey and site test at Matagorda Bay. He has also led several more recent archaeological projects in the bay. Along with historical documents, maps and geographic information systems (GIS) map overlays, data collected and lessons learned during these projects provided invaluable background when planning the location and excavation of the Belle.
At first, the La Salle team employed GPS to guide their research vessel while searching for the Belle. For this purpose, Arnold recalls, "the Trimble NT200D's ability to receive the U.S. Coast Guard's differential GPS (DGPS) signal for greater accuracy, display information graphically, record data and interface with a PC were vital." A feature of the NT200D that was key during the initial effort to locate the Belle, says Arnold, was its built-in program that creates a search pattern with parallel search lines, requiring only four data points: starting point, initial heading, specification of left or right turn and distance between search lines. Without this capability, notes Arnold, "we would have had to generate a search pattern on paper using a map, identifying the beginning and ending point of each individual search line. It would have been inaccurate, time-consuming and difficult.
"DGPS also proved particularly useful by applying a differential correction to magnetometer surveys to achieve 3-meter accuracy," Arnold adds. "On a previous survey, we were using an older radar-based system which is leased at a monthly cost of $10,000; an outright purchase would have amounted to 70 or 80 thousand dollars. However, Arnold stresses, "the Trimble receiver gives the same accuracy - without the need to set up and tear down multiple line-of-sight radar transponders every day - for an investment of under $4,000."
The NT200D was used in conjunction with a Geometrics (Sunnyvale, Calif.) 866 proton precession magnetometer. Both units were connected to Compaq (Houston) 486-class notebook computers, which recorded GPS and magnetometer data points at the rate of once per second. This setup, recalls Arnold, allowed "remarkably fast coverage" of the target area.
When examining magnetometer data, the goal is to find anomalous patterns on parallel survey tracks. Determining the spacing between these tracks is crucial to the success of the survey, and is sensitive to a number of factors, including the size of the ship, the period in which it was built (earlier hulls contain less iron) and the amount of subsurface turbulence in the target area (higher energies mean greater scattering and, therefore, smaller and more numerous anomalies).
The target site was first located as a magnetometric anomaly while conducting initial survey. The anomaly was identified on three of twelve 4200m search lines spaced 30m from one another at a depth of 1- to 4-meters. Subsequently, a more focused, higher-resolution magnetometer survey pinpointed one primary and three secondary anomalies.
The primary anomaly proved to be the Belle - that is, 25-30 percent of the hull along with contents. (The secondary anomalies, assumed to be related to the primary site, will be examined at a later date.) The primary anomaly was subsequently relocated by magnetometer, and the site was defined with buoys. After relocating the site, the La Salle team continued to use the NT200D during excavation, allowing the team to remove the buoys. "The Belle site was located some 15 miles from the port of Palacios, where we had established our headquarters" Arnold explains. "Precise positioning was especially critical for navigating between port and site under low-visibility conditions, such as at night or under heavy fog. Moreover, combining DGPS with radar gave me a new level of confidence in piloting the boat."
After a series of dive team investigations, a key find was made: an ornately decorated cast bronze cannon with lifting handles, an object acknowledged to be of early manufacture. Recovering the cannon was critical to determining whether or not the find was actually the Belle. Moreover, the site was well-known, and there were security concerns that made it imperative that the cannon be excavated.
By identifying the cannon's construction, decorations and inscriptions, and comparing them with historical records and other recovered cannons from the same period, it was concluded that it did, indeed, come from the Belle. The team then proceeded to painstakingly recover the wealth of artifacts found at the site, concluding with the spectacular remains of the hull itself.
It was July 13, 1995 - curiously, a day before Bastille Day - and La Salle had risen from its watery grave.
Arnold is emphatic that the victory belongs in part to GPS technology, concluding that "no archaeologist - marine or land - should ever go out into the field without GPS." About the Author:
Stuart Dambrot is a senior copywriter for Trimble Navigation. He may be reached at 408-481-8577. Back |
