Mapping a National Memorial
Today, what remains of the USS Arizona lies submerged in 38 feet of water in exactly the same position as it did after the attack more than 60 years ago. Its superstructure and most of its upper deck components have been removed. Only portions of its mainmast and a gun turret rise above the surface. The ship has been designated a national memorial and is administered by the National Park Service (NPS) to honor those who lost their lives at Pearl Harbor on that dreadful day.
NPS commits extensive resources to preserving and protecting the wreck as a cultural resource of the United States. While continued saltwater corrosion is inevitable, NPS is prepared to take corrective action, if possible, to prevent the Arizona from degrading to the point of collapsing in on itself. A structural failure of this magnitude could prove environmentally hazardous if the fuel oil still inside the warship spills into the harbor.
"The ship is in surprisingly good condition because a concretion composed of iron corrosion products and other natural materials have encrusted much of the surface, essentially protecting it from exposure to the seawater," says Matthew Russell, an archaeologist with the NPS Submerged Resources Center (SRC) in New Mexico.
Beginning in the 1980s, SRC staff members, in conjunction with USS Arizona Memorial staff members, devised a plan to map the ship's current condition in order to develop a base line and to be able to monitor the ship over time. Having an initial base line measurement would alert them to any movement of the ship and help predict a potential structural collapse. Not only did the initial mapping provide monitoring capabilities, it also facilitated the locating and logging of numerous artifacts both inside the ship and on its decks. From this information, a database will be constructed to aid in the archaeological study of the battleship and its role in one of the most infamous moments in U.S. history.
Integrating GPS Data Into a GISRecently, the NPS assembled the map, feature and attribute data into a centralized web-based Geographic Information System (GIS) located in the NPS NISC-GISD (National Information Systems Center, GIS Division) office in Lakewood, Colorado. At present, it is only available to NPS scientists and researchers, but it ultimately will be made accessible to the public via the Internet.
In 2001, NPS personnel dove into the site waters to initially establish control points and perform survey-grade mapping of the wreck with GPS to geo-rectify data for the GIS. Then, in late 2003, divers returned to re-survey those points to compare with the earlier data and assess any structural deterioration.
The USS Arizona GIS project offers valuable insights for surveying and mapping crews working in challenging conditions. Along with highlighting underwater GPS mapping techniques first developed by SRC and refined at Pearl Harbor, the project demonstrates how survey-grade GPS equipment can be combined with mobile GIS technology to facilitate mapping in a harsh environment.
Mapping the ArizonaDetailed mapping of the wreck began in the early 1980s and took several years to complete during hundreds of dives by NPS and U.S. Navy personnel. GPS wasn't available to them then, so the only options for divers to use under water were low-tech and extremely labor intensive. Divers used nylon string, clothespins, plastic tape measures, an ordinary protractor and the process of trilateration to locate thousands of points on the hull, decks and superstructure. From these measurements, the divers made detailed sketch maps of the sunken hull. The divers had a detailed list of required measurements. They also identified and described 400 features on the decks, many of them pieces of debris; the features were each assigned identification numbers and added to the sketch map.
"Today, surveying the USS Arizona takes two days and a team of seven people," says Mark Duffy, an NPS geographer. "GPS technology has revolutionized the way projects like these can be surveyed and mapped."
The information gained by the divers formed the core of what became the USS Arizona GIS. Using original ship plans, the 1980s data and new information gathered by SCR divers, Northrop Grumman IT (Lakewood, Colo.), a division of defense contractor Northrop Grumman (Los Angeles, Calif.), built the GIS under contract to NPS.
"Developing a GIS for a ship is similar to creating one for a city or utility except that the Arizona is only 608 feet long and 97 feet wide," says Matt Brown, GIS solutions engineer for Northrop Grumman. "The challenge this presents is that every point must be located with survey-grade accuracy of one to 10 centimeters."
In the 2001 Arizona mapping project, NPS joined forces with the U.S. Army's 29th Engineer Battalion Survey Platoon to survey the battleship. At this stage, they used Trimble (Sunnyvale, Calif.) 4000SSE receivers. Although vastly more efficient than previous mapping technology, the process still required two minutes of survey time to achieve the required accuracy and post-processing of the measurement data. Two minutes is a long time in such a dynamic environment.
"The objective of the 2001 dives was twofold," explains SRC's Russell. "Approximately 35 points were located on the wreck for use in rectifying the 1980s sketch map. The divers also established and marked eight super-points that would serve as the base line for measuring movement of the ship."
For the 2001 project, the team put the GPS base station at a survey monument on land near the memorial's visitors' center. Two basic types of points were surveyed using post-process kinematic GPS techniques. Kinematic points were taken on recognizable features of the sketch map done in the 1980s and "super-points," which would be used as control points to monitor the wreck into the future.
Divers examined the sketch map to pinpoint vessel features that could easily be found underwater to serve as kinematic points. These features had to be on a relatively level surface of the deck to hold the tripod. To establish the super-points, divers selected eight evenly spaced locations across the surface of the deck. They scraped away layers of rust and marine growth and then fired stainless steel nails into the deck to mark the points.
The team's biggest obstacle was keeping the tripod steady on a specific point while it was submerged in the constantly moving water. The post-process kinematic GPS technique necessitated multiple GPS satellite cycle counts to correct the rover unit's measurements with the base station's recording to achieve centimeter-level accuracy. Thus, all points had to be occupied for two minutes in order for the GPS receiver to make the required number of satellite observations.
"It wasn't easy holding the tripod on the point when the currents were moving," Russell says, "but we got the measurements we wanted."
Using ERDAS IMAGINE (Leica Geosystems GIS & Mapping Division, Atlanta, Ga.) imaging software, the NPS GIS office in Colorado utilized the coordinates of the points to geo-rectify the scanned 1980s sketch map and original ship construction plans. After the various images or maps were rectified into a common frame of reference, they lined up in the GIS and could be overlaid. NPS then delivered these data sets to Northrop Grumman engineers for continued development of the GIS. Northrop Grumman engineers vectorized and attributed all of the original ship plans for use in the GIS.
"At that point, the GIS had base maps of what the ship looked like before the attack and what it looks like today," says Northrop Grumman's Brown.
Using Mobile GISIn 2003, SRC consulted with Trimble to create a plan for further operations on the site. Trimble's recommendations included the use of a Trimble survey-grade 5700 GPS base station and rover units as well as TerraSync GIS data collection software. (TerraSync mobile GIS software can integrate GIS data collection capabilities with survey-grade GPS mapping.) The 5700 Total Station GPS receiver system allowed the roving receiver to communicate with the base receiver via radio. Differential correction was accomplished in real-time, thus eliminating the need for post-processing and greatly accelerating data acquisition.
"Real-time radio correction reduces the time needed to occupy a point from two minutes to just five seconds and increases the accuracy of [the] data," Duffy says. "That makes a tremendous difference in a dynamic environment [like] water."
During 2003 operations, SRC used TerraSync to display GIS location and map data in the field on a Trimble/TDS Recon, which can be used under water. Having the GIS data with them for dive operations ultimately saved the project from losing two years' worth of position data, according to Russell.
"When we got down to the ship, we were stunned to find that the stainless steel nails had completely corroded in just two years," Russell says. "The divers couldn't locate our super-points."
"The guys in the dive boat had the whole Arizona GIS in the palm of their hands. It was invaluable to us that we could look at data on site and make decisions right then," Russell says. "By viewing the data collector screen, they could see how close the divers were to the super-points on the deck, and they radioed instructions to them to move the tripod until they finally hit the right spot. The nails looked like smudges on the metal, but we found seven of the eight."
The team re-surveyed the super-points and updated the position data directly in the mobile GIS. The data provided instant verification that the ship had not moved since it was last surveyed. Immediately, eight new base line points were surveyed and established using plastic disks glued into place to replace the 2001 points. The team hopes that plastic will withstand the harsh conditions better than the nails.
"Only having to occupy a point for five seconds with the GPS made our jobs much easier and faster," Russell says. "This improved the accuracy of our 2003 data collection over the 2001 data because the tripod moved much less in five seconds than it would in two minutes [of point occupation]."
For the rest of the 2003 dive, the team used a small, remotely operated vehicle that was able to photograph conditions inside the ship by slipping through small portholes and hatches. This information will be included in the GIS for archaeological and structural assessment purposes.
"This project was really made possible by new survey technology," Duffy says. "Using centimeter-level RTK is the only way to monitor the changes of the USS Arizona due to the environment it is in."
Putting the USS Arizona on the WebSince the data gathered from the 2003 dives validated that no movement had occurred in the structure of the Arizona since 2001, plans about what to do if the ship begins to collapse have been put on hold.
In the meantime, Northrop Grumman engineers continue to build the USS Arizona GIS map layers for every deck with associated attributes, both current and historical. For example, portable items found on the decks or bulkhead conditions captured by the remotely operated vehicle in ship compartments have been recorded in the GIS. Additionally, the GIS team is scanning more than 2,000 engineering drawings of individual rooms and attaching them to the base map. More than 3,000 historical and current photographs, images and documents will ultimately be included in the GIS, completely searchable either by name or location.
Funding permitting, NPS will publish the GIS on its website, giving the general public and interested historical organizations access to view and study through a state-of-the-art web-based GIS system for the warship. This is just one of numerous examples of how technological advancements have had a significant impact on historical preservation.