By Brian Miller Features Editor When a natural disaster strikes, information is the lifeblood of relief and recovery efforts. But with electricity out, bridges washed away or shaken, and general chaos among victims, disaster managers have had difficulty pinpointing the precise location of various features and occurrences. But in California, which has a reputation for periodic man-made and natural disasters, crews have begun using differential global positioning system (DGPS) to record the precise location of unsafe bridges, buildings and intersections to help coordinate immediate disaster response as well as keep a record for future analysis. The California Governor's Office of Emergency Services (OES), which bought about 100 Trimble Scoutmaster GPS receivers, issued units to field workers after the January 1994 Northridge earthquake. OES response personnel carry a GPS unit about the size of a cellular telephone and have a differential unit the size of a pager cabled to the GPS unit. Differential services were provided by Differential Corrections Inc. of Cupertino, Calif., after the Northridge quake. In Northridge, the exact location of cracked bridges, broken gas or water lines and unsteady buildings was recorded. The data was later loaded into a geographic information system (GIS) for spatial analysis and recovery planning. OES is using Arc./Info, Arc/View from ESRI, based in Redlands, Calif.
INADEQUATE ADDRESSES A key advantage to using DGPS is that the locations are determined with latitude, longitude and elevation. This is important because addresses have been the main method for recording geographic information. The problem with this is that features such as bridges don't have addresses, making it difficult for crews to communicate precise locations. Sometimes addresses are impossible to use even in residential areas hit by a disaster. For example, after Hurricane Andrew ripped through Florida in 1992, recording damage location was made difficult because street signs in many areas had been blown away. Before GPS and GIS, disaster recovery personnel would use physical maps and make guesses as to the location of some of the damage areas. Workers would use paper and text descriptions to indicate what was damaged and its location. This data was then combined with other reports and cross referenced by hand. This process made for a time-consuming chore when time was of the essence. With the data loaded onto a GIS from DGPS, not only is location data more accurate, but it is easier to read than the old paper maps and text. "We use it for visualizing damaged areas," said Terrie Monaghan, OES program manager for public assistance. "And if we don't know where it is at, we can look at a map, click on that point, and know exactly where and what it is." Monaghan said that the system can also be used in cooperation with local authorities using GIS. If the county has latitude and longitude for all its bridges, for example, data collection from damaged structures can be plugged into the system and shown on a GIS map.
TRIAL AND ERROR The new system is much easier and more mobile than the "old" method used in post-disaster assessment by OES personnel. Laptop computers were used by crews during the Los Angeles riots in 1992, and location data had to be entered in text form. OES also had state Forestry Department aircraft fly over the strife-ridden area to take photographs in an attempt to determine what was damaged and where. "This proved to not be the most effective way," Monaghan said. "Then we drove the streets during the riots, and would stop at intersections and try to type-in data as text." Monaghan said that when OES was putting together a data collection system, they thought about using pen-based computers for writing text information in addition to taking a DGPS reading, but decided against this route. "We found that they're not feasible because the units had a hard time reading writing." The device "has its place but we're not using them because we had wordy reports." But with DGPS, the precise, "ultimate" location is determined, recorded and time-stamped. The latitude, longitude and elevation are determined by the GPS unit, which has accuracy within 10 feet when differential is used. The system worked great for OES after the Northridge temblor, and was also used during last winter's flooding, which washed out highway bridges and other infrastructure. "I have used it, and once it is set up, it's wonderful," said Monaghan. "It allows mobile access to damage sites, and transfers differential GPS data for development of precision GIS maps of damage areas," she said.
DIFFERENTIAL ACCURACY OES is using differential GPS because it gives more accurate readings than straight GPS. Because the Defense Department scrambles GPS signals (called selective availability), regular GPS gets accuracy within about 100 meters, though readings are usually accurate within a few meters. Differential GPS, meanwhile, uses a known point to determine how inaccurate the satellite signals are at a given time. A user measuring a new point receives a signal both from the satellite and the known point. The known point normally uses FM subclass radio signals to send correction data to the field user taking location measurements. Triangulation plus signal correction is calculated by GPS units to achieve accuracy within a few centimeters.
HISTORICAL DATA The DGPS system also enabled OES to store information digitally for future reference. If a bridge washes out every rainy season, then OES can prepare contingency plans for the next disaster - or try to get the situation on the ground changed, such as rerouting drainage systems. "We want to have this information and use it for the future," Monaghan said. "And by having it electronic, the data is more accessible and can be cross referenced much easier than having paper in a file cabinet," she said. "This is part of a bigger project. Ultimately we'd like to be more efficient in the disaster business," Monaghan said. "This is just a piece of the pie of what we're doing for public assistance after disasters."
