data from the two receivers and applies corrections to the remote files. Corrected files imported to the GIS are first converted by a utility program in the software to a language understood by AutoCAD, Arc/Info, MapInfo and other databases.
Currently in the developmental research phase of the pilot program, DGPS is being evaluated with a broad range of applications for the bureaus of Wildlife, Fisheries, and Environmental Protection. The project team, for example, mapped Tern and Piping Plover communities on Long Island. Both populations are declining along the Atlantic Coast and the species have been put on the state's endangered list. Other endangered or threatened species habitats being mapped by the team include the Massasauga Rattlesnake, found in wetlands, and the Eastern Timber Rattlesnake, which is found mainly in mountainous areas.
Crocoll explained that wetlands are plotted on New York state planimetric maps using 1:24,000 scale, with thick lines indicating the approximate outer edge of the wetland. "Based on the scale of the map, that line could be 50-feet wide," he said. But "GPS is going to give us an accuracy of 2 [meters] to 5 meters."
Differentially-corrected static files taken in open terrain are producing this kind of accuracy with occupation times of nearly five minutes. Forested areas with heavy canopy require longer occupation times.
Data collected can be checked by a crew still in the field using a portable PC, Ozard said. "The flexibility is especially useful because it lets us check the accuracy of our product before we return to the home office. We can also differentially correct our data from a remote office and produce an output map before returning home."
BASE STATION RANGES
Part of the DGPS test includes verifying the accuracy of known horizontal control stations. When a National Geodetic Survey marker is in the area, the team will take one or two static files. If those correct to better than 5-meter accuracy, they are fairly confident that the data will be in the same accuracy range. Most of the tests thus far have shown accuracies within 5 meters, Ozard said.
Another test is to see how far from a base station DGPS measurements can be taken while maintaining accuracy within 5 meters. Points taken from as far away as 311 miles from a station have produced errors of less than 3 meters. "The accuracies are better than those required for wildlife and land management applications," Ozard said. "It looks like one base station will adequately cover the entire state."
Although it will take another year to complete the evaluation, Ozard believes DGPS will be a very useful tool in determining locations of rare and endangered species, particularly in areas without landmarks. "It should also assist us in determining the movements of animals, their home range, and how far they travel in various time periods. Another practical benefit will be using mobile files to map locations of smaller communities and habitats that cannot be determined from aerial photographs."
Following training and product evaluation, the New York project leaders chose a Ranger, 12-channel, all-in-view Base Station from Ashtech, and a package of three Magellan NAV 5000 PRO GPS Receivers, Hewlett-Packard HP-95LX Dataloggers, and Magellan's mission-planning, data-collection and post-processing software. The NAV 5000 units were subsequently upgraded to the more powerful ProMark V's. Their expanded data-storage capacity eliminates the need for external data recorders. Important factors in the selection of remote receivers were portability, weight and compactness. "We were looking for receivers that could easily be carried through rugged terrain and brush," Crocoll explained.