New York is experimenting with differential GPS, which offers greater accuracy and translates to better GIS data for user agencies.

LATHAM, N.Y. - The latest step in the evolution of global positioning system technology is being tested by a New York state department which maps and analyzes endangered species habitat. The Fish & Wildlife Division of New York's Environmental Conservation Department is conducting a pilot program to determine the feasibility of using a differential global positioning system (DGPS) to support a broad range of geographic information-based applications in ecosystem management.

One of the main potential applications for this technology is mapping New York's 2.5 million acres of wetlands. Other applications include mapping forest areas, hiking trails, rare plant and animal communities and monitoring toxic-substance areas.

The pilot program - directed by Senior Wildlife Biologist John Ozard and Senior Fish and Wildlife Ecologist Scott Crocoll - was prompted by the need to ensure accuracy and currency in the division's geographic information system (GIS) and other biodiversity databases. The division was also looking for a faster, more accurate and economical method of mapping and data collection.

PROBLEMS WITH EXISTING METHODS

According to the project's leaders, existing methods for mapping wetlands and locating species habitats are slow, error prone, and costly in manpower. "Where GPS is going to come into wetlands mapping," explained GIS Project Coordinator Wayne Richter, "is in the data gathering and data updating stage."

"For instance, freshwater wetlands have traditionally been mapped by air photography or ground surveys," Richter said. "Boundaries were then transferred to a quad map by estimating locations. Surveys conducted with GPS will be used as a basis to amend maps already in the GIS, [but] with much greater accuracy."

DIFFERENTIAL GPS

The differential in DGPS is a referencing technique used to overcome natural and artificial errors in a GPS. Natural errors result from atmospheric conditions, timing differences, and minor perturbations in satellite orbits. Another source of error, called selective availability (SA), is randomly activated by the Defense Department to degrade the accuracy of GPS for civilian use. The Pentagon's SA is ostensibly motivated by national security concerns.

Raw GPS with SA activated produces accuracies of 100 meters; without SA, predictable accuracies for civilian use are between 20 meters and 30 meters. DGPS, however, can produce accuracies in the millimeter range, depending on the type of measurement used by the receiver.

Differentially-corrected static files obtained with pseudorange measurement produce accuracies of between 2 and 5 meters. The higher-price carrier-phase measurement, an option with the Magellan ProMark V, produces differentially-corrected accuracies in the sub-meter range.

The differential principle has been used in electronic navigation systems for years. In DGPS, a computer measures the differences between the known geographic location of a GPS base station and its satellite-reported positions, and generates time-stamped corrections, which can be transmitted as real-time corrections to remote receivers within a 300-mile radius, or downloaded to a hard disk for post processing (applying differential corrections after remote files have been collected).

POST PROCESSING - FROM GPS TO GIS

The base station, located at the agency's Wildlife Resources Center in Albany, takes a fix each second, 10 hours a day, seven days a week, and stores the data on an internal file. Every hour, a computer automatically downloads the file to its hard disk, then clears the receiver's memory. The base station's recording rate and hours of operation are user selectable and can be changed locally, or remotely by PC, modem and telephone.

GPS files collected in the field can be post-processed at the Resources Center or from remote locations. Files from the remote receiver and the base station are first converted to RINEX, a receiver-independent exchange format that enables post-processing software to work with files from different brand GPS receivers. The software then time-matches

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