OSEPH W. PFIEFER, the battalion chief who was in charge of the fire department's planning and strategy at the World Trade Center site, realized within days of the Sept. 11 attack that recovery workers were having trouble recording the location of evidence.

The workers had to sight landmarks to gauge where they were within the 16-acre site, which was broken into blocks 75 feet by 75 feet for mapping. They used paper to take notes on the evidence — from metal fragments to human remains — before it was removed to a landfill, a warehouse or the medical examiner's office. The information was later entered into a database by hand.

The relative location of evidence is seen as crucial to police detectives pursuing the criminal investigation, to engineers determining how the buildings fell and to fire officials reviewing their tactics in responding to the attack.

Chief Pfiefer has a special interest in the last issue. He was among the first battalion chiefs on the scene and lost his younger brother, also a firefighter, in the towers' collapse. The weekend after the attack, he met with technology companies to come up with a more accurate solution.

He selected hand-held computers from Symbol Technologies (news/quote) equipped with devices from LinksPoint using the Global Positioning System, or G.P.S.

Three days later, equipped with the computers, a team of six firefighters was on the scene from Engine 10, Ladder 10, a company just across Liberty Street that lost five firefighters in the disaster. They used the computers to enter a description of each item and give it an identifying number and bar code.

The G.P.S. devices automatically record the location with an accuracy of three to nine feet. The data is then transferred to nearby laptops connected with a database.

"We had too many steps before and were only getting a vague location," Chief Pfiefer said this week in an interview at the Fire Department's command center, a few blocks north of the trade center site. "This solves the problem of all the scribing and transcribing, and gives a better location."

G.P.S., which relies on making connections to at least three satellites to pinpoint a location, generally has not worked very well in Manhattan because of the canyon effect of the city's skyscrapers. But Gregory J. Fucheck, a vice president at LinksPoint, a wireless-technology company in Norwalk, Conn., said the large space created by the collapse of the two World Trade Center towers opened up the skyline for the devices to connect to satellites.

G.P.S. devices are limited in their accuracy by the Defense Department, which controls the satellites and reserves the most accurate readings for military operations.

Commercial G.P.S. has generally been used to map items like utility systems. The data, as is being done at the trade center, is then pumped into a database using Geographic Information System technology, or G.I.S., with mapping software.

At the trade center site, G.P.S. and G.I.S. are being used to track trucks leaving the site with scrap metal to prevent theft, to map fires and other hazards underneath the site and for the investigation, said Vincent Luciano, a vice president for mobile computing at Symbol, which is based in Holtsville, N.Y.

There have been some concerns at the site about the accuracy of hand-held G.P.S. devices. More accurate alternatives, like a backpack with a G.P.S. antenna, were examined. But officials decided the equipment would be too cumbersome at the difficult site.

"What people forget is it is a very dangerous site and that you need a device that is very portable and easy to use," said Capt. Justin Werner, the head of the Fire Department's mapping unit.

There are still some spots where G.P.S. coverage does not work on the edge of the trade center site. Officials are examining the idea of hoisting an antenna on 3 World Financial Center, across the highway from the trade center site, that would give them a clearer shot to satellites so as to improve reception of the satellites' radio signals.

Fire officials are using the data to create maps, some littered with thousands of little yellow and red dots that can be clicked on a computer for more detail, to mark everything from human remains and clothing to parts of the planes and fire equipment.

Officials hope that in addition to helping with the investigations, the information will be useful to victims' families.

"I know that for some of the families and the friends," said Chief Pfiefer, whose brother, Lt. Kevin Pfiefer, has yet to be found, "to know where their loved ones were found will be important."