OKLAHOMA CITY -- At 9:02 a.m. on Wednesday, April 19, 1995, a truck bomb went off in Oklahoma City, destroying the Alfred P. Murrah Federal Building and most of the people inside.

"I'll never forget that morning," said Deral Paulk, president of Topographic Land Surveyors of Oklahoma. "My office is nearly five miles away from the federal building, but we heard and felt the blast. It shook the foundations of our building. The smoke from the blast could be seen from all over town. At first we all thought that maybe a plane had crashed. Everything went crazy."

Shortly after the blast, representatives of the FBI contacted the city surveyor, Rich Heimz, requesting state plane data against which to measure shifting of not only the federal building, but many others damaged by the blast in the downtown area. These data would be used for accurate mapping of damage as well as for mapping points for evidence gathering.

Heimz felt that it would take four to five days to perform the measurements with the methods available to the city at the time. Furthermore, according to Paulk, the amount of equipment and the confusion in the area would make conventional surveying more time-consuming and dangerous.

But the FBI needed the information within 24 hours. "That's when they came to us," Paulk explained. "After we were given a good briefing on what they wanted, we determined that if we used our Trimble GPS units, we could do the job."

Paulk said GPS methods could reduce surveying time as well as the number of personnel inside the controlled area. "There was no way we could have performed the jobs they needed within the time they needed it without GPS."

Within hours of the blast Paulk and two of his colleagues were at work taking GPS measurements.

THE WAR ZONE

"I'll never forget working down there," Paulk said. "It was like a war zone." There was thick black smoke everywhere. A car hood hung from the limb of a tree. Debris, rocks, burned cars, glass, fire and water covered the streets.

During Paulk's preliminary trip to the site he determined obstructions and spots that would be safe for later occupation. Light points were set and an almanac was gathered with a 12-channel receiver. This was downloaded into a laptop and mission planning was accomplished 'on the spot' for the next day's survey.

Three Oklahoma City monuments within a five-mile radius were picked for control points. These were previously surveyed with GPS for a photo control project. Using dual frequency receivers, a fast-static survey was performed on each point within the project area. "We used both static and fast-static techniques," Paulk explained. "We set eight control points, used four of their points as controls and we had a lot of quality control steps throughout the project. We accomplished everything within 24 hours, which included taking an ASPEN unit on a drive through downtown and running planning software."

The data was then downloaded to a laptop and processed 'on the spot' to ensure that enough valid data was collected. All baselines passed the 'Iono Free Fixed' process, ensuring the quality of the measurements. This data was then fixed to one point and a second test was run to ensure that the local monuments were in agreement. After each passed the test, a dual height mode adjustment and geoid model were built. The data was sent through a final adjustment at a two sigma level, which it passed. Data was transformed to the local base coordinate system, then outputted in a format that could be loaded into a standard data collector for a conventional total station. Several points were then 'shot' between for a final quality check.

"The morning after the blast we went down with the FBI, and

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