"When the request for proposals came out [from the university], we could hardly believe it," says Patrick Hastings, PS, principal in charge of survey operations at Midwestern. "Nobody expected the university to undertake stadium renovations for several years." The university needed a site topographic survey and additional building surveys prepared inside and around the stadium. "We sat down and looked at the benefits of using traditional surveying methods versus using our laser scanner, and calculated the cost of each," Hastings says. "We estimated the cost between the two methods to be nearly identical. What made us decide to use the laser scanner, among other things, was that the requirements for the survey were to locate all accessible structures. With the laser scanner, everything was accessible." Midwestern was, however, concerned that our price might be higher than the competition so we decided to invite the engineer in charge of stadium renovations to our office for a demonstration of the scanner's benefits. A few days later, we received notice that two engineers would be coming to our office. Rather than showcasing one of our previous projects, we quickly headed to the stadium and did a sample scan. We then created a demonstration for the engineers. They were impressed and agreed that there were some real benefits to the laser scanning technique, such as documenting all structures safely from the ground. However, even though they were impressed, they were not willing to award the entire project to us just yet. Instead, they asked us to do a pilot project on half of the seating bowl.
We set up the scanner, and while it went through its initialization process, we set up the control points (scan targets). These targets were located using a Leica Geosystems TCRA1103+ total station (Leica Geosys-tems, Norcross, Ga.), which not only gives real-world coordinates to the scan data but also serves as an independent check on the scanner's accuracy. Once everything was set up, we scanned a target to serve as a backsight check. When the scan was complete, this target was scanned again to ensure there were no errors with the scanner or its setup while it was scanning. This procedure wasn't always needed with the Cyrax 2500 because most of those scans lasted less than 30 minutes. The increased field of view of the HDS 3000 means that scans could last much longer and could be more prone to errors. Our first scan from the press box lasted almost four hours and contained 20 million points, but it had some errors. When the backsight target was scanned at the end of the scan and compared to the initial scan, it had moved 0.2 feet, much more than the 0.02 feet expected from previous experience. After we checked to make sure the target hadn't moved and rechecked the scanner setup to ensure nothing had slipped and it was still level, we were a little frustrated. We consulted Leica's HDS support, and determined that slight movements, due to factors such as thermal expansion or the tripod settling (the scanner weighs 35 lbs), can have an effect on the scan data. Unlike modern total stations, the HDS 3000 doesn't yet have a vertical or horizontal axis compensator, so even slight movements can greatly affect the results. To remedy this, we limit scan times, which in turn limits the slight movements that may occur. A setup may still take four hours, but it will be comprised of many individual scans. Still, it only took two days for one scanner operator to scan the 23 walkways from the press box and from within the stands. In all, the scans generated approximately 30 million points. But the university did not want just a point cloud; they wanted traditional drawings showing cross sections through the stadium.