Before scanning, Kuypers established a control network by setting laser-scan targets throughout the main ticketing areas and long halls of the terminal. Control targets were located every four or five scans, with intermediate (non-control) targets used for feature alignments. Kuypers used a conventional spirit level to transfer elevations to control targets, and a conventional total station to traverse through the control targets, “the same principle we would use in conventional survey, a primary traverse and then a secondary traverse,” said Kuypers. Once scanning started, he took care to capture common features to ensure all the smaller areas could be aligned into the overall scan data.
“The survey field book was important, too.” said Stacy. Kuypers “logged daily scan activities to overcome the necessity of ‘jumping around' as access was obtained for controlled areas.” As he scanned, he had to adjust daily scan activities to accommodate access and provide a continuous workflow for the scanning. The field book was used to “keep track of where we had been, and where we had to go.”Equipment and Processing
When Woolpert elected to add laser scanning to its arsenal of survey tools, “we looked at every laser scanner on the market,” said Stacy. “We decided we needed a scanner that would work well both indoors and outdoors. We wanted to have something that was versatile enough that, as we entered the laser scanning market, we could perform a number of different types of projects.”
After careful study, the firm settled on the Riegl LMS-Z360 scanner from Riegl USA Inc., based in Orlando. Fla. “We've been involved with doing buildings and as-builts, adding data to clients” GIS systems, and DOT work,” said Stacy. “So that's one of the reasons we decided on the Riegl.”
How did the Riegl scanner perform? An attractive feature of this device is that it “works within a 90-degree scan window, from +50 to -40 degrees,” said Stacy. “It can pivot upward to collect ceiling data without the operator having to move or readjust the scanner.”
Kuypers explains that this allows more data to be captured in fewer scans than with scanners having a narrower field of view. “What led us to Riegl was the desire for a 360-degree scan,” he said. “Why do 15 scans when a 360-degree field-of-view scanner gives you the same results with just four or five 90-degree scans?”
Also important was the calibrated 6-megapixel digital color camera integrated within the laser scanner. This provides a color digital image of the scan scene that can be overlaid on the laser-scan point cloud, which results in a scalable orthophoto similar to those generated for aerial applications. The additional data make it easier to complete feature extraction processes. “The color data are very useful,” said Kuypers. “We used the digital photos to clarify what we were looking at in the point cloud.”
Calvin Johnson, Woolpert's survey/CAD manager explains that when he was processing the laser-scan data, the color images helped him in “picking out small details” such as the PME features required in the data deliverables.
How about accuracy? “We're surprised how clean the data was,” said Stacy. “I'm typically a skeptical person, so when we went out and did our first laser-scanning test on a bridge, we had our total station as well. We measured points with both [instruments] and compared them, and found we were getting survey-grade accuracy everywhere we measured when we aligned the [laser-scan] point clouds, as long as we did our due diligence on survey control.”
Johnson processed the data through an involved procedure that aligned all of the scans, extracted PME features, and created CAD drawings. The last steps were quality control and finalizing the deliverable:Build the aligned geo-referenced footprint of the entire buildingComplete the floor plan with door swings, windows, and overhangs for the entire buildingComplete the ceiling plan for the entire buildingAdd PME features to complete the project.SPEEDING DATA FLOW
Had Woolpert used traditional survey methods in the San Diego Airport project, the work process would have started with digitally scanning existing, out-of-date paper drawings. “Then a two-person crew would have gone into the facility, redrawn the plans on pen tablet computers as they took physical measurements with mechanical and electronic tape, taken that data back to the office, and cleaned it up,” said Stacy. With this approach, field data collection would have required some 700 hours of work by each of the two people. Instead, with laser scanning, data capture required Kuypers alone to make just two trips of 10 days each, working an average of 12 hours a day. The trips were separated by a week in the office to check the quality of the data from the first trip.
Further savings came after scanning, with office time needed to produce 2-D plans in just 425 hours, according to Johnson. Had conventional measurement methods been used, Johnson said this much time would have been needed just for CAD cleanup, to complete the drawings that would have been compiled in the field on pen-top computers.
Still, 3-D laser scanning is not always the most effective solution for creating 2-D floor plans. “Many simple jobs are best done with a total station,” said Stacy. “For example, measuring the floor plan and dimensions of a ‘big-box' retail facility that doesn't have complex interiors or curved walls. That's especially true if no feature extraction is required. The more complicated the space, the more a job lends itself to laser scanning.”
— Jenkins is senior analyst at Spar Point Research LLC, based in Danvers, Mass.Benefits of 3-D laser scanning over conventional surveyingMore accurate and cost-effective method for capturing existing conditionsIncreased safety through remote measurement, which keeps field personnel out of dangerous environmentsHigh-resolution and true-color dataGeo-referenced digital photographs that allow work in a point cloud—click on an area to see the corresponding digital photo in a different window.Increased survey accuracyHigh-volume field data collectionMinimal interruption to the existing environment. Using an eye-safe laser allows work during everyday activities on the scene; post-processing software makes it possible to filter out any people who pass through the scene during data capture.
These benefits are according to Kevin Stacy, a project manager in Woolpert's surveying/global positioning system group.