tion, the architect required a second point cloud that would include the building’s surroundings including trees and buildings across the street. To capture the entire building and meet that precision, Vestbø estab- lished seven georeferenced scan locations, including roof corners, street-level building corners and the back of the building, and set up an additional 45 scan stations; those non-registered scans would be tied into the georeferenced scans in post processing. Setting the SX10 on the roof positions, Vestbø first scanned all of the roof’s features including picnic tables and benches, utilites, raised structures, air vents, and fencing along the external edges. From that vantage point, he also had clear sight to scan the courtyards and back- yard features, including a glassed-roof atrium, 30 m (98 ft) below. He then moved to the street, and methodically moved around the BT struc- ture, setting up and leveling the SX10 and capturing a full dome scan of every detail of the façade, as well as the courtyards and backyards. At scanning distances ranging from 2 m to 30 m, each scan took 12 minutes to complete and then Vestbø spent another three minutes using the scanning total station’s built-in camera to take a series of photos. After 13 hours of scanning, Vestbø collected 260 million data points with a 40-60 percent overlap–– fortunately, not a snowflake fell. “The ability to switch between total station surveying and scanning with one instrument al-lows me to be so much more efficient, both in time and costs,” says Vestbø. “The scanner is sur-prisingly quick and quite precise, and with so many overlapping points, it makes it much easier to process the data and create an accurate point cloud. It also automatically registers the data, so you don’t have to georeference the point cloud during post-processing, which is a huge time savings.” Tying it all together Back in the office, Vestbø used the drag-and-drop nature of Trimble Business Center (TBC) software to seamlessly integrate the GNSS and scanning data and then he colorized the scan. He then used the seven georeferenced scans to spatially anchor the 45 unregistered scans. He manually delineated a window of referenced points and the software automatically georeferenced each scan one by one to create a spatially accurate 3D point cloud––a process that took about two minutes per scan.
says Vestbø. “What was unpredictable was the weather and typically in Norway we have a lot of snow this time of year. Snow can produce extraneous noise in the point cloud. So, I had to choose my scanning days wisely and work really efficiently.” With the risk of inclement weather, Vestbø chose to capture the roof first. Resembling a thick “W”, the flat roof has two long rectangular ends running vertically, a wide horizontal strip across the length of the building, and an elevated rectangular block in the middle. Dotted around the surfaces are various utility structures and a cluster of picnic tables and benches. Using a Trimble R10 GNSS receiver, he established site control by setting three temporary fixed points on the roof. He observed each point for about one minute, waited 15 minutes, and then repeated the same process two more times. Tying into Norway’s CPOS GNSS measurement correction service, he obtained an initial overall accuracy between 8-14 mm (0.1 in - 0.6 in) in plane view and 2-3 cm (0.7 in - 1 in) accuracy in vertical. The R10 data was saved using Trimble’s Ac- cess™ field software on his Trimble TSC3 data controller. Using the Trimble SX10’s total station functionality, he then occupied the GNSS points and set two other control points on the roof. Combin- ing the two measurements not only created a tighter control network of 3 mm, but georeferencing the SX10 setups enabled him to ensure he had good scan coverage before leaving the field as well as reducing both field and processing time. “The integrated survey workflow provided through Trimble Access makes it really easy to move from one instrument to another and sync data between the field and the office,” says Vestbø. “When you’re working against nature, that’s really helpful.” Focused on the façade Per project specifications, Geomatikk Survey needed to scan the BT building’s façade, roof, courtyards, and backyard features and produce a point cloud with an overall accuracy of 2 mm (0.07 in). In addi- Looking down on the roof and front façade through the colorized 2-mm point cloud of the BT building. Photo credit: Geomatikk Survey
The SX10 atop the UiB building, with a clear view of Bergen. Fortunately, not a snowflake fell during the scanning of the building. Photo credit: Geomatikk Survey
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february 2020
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