The Fast Lane
January 29, 2009
The California Department of Transportation (Caltrans) is one of the largest engineering and surveying organizations in the United States. The department manages more than 45,000 miles of California’s highway and freeway lanes, provides intercity rail services, permits more than 400 public-use airports and special-use hospital heliports, and works with local agencies to improve mobility across the state.
With stewardship as one of its primary goals, Caltrans seeks to ensure that transportation tax dollars are well spent−in part by phasing in new technologies that deliver strategic benefits. For example, after testing and evaluating various systems to improve interoperability with existing data collection hardware and software, Caltrans began equipping most of its more than 100 survey crews with Trimble S6 DR (direct reflex) robotic total stations in 2007. Three-dimensional scanning is the department’s next initiative. “We expect scanning will become the preferred method for remote surveying data collection along transportation corridors, and we’re already doing a lot of it,” says Caltrans Transportation Surveyor John L. Gilmore III, PLS. “We’re also interested in the integrated use of photos and controllers with live video streaming.”
In early 2008, Caltrans tested a Trimble VX Spatial Station from California Surveying and Drafting Supply based in Sacramento. After crews worked with the instrument on various projects over several months, Caltrans purchased a Trimble VX in July 2008 for use by districts in northern California, where one of its crews had become expert in using the new technology. At the top of the benefits list was versatility--according to Gilmore, the VX combines robotic, reflectorless and scanning technology (up to 15 points per second can be gathered in scan areas defined by the operator) in a way that allows seamless switching between modes.
“The versatility means you can use it for anything,” Gilmore says. “If you need to do scanning, you scan, and if you need to do total station work, you can do just that. It’s not going to replace total stations for most crews, but for crews that do certain types of work, like computing quantities, it’s a great instrument for daily use and the only instrument they’ll need.
“For us, the VX is just another tool in the toolbox,” he adds, “Whatever the job calls for, we’re ready with the right instrument.”
Timesaving TechnologyAs one might expect, the most common task for Caltrans field crews is topographic surveying of existing highways and freeways. Roadway profiles and elevations are among the most frequent office outputs. For consistency, the field- and office-work techniques for roadway surveys are rigidly codified, and one requirement is the location of typical breaklines--for instance, at the edge of pavement. The low angle of incidence between the flat, smooth road surfaces and conventional scanners generally limits the range of scanners for this type of work. Additionally, determining breaklines in large point clouds can be tedious, and because it adds a new step to the work flow, getting office personnel to work without pre-identified, clean breaklines is not always an easy task.
The Trimble VX addresses all of these issues. Since the VX is operated by a wireless data controller with a live video feed from the instrument’s point of view, it can be used on a tall tripod to improve the scanning range for edge-of-pavement scanning. “We know that one of Caltrans’ consultants uses 13-foot tripods so that they can use reflectorless technology at longer distances,” Gilmore says, “and we’ll be able to do the same thing with the VX.” This capability also makes conventional total station work more efficient because a taller tripod can see over more obstructions. Taller tripods mean fewer setups, which saves time and money.
Gilmore points out another advantage of the video interface: “Like they say, a picture is worth a thousand words, and that’s certainly true for us. Our crews do a lot of topos for preliminary design work, and after the survey, they get a lot of calls about particular shots--where they were taken, what the site actually looks like, things like that. With the VX, photos can show what the instrument was pointed at and also show where the shots were taken with the point numbers. The photos can be included with the deliverables, so now the design engineers have pictures with point numbers that answer most questions. That’s pretty cool, and it saves a lot of time and [prevents] errors.”
The Trimble VX also resolves the breakline challenge. “With any scanner, it’s hard to define the line where two planes come together,” says Crew Chief Eric Vance, District 1-Ukiah, who also ran the scanner program for his previous employer. “But the VX works well with the way Caltrans shoots and labels breaklines. We can just go into total station mode and take the precise breakline shots we need and go back into scanning.”
Vance says the technique also works well when surveying structures. “We surveyed a very large bridge, and it took awhile even with the VX. But it was very nice to set up to the side, away from the roadway, and then scan the abutment quickly,” he says. “After that, we went to direct reflex mode and shot things like the soffit and edge of deck very precisely. This definitely saves us time compared to even a good reflectorless total station.”
To further streamline work flow, Caltrans uses RealWorks Survey, Trimble’s survey office software. “I’ve used a lot of different scan data managers,” Vance says, “and downloading with RealWorks is very straightforward by comparison. I can just plug the data collector in and it pulls the job file right through with no conversions or anything funky. And then I can get right into working with the data.”
Improved SafetySafety is another big advantage of scanning in general and of this instrument in particular. In addition to alleviating traffic concerns, the Trimble VX lets crews quickly cover cut slopes with enough shots for accurate volume calculations. It also provides an efficient way to collect topographic information on unstable slopes where rockfalls or small landslides occur unpredictably. In fact, one of the first projects Caltrans handled with the leased VX was a landslide in Oroville.
“On our last job with the VX,” says Vance, “we ran normal breaklines like curb and gutter, edge of pavement, etc., in total station mode. Then we switched to scanning to survey a slope that was heavily eroded. There were a lot of erosion channels, and at a foot or so deep they were significant enough that I felt we had to get them all. The VX was perfect. We set the scanner to take shots every half foot or so and were done that day.”
According to Vance, this project would have taken three or four days with a total station, and it would have posed substantially more safety risks for the crew.
Beyond RoadsOne new area of work being opened up by scanning is the preservation of historic building detail. As a test, a Caltrans survey crew used the Trimble VX to scan and photograph a historic incinerator building in the Presidio of San Francisco. “We just scanned the outside,” Gilmore explains, “and we needed several setups thanks to trees in the area. There were a lot of hills and we had to do some hiking, but the VX did just fine. A bigger instrument would have been much harder to work with.” Gilmore expects that Caltrans will eventually use such data to produce 3D models and photos along with its conventional use in their everyday work flow.
After months of use, the Trimble VX Spatial Station is standing up to the scrutiny of Caltrans surveyors and is adding value that is making 3D scanning a mainstream tool for use on one of the world’s most impressive highway systems.
“It’s great to have different ways to collect data, and to be able to scan when appropriate. I just pick the tool that works best,” Vance says. “It’s like having two instruments in one.”