Combining bathymetry and lidar data for a fast-turnaround topographic survey

Author: Sam Pfeifle

As it arrives at the state line separating Washington from Idaho, the Snake River forks, one tine flowing to the south as the Snake along the border, the other heading due east as the beginning of the Clearwater. Anyone traveling east on a boat would quickly encounter the Clearwater Memorial Bridge, a roughly 228-metre-long bridge dedicated to veterans of WWII.

这座桥开业于1951年春季，被联邦政府指定为十多年前，主要是因为养活了它的道路，这些道路不再适合每天处理桥梁的交通。

2019年3月，即建造建造近70年后，爱达荷州交通运输部委托了一对工程公司，对桥梁及周边地区进行了完整的地形调查，作为一个多年项目的一部分，以取代其和周围的道路，以确保几代人的安全有效运输。

The job fell first to恐怖工程师, which has 15 offices throughout the western United States, including three in Idaho. Collecting survey data for the bridge and surrounding roadways was a standard job for their mobile mapping unit, said Lonnie Olson, principal and survey manager at Horrocks, but the project also called for bathymetric data for the river bottom and bridge substructure.

奥尔森说：“他们想看看桥桩周围的冲刷。”“他们正在寻找的途径之一是保持子结构和大部分桥梁本身。”

He knew that adding multibeam sonar data to the mobile mapping cloud data would give the resulting topo consistent, fine-grained, accurate data from the river bottom on up, but there was a problem: Horrocks doesn’t have a multibeam sonar unit.

“In Utah, we don’t have any big rivers, and most of the lakes don’t require much bathymetric data,” Olson said. “We normally just do single-beam sonar, which isn’t what we were looking for here.”

幸运的是Spicer Group有一个用于这种桥梁尺寸分析和液压建模的多光束单元。更好的是，Spicer的声纳船还装有Leica Pegasus:Two移动传感器平台的上面and below-waterline mobile mapping, the same mobile scanning platform Horrocks invested in 18 months prior for its roadway mapping.

Olson decided to reach out to Spicer project surveyor Nathan Shepherd and combine forces on the job—the first time he’d ever subcontracted out to another surveying team.

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一天完成

恐怖将驱动桥梁和周围的道路。Spicer会驱车河流以将所有东西放在水线，河岸和桥梁的下面。然后，他们将数据拼接在一起，提取平面计算学以创建表面模型和地形调查。

Because the Pegasus is a sensor platform capable of ingesting data from multiple sources, the multibeam and lidar information could be integrated into a single point cloud.

“The mount for the Pegasus is bolted to the boat,” Shepherd said, “You can’t just go out and rent a boat and make it work.”

While the Pegasus’ lidar unit could grab everything with just a single pass back and forth under the bridge and along the riverbanks, the multibeam is a bit more like mowing the lawn, Shepherd explained. The team needed 40 or 50 passes with the multibeam to get the bottom of the river and the data necessary for all the clearance elevations and hydraulic study.

Still, it was all done in a day.

All told, it took about three days to set up the full control network for the job, then four days onsite preparing and doing the actual scanning, for a total of seven days in the field— significant time savings compared to traditional methods.

“Ten years ago,” said Olson, “you would have had to cross-section every 50 feet with a robotic total station and shoot in some key points of the structure, and you’d have to spend probably three to four weeks onsite doing control and collection.”

As for the underwater work, “you’d probably have a cross-section every 250 feet, maybe a dozen total measurements with single-beam sonar, and then a model from that,” said Shepherd. “And they wouldn’t have been able to get the scour. There would be no way to know about that. They would have had to send divers down to visually look at it.

所以,大约三分之一的the time,” Shepherd summarised, “and I don’t know how many hundreds of times more data.”

更好？奥尔森说：“当我将所有这些都放在一起时，我们的云层排成一排，而Spicer和我们的上下数据之间的精度不足。您再也找不到任何其他方法。您可以实现它，但成本将是指数级的。”

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Mobile Mapping’s Back-office Effect

You’ve probably heard the horror stories: “Mobile mapping creates so much data it’s impossible to manage!” “Who needs a survey team when all you have to do is drive a scanner around?”

当然，搬进移动映射的确需要后台调整，但是如果您为他们做准备并为您的员工提供培训，它们应该相对易于管理。

恐怖工程师has been doing laser scanning for roughly eight years but just moved into mobile scanning about 18 months ago with the purchase of the Pegasus: Two platforms from Leica Geosystems, part of Hexagon. Horrocks’ principal and survey manager Lonnie Olson said it absolutely affected his workforce.

他说：“我们的一些现场工作人员在室内过渡。”“而且我有一支由前高级CAD技术组成的团队，我们已经训练过进行调查工作。我们已经能够在内部完成大多数后处理工作。”

They also recruited recent graduates from local geomatics programs and started training them as soon as they were on board to work with mobile point-cloud data.

“Now we have a team of five people,” Olson said, “who do nothing but extract data from point clouds all day, every day.”

Leica Geosystems’ Jason Ellis said Horrocks’ experience is typical, and the movement of field staff into the back office should be seen as a smart move and not a conciliation.

埃利斯说：“从现场到办公室的连续性始终是一个挑战。”“很难传达现场所做的事情以及如何处理。”

By having experienced field staff cross-trained into experts at working with the data, you have people who can intuit what the field staffers were trying to accomplish as they sort out the important data.

同样，埃利斯说，让在现场操作扫描仪的人也能够使用该软件，这可能非常有帮助。

“I see a lot of people doing post-processing in the field so they know they’re good and can pack everything up or know they need to rescan if they’ve had an issue,” he said. “A lot of field people are starting to take on the role of analysis, and that’s a good thing. You’ve got sharper people in the field who can be that much more productive.”

Cross-training also helps fill the labour gap in the geospatial profession. “It’s hard to find good people, so if the team doing the collection in the field can manage the majority of the post-processing and get to the point where it can be certified, that helps keep projects on track,” Ellis said.

至于这项技术，奥尔森说，无需对此强调。只要确保您拥有很多多端的USB驱动器，这些驱动器现在非常便宜，并且在硬件上也不便宜。

“您的标准CAD机器可以很好地处理这些东西，”奥尔森说。“该软件现在开始赶上。您真的不需要太多才能开始。很简单。即使您的基本笔记本电脑也可以处理。当然，您需要一台体面的计算机，但我们是一家工程公司，所以我们还是拥有它们。”

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A Seamless Workflow

With all the post-processing done in Pegasus Manager, the workflow was straightforward for combining the datasets. Teams running both vehicles were able to check the data and imagery in real time for coverage, then use the trajectory adjustment module to bring all the data together and constrain for survey control. The Pegasus software uses automated feature extraction and other automated functions to make post-processing fast and efficient.

From there, Horrocks was able to deliver the MicroStation and TopoDOT files to the Idaho DoT. (The software exports to all common industry standard formats.)

“To crunch all that data, it was about a week and a half for a team of four people to extract the full survey,” said Olson. “It’s a lot faster than it was a few years ago.”

Leica Geosystems’ Jason Ellis provided technical support on the job, as he’s been doing for survey teams for decades. Even he was impressed by the speed of collection in this case.

Thanks to their complementary expertise, expectation management and good communication, the two engineering firms partnered for success. Given strong communication ahead of time and relatively smooth operations, the teams were able to combine bathymetry and lidar data without a headache.

A full version of this article was published inXYHT杂志in Oct. 2019.