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.

The bridge, opened in the spring of 1951, was designated ‘functionally obsolete’ by the federal government more than a decade ago, largely because of the roads that feed it, which are no longer suited to the traffic the bridge handles daily.

2019年3月，即建造建造近70年后，爱达荷州交通运输部委托一对工程公司，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。

这项工作首先Horrocks Engineers，在整个美国西部设有15个办事处，其中包括爱达荷州的三个办事处。Horrocks的校长和调查经理Lonnie Olson说，收集桥梁和周围道路的调查数据是其移动地图部门的标准工作，但该项目还要求为河底和桥梁子结构提供测深数据。

“They wanted a look at the scour around the bridge pilings,” Olson said. “One of the avenues they’re looking at is to keep the substructure and much of the bridge itself.”

他知道，在移动映射云数据中添加多层声纳数据将使最终的TOPO一致，细粒度，准确的数据从河流底部开始，但存在一个问题：Horrocks没有一个多层声纳单元。

“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 Grouphas a multibeam unit for just this kind of bridge-scour analysis and hydraulic modelling. Even better, Spicer’s sonar vessel is also fitted with aLeica Pegasus:Twomobile sensor platform for its above- 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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Done in a Day

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.

尽管如此，一切都在一天之内完成。

总而言之，大约花了三天的时间来建立工作的完整控制网络，然后在现场准备和进行实际扫描四天，总共在现场进行了7天 - 与传统方法相比，大量节省时间。

“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.

So, it’s about one-third of the time,” Shepherd summarised, “and I don’t know how many hundreds of times more data.”

更好吗?说:“当我把它放在一起Olson, “our clouds lined up with a less-than-centimetre accuracy between Spicer’s and our upper and lower data. You would never find that kind of accuracy any other way. You could achieve it, but the cost would be exponential.”

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Mobile Mapping’s Back-office Effect

您可能已经听过恐怖故事：“移动映射创建了很多数据，以至于无法管理！”“当您要做的就是驾驶扫描仪时，谁需要一个调查团队？”

Of course, it’s true that moving into mobile mapping is going to require back-office adjustments, but they should be relatively manageable if you prepare for them and offer training to your workforce.

Horrocks Engineers 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.

“Some of our field crews transitioned indoors,” he said. “And I have a team of former high-ranging CAD techs that we’ve trained to do survey work. We’ve been able to do most of the post-processing work in-house.”

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的Jason Ellis说，Horrocks的经历是典型的，现场工作人员进入后台的运动应被视为明智的举动，而不是和解。

“The continuity from the field to the office is always a challenge,” Ellis said. “It’s hard to convey what’s been done in the field and how it needs to be processed.”

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.

Similarly, said Ellis, it can be hugely helpful to have the people who are out in the field operating the scanners also able to work the software.

“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.”

交叉训练还有助于填补地理空间专业的劳动差距。埃利斯说：“很难找到好人，因此，如果在该领域进行集合的团队可以管理大多数后处理，并达到可以认证的地步，这有助于保持项目的良好状态。”

As for the tech, Olson said there’s no need to stress about that. Just make sure you’ve got a lot of multi-terabyte USB drives, which are now extremely inexpensive, and don’t go cheap on your hardware.

“Your standard CAD machine handles this stuff just fine,” Olson said. “The software is starting to catch up now. You don’t really need much to get going. It’s pretty simple. Even your basic laptop can handle it. Sure, you need a decent computer, but we’re an engineering firm, so we had them anyway.”

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无缝工作流程

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.

从那里开始，Horrocks能够将微观的文件和TopoDot文件传递到爱达荷州。（软件出口到所有通用行业标准格式。）

“压缩处理所有数据,大约一个星期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.

本文的完整版本发表在xyHt magazinein Oct. 2019.