Combining bathymetry and lidar data for a fast-turnaround topographic survey
Case study
Author: Sam Pfeifle
当它到达州线，将华盛顿与爱达荷州分隔开来时，蛇河叉，一个tine流向南部，沿边界的蛇流向南部，另一个尖端作为蛇的开始。任何乘船向东行驶的人都会迅速遇到Clearwater Memorial Bridge，这是一座大约228米长的桥梁，专用于第二次世界大战的退伍军人。
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.
In March of 2019, almost 70 years after its construction began, the Idaho Department of Transportation entrusted a pair of engineering companies with creating a full topographic survey of the bridge and surrounding area as part of a multiyear project to replace it and surrounding roadways, ensuring safe and efficient transportation for generations to come.
The job fell first toHorrocks Engineers, 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。”
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.”
Luckily,Spicer组has 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:Two移动传感器平台用于其高于水线的移动映射，同一移动扫描平台Horrocks在其道路映射的前18个月投资了。
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.
Done in a Day
Horrocks would drive the bridge and surrounding roadways. Spicer would drive the river to get everything below waterline, the riverbanks, and the underside of the bridge. Then they’d stitch the data together, extracting the planometrics to create a surface model and topographic survey.
由于Pegasus是一个能够从多个来源摄入数据的传感器平台，因此可以将多光束和LIDAR信息集成到单点云中。
“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,它s about one-third of the time,” Shepherd summarised, “and I don’t know how many hundreds of times more data.”
Even better? “When I pulled it all together,” said 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.”
移动映射的后台效果
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。
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.
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。”
他们还招募了本地地理学计划的近期毕业生，并在他们加入移动点云数据后立即开始培训。
奥尔森说：“现在我们有一个由五个人组成的团队，除了每天整天从点云中提取数据外什么都不做。”
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.
“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.”
通过经验丰富的现场人员交叉培训，可以与数据合作，您可以在整理重要数据时会直觉在现场工作人员试图完成的工作。
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。
他说：“我看到很多人在现场进行后期处理，因此他们知道自己很好，可以打包一切，或者知道如果他们遇到问题，他们需要重新恢复。”“许多野外人员都开始扮演分析的角色，这是一件好事。您在该领域的人们的生产力要高得多。”
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.
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.”
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.)
奥尔森说：“为了解决所有这些数据，四人组成的团队大约需要一周半的时间来提取完整的调查。”“它比几年前快得多。”
Leica Geosystems的Jason Ellis为这份工作提供了技术支持，因为他已经为调查团队做了数十年。在这种情况下，他也对收集速度印象深刻。
由于他们的互补专业知识，期望管理和良好的沟通，这两家工程公司取得了成功。鉴于提前进行良好的沟通和相对平稳的操作，团队能够在没有头痛的情况下将测深和激光雷达数据组合在一起。
本文的完整版本发表在xyHt magazine2019年10月。
