A 2.397Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE
In March 1913, a devastating flood forever altered the landscape of the Ohio and Muskingum valleys. More than 400 people were killed, and thousands of homes were destroyed. In the wake of this disaster, state officials and government agencies established the Muskingum Watershed Conservancy District (MWCD) and built a system of dams and reservoirs in the watersheds of three main tributaries of the Muskingum River–the Walhonding River, the Tuscarawas River and Wills Creek–to provide flood control throughout the basin. Owned by the federal government and operated by the U.S. Army Corps of Engineers (USACE) in cooperation with the MWCD, these dams have saved the region billions of dollars in potential damage from flooding.
Mohawk Dam, which was constructed between April 1835 and September 1837, stands sentry along the Walhonding River in Jefferson Township, Ohio. A dry dam that was originally expected to be in service for 50 years, the structure is showing its age. A history of excessive uncontrolled seepage has negatively affected its structural stability. Sedimentation and erosion have weakened the dam to the point that several safety issues have developed.
The dam was included in a USACE Dam Safety Program that periodically inspects and evaluates dams to prevent risks to people or the surrounding property and environment. The safety program listed the Mohawk Dam as an urgent project due to the critical nature of the issues. Before any repair work could be performed, however, the USACE had to know exactly what they were dealing with. If any deformation existed along the face of the land dam, they needed to be able to identify the precise location and extent of the damage, and they also wanted to evaluate the dam’s design in context with the surrounding landscape.
In early February 2014, Mack McCarty, PLS, at USACE, Huntington District, hired West Virginia-based Allegheny Surveys, Inc. to create a detailed assessment of the 110acre site with 3D laser scanning. "The Corps found significant errors in previous aerial mapping information of the site," McCarty says. "We felt that laser scanning would best capture the site’s true conditions."
A Strategic Scan Plan
Damon Wilkewitz, PLS, 3D laser scanning manager at Allegheny Surveys, devised a plan of action for the site that involved using an RTK GPS system to locate the center line of ditches and culvert inlet and outlet elevations, and then scanning the site with the firm’s Leica ScanStation C10 laser scanner. By incorporating the RTK GPS locations with the scan data, the crew was able to minimize their scanner setups.
The project was on the Ohio State Plane Coordinate System, so there were grid factor considerations. The size of the site also presented the need for special consideration. Including the entire site in a single scan world would create a point cloud file that was difficult to manage. The crew addressed this by dividing the site into five separate scan areas:
• The campground area
• A large spillway area
• The focus area, which included the inlet and outlet structures
• The house area
• The large land dam
The USACE established more than 30 new control points just for this survey. To provide even tighter control, Wilkewitz and his crew set nails in the ground in intermediate locations. They used targets on the control points, resected to the known control with the C10 laser scanner, and then registered the point clouds.
Each of the five areas was tied to its respective control point coordinates, so it all registered exactly in the right place. "I combined all five point cloud areas to map the entire site in CAD," Wilkewitz explains. "But in the field, and for processing, I find that dividing the site up into manageable sizes helps."
A Safe & Highly Accurate Survey
With traditional surveying methods, a rodman would have had to position himself in precarious locations, such as on the top of the high wall, the high road bank or a steep, rocky slope, and carefully document the instrument heights and rod elevations in each location. Laser scanning eliminated these safety risks while making it much easier to capture highly accurate data.
The scanning crew was well versed in the use of laser scanning technology and target placement. Armed with that knowledge, their surveying experience, and their excellent communications skills, they were able to complete the scans safely and quickly. "Having quality, trained employees is one of the keys to the success of any project," Wilkewitz says. "McCarty was on site for two of the four days of scanning, and he commented more than once on the efficient performance of our crew."
Since the Mohawk Dam project was Huntington District’s first experience with laser scanning, they requested several cross sections using conventional survey methods to verify the laser scan data. "Some of our scan registrations were giving us mean absolute errors of just 0.027′, 0.017′, and 0.019′," Wilkewitz says. "I was very pleased with the closure accuracy compared to what it would have been using conventional equipment."
For the Mohawk Dam project, Allegheny managed all the point cloud data using Leica Cyclone software. The team then used 3DReshaper and AutoCAD software to create the site map and many of the project deliverables. These deliverables included:
• A completely contoured and detailed site map
• A digital terrain model of the site
• All the cross section data
• All the Cyclone laser scanning files
• The final map converted to MicroStation software
• Leica TruViews
The Leica TruViews were not part of the original project scope; however, Wilkewitz wanted to show the Huntington District representatives all 65 of the setups and demonstrate how to use them in order to view the point cloud data. "I felt that just giving them a digital terrain model wouldn’t let them examine every possible area of interest they may have," he says. "I wanted the Corps to be able to access true X, Y, and Z values off the point cloud so that they could check for elevation differences and subtle variances of the site."
Reduced Labor & Project Costs
Although laser scanning technology has been in use for more than a decade, it is still underutilized as a survey method, often due to misperceptions about cost. Wilkewitz notes that every project the firm has completed with laser scanning rather than traditional surveying has saved clients money by allowing the crew to finish the job more quickly and with less labor. The Mohawk Dam project was no exception. Allegheny’s one-person GPS RTK crew and two-person scanning team were able to survey the 110-acre site in just four days and deliver the entire project in one month. With traditional methods, the time on the project could easily have been two or three times longer with at least twice as many people.
The USACE Huntington District office already knew that the laser scanning process cost a third of the aerial survey they had done while providing greater accuracy. When they compared it to the cost of a traditional survey, the expenses were significantly less.
"The Corps also appreciated the fact that they didn’t need to purchase any special software or expensive licenses or spend hours in training so they could see the results of the scans," Wilkewitz says.
Increased Business T
hrough Successful Scanning
Wilkewitz says using the ScanStation C10 on a project of this size demonstrated the effectiveness of the technology in reducing costs while providing greater accuracy, which are both important factors when addressing serious safety risks to the people, property, and environment in the areas surrounding aging dam infrastructure. It also demonstrated to Allegheny how much the laser scanning technology has positively affected their workflows.
"The project went smoothly, and we never encountered any big challenges," Wilkewitz says. "The C10’s range and accuracy made it the perfect tool for the job. We had a good plan, and we were able to execute it by locating the entire site in a methodical and efficient way."
Without the laser scanner, Allegheny would have needed to employ a three-person survey crew to locate the entire site, which included ditches, concrete walls, power poles, utility lines, and other details. Back in the office, connecting the break lines in CAD would have added even more time to the project. "We were excited to see just how quickly and accurately we could produce a map for such a large area using almost nothing but the laser scanner," Wilkewitz says.
The extra deliverables Allegheny is able to offer clients due to their use of the Leica Geosystems hardware and software solutions have also provided the company with a valuable side benefit. "Clients often don’t understand the full scope of the work from the map," Wilkewitz says. "When we present the point clouds, TruViews, modeling, and animations, these additional deliverables give our clients more project ideas, which increases our business."
Beth Wilson is a freelance writer based in Bremerton, Wash. For more information about Allegheny Surveys, visit alleghenysurveys.com. A video about the project can be found at youtube.com/ watch?v=mcN4zoOYQ8I. To learn more about 3D laser scanning solutions, visit leicageosystems.us.
Allegheny Surveys, Inc. focuses on surveying for real estate, construction, oil, gas and coal, and other applications. Since Allegheny’s president, Marshall Robinson, began the company 25 years ago, the firm has experienced considerable growth. The company currently has five offices in West Virginia and Ohio and 62 employees, including 15 professional licensed land surveyors and one professional engineer on staff.
When Robinson first discovered the potential for laser scanning techniques in 2011, he began shopping around for the best options, eventually determining that the Leica ScanStation C10 offered the best quality and software stability on the market. "The C10 gives us a good range of targeting and a quality data return," Wilkewitz explains.
The Mohawk Dam site survey represented a culmination of the scanning team’s field and office capabilities. Wilkewitz gave a presentation on the survey at the HxGN LIVE 2014 conference in Las Vegas, and at the last minute decided to enter the deliverables in the Civil/Survey category of the 2014 HDS Plan Contest. With 11 impressive entries, it was the most challenging category of the 2014 contest.
After three rounds of voting, Allegheny Surveys’ plan of the Mohawk Dam site survey was selected as the first place winner based on their creative use of point clouds and the overall usefulness of the plan. "As the old saying goes, a picture is worth a thousand words," Wilkewitz says. "Our clients need mapping that is clear, precise and tells the tale. Leica Geosystems helps us do everything we need from start to finish and deliver a good quality product. Every project we’ve done with laser scanning, our clients have been thrilled with the speed and accuracy of the mapping and the final product."
To see all the Civil/Survey entries in the 2014 HDS Plan Contest, download the ebook at http://hds.leica-geosystems.us/2014_HDS_Contest/civil.php.
A 2.397Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE