An innovative technique is taking the place of a traditional survey team in preparing a water main replacement project on Broadway Boulevard this year as Bartlett and West company engineers will use LiDAR (Light Detection and Ranging) to compose a map of the area.
“We are going to replace a main that is in Broadway right now and MoDOT has plans here in the next year or so to do some resurfacing work on Broadway,” said Charles Brosch, Sedalia Water Department General Manager. “This new technology does such a good job and is much less time consuming than traditional survey work. So we contacted Bartlett and West and they agreed to bring this down and get started on it.”
A LIDAR instrument principally consists of a laser, a scanner, and a specialized GPS receiver. The laser beam is shot from a unit, bounces off a target and the data returned is recorded to produce a 3D image.
“It sends out a beam that reflects back. For example, concrete would reflect back differently than this wooden pole. With that, it reflects back a different signal for a different material surface which relates to a different color scheme, so we can pick up different features,” said Mike Logston, engineer for Bartlett and West.
According to the National Oceanic and Atmospheric Administration (NOAA), when an airborne laser is pointed at a targeted area on the ground, the beam of light is reflected by the surface it encounters. A sensor records this reflected light to measure a range. When laser ranges are combined with position and orientation data generated from integrated GPS and Inertial Measurement Unit systems, scan angles, and calibration data, the result is a dense, detail-rich group of elevation points, called a “point cloud.”
Engineers with Bartlett and West say the biggest benefit to using LiDAR is that a survey team is not forced to stand in heavy traffic along busy thoroughfares. The scans can be completed while driving, so it is also much faster at collecting the data than a tradition survey team.
“It (LiDAR) can do it a high speed—you can actually scan with this unit at 75 mph down the highway,” said Pete Wiens of Bartlett and West. “It averages approximately 1.1 million points per second, so it creates a realistic 3D image of whatever you’re driving past.”
LiDAR was developed in the early 1960s and used primarily in meteorology. As the technology advanced, it was used in military applications and radar guns used by law enforcement. In recent years, engineers say the technology has advanced to the point that it is practical for making a base map of an area for construction projects.
“The mobile units have been around for six-seven years, but they have been really trying to perfect the technology,” said Wiens. “This (unit) is less than centimeter accuracy— you’re talking millimeters when you’re driving down the highway at 75 mph. They have worked out the bugs. You still need traditional survey for the control and getting stuff laid out.”
While LiDAR can create a map much faster than a traditional survey, the time used in interpreting the data is much greater, so its cost effectiveness must be judged on a case-by-case basis. On Friday, engineers will compose a scan of Broadway from Warren Street to Grand Avenue for a cost of $1,000. The cost of the entire engineering report for the project will be much greater, however, when man-hours and other costs are factored in. The technique is also not practical in every application, such as mapping projects in rural area where there are few features.
“We shot six blocks in the few minutes you guys were waiting on us,” said Gary Davis, engineer for Bartlett and West. “But that is just a little piece of it. Now we have to bring it back to the office and process all the data. You still have a guy in the office, looking at the data, picking out all the features and inventorying the information we are looking for.”
For more information on LiDAR, its applications and history visit the NOAA LiDAR website at oceanservice.noaa.gov/facts/lidar.html