Mobile Lidar Supports Stringless Paving
Stringless paving is a hallmark for maximizing taxpayer investment through new-found efficiency. To understand just how financially effective stringless paving is, first you have to understand the precarious nature of the string line paving it replaces. String lines are essentially long lines of thread held up by steaks that are used to set the grade of pavement. By eliminating the use of a string line, contractors save time and money as the string line does not need to be constantly verified due to it getting bumped or moved. Also, dump trucks accessing the paver do not have to back down long stretches of grade between these string lines to get to the paver, which is a safety hazard. Those savings are passed along to the public agency and help address the need for financial effectiveness.
|In 2013 the Minnesota Department of Transportation pursued their first mobile lidar project with the help of Continental Mapping as part of the Highway 23 reconstruction project.|
To the casual observer, Highway 23 in the heart of the Minnesota grasslands may seem like a boring stretch of highway. However, this 22 mile stretch of road demonstrated the value of high accuracy mobile lidar to support stringless paving. Continental Mapping utilized mobile lidar to create a very high accuracy surface that can be fed into slip-form pavers to control the concrete quantity, depth, and surface elevations with high precision. This improves efficiency and saves taxpayer money.
An additional benefit beyond increased efficiency during construction and overall financial effectiveness is the high accuracy of the geospatial data derived from mobile lidar. Continental Mapping completed thorough survey control to achieve an accuracy of 1 cm at 1 Sigma. Those efforts included RTK placement of 222 registration points (every 500 feet) along alternating sides of the road with digital leveling using Trimble R8GNSS dual receivers and DiNi digital levels turned throughout. All loops closed within 0.6–1.2 cm. Following acquisition, the team completed post processing including removal of dynamic data, performed break line extraction, completed QA/QC, and finalized deliverables.
A mobile lidar project is only as solid as its base station. Selecting a solid RTK base location is paramount.
The success of the Highway 23 project, in part, led to Continental Mapping assisting MnDOT with a similar job along 20 miles of Interstate 35 between Duluth and the Twin Cities. MnDOT required the project be completely delivered within two months from the notice to proceed — a very aggressive schedule. To mitigate concerns about performance and quality, Continental Mapping leveraged the speed of mobile lidar field data collection and developed unique approaches to data processing and quality control to meet the deadline. One such example was generating stereo pairs from the lidar data and viewing them in 3D. This greatly assists in the QC process. These stereo pairs were delivered early and reviewed by MnDOT staff and alleviated any lingering concerns. Continental Mapping’s job performance on the I-35 project brought with it several high scores from MnDOT’s performance evaluation.
Projects like the I-35 job are critical to addressing MAP-21 mandates for improving the life-cycle of the federal highway system. These roadways carry approximately 60% of Minnesota traffic, despite the fact that they only account for 8.5% of the total roadway mileage in the state. The MAP-21 programs award funding based on the performance of the agency applying.