When every millimetre counts
Author: Tamas Safar
When thinking of common laser scanning applications, capturing intricate 3D objects or structures comes to mind. Flat, dark surfaces, just like the asphalt of the Zalaegerszeg test track in western Hungary, do not escape, however, the realm of High Definition Surveying (HDS). A 3D smart digital reality of this unique proving ground was created to ensure the track offers driving stability not only for conventional automobiles, but also for autonomous vehicles and electric vehicles.
With the Zalaegerszeg Automotive Proving Ground, Hungary provides an unparalleled venue with the highest standards. The world’s leading automotive companies, research teams and universities, will test here from breaking systems, running gears and tires to onboard navigation systems; as well as testing self-driving vehicles in an especially urban-built environment.
HDS professionals from Hírös Geo Kft assessed the quality of the pavement structure and flatness by analysing 3D point clouds generated by laser scanning.
Point clouds for pavement
Engineering teams need to produce the best possible quality pavement with an even surface so teams can trial automobiles. The dynamic platform is the area of the test track system where the cornering grip/slip friction and overturning stability of the self-driving vehicles incoming at high speed will be tested. Hírös Geo Kft, a Hungarian land surveying, engineering and utility geodesy company, was commissioned for the quality control of the 10 centimetres thick asphalt base layer of the 300-metre diameter circular dynamic platform.
The accuracy requirement was very strict – Hírös Geo Kft needed proof that the top surface of the asphalt base layer deviated only few millimetres relative to the planned plane surface at any of its tested points.
Hírös Geo Kft chose terrestrial laser scanning to measure the asphalt deviation. The specialists carrying out the measurement presented the divergences from the measured surface in the nodes of a regular 2 cm x 2 cm grid covering the dynamic platform. All measurements fitted to the Hungarian Unified National Projection measuring coordinate system.
Using the Leica ScanStation P40, the detailed survey of the area, consisting of 70,650 square metres, took a full working day. A single setup yielded high-quality data, corresponding to the planned sampling density, within a radius of 40 meters around the laser scanner. 48 setups were collected and linked by using the traverse surveying method. This resulted in a 3D point cloud of about 3.45 billion measure points. Point clouds were evaluated in the Leica Cyclone REGISTER software after a basic cleaning process.
“It would be impossible to carry out the control measurement with traditional surveying technology. The ScanStation P40 gives us freedom and flexibility to produce the needed amount of data, scaled directly to the project requirements,” said Barna Bodri surveying specialist at Hírös Geo Kft.
As flat as a strap
The high-precision registration of the setups was perhaps the biggest challenge. The process could not be assisted by the 3D point cloud post processing software’s recognition of “identical surfaces” (auto-alignment and visual alignment of the point clouds) because the asphalt is a monotonous, flat, dark surface without any points of orientation. Instead, black-and-white targets above the ground control points enabled the registration to be globally accurate to 1.8 mm.
The evaluated data clearly revealed that the upper surface of the asphalt layer has a match of nearly 90 per cent, provided that the deviation of the measured points cannot be greater than a few millimetres to the best fit plane. To lay the asphalt layers, operators relied on the Leica iCON iGG3 to guide the grader with precise control of the blade. Thanks to the 3D coordinates, it was easy to define and stake out in the field which areas need milling before the binding layer was placed. From here, the milling is an easy process using automated iCON machine control.
What caught the eye of experts at the Hungarian civil engineering company in charge of building the automotive proving ground was the presence of indirect signs of strip-like elongations, parallel to the direction of the travel, of the areas outside the specified tolerance exactly where the edges of the asphalt tracks meet. Protruding surfaces are related to the temperature of the asphalt when spread – an asphalt track already laid is in the cooling phase when the connecting strip is deposited at elevated temperatures. The higher the difference in temperature between the joining surfaces, the more difficult it is for the road roller to smoothen the connection.
Creating digital realities
Reality capture allowed HDS experts in Hungary to measure and verify the surface uniformity with a 3D georeferenced model, optimising the construction workflows and digitally sharing information between the field and office.
A robust communication to a correction service is needed but cannot always be relied upon. Despite a narrow window for satellite visibility and poor coverage of the area by mobile network during the survey, the highly-sensitive board and computer core of the smart antenna delivered reliable centimetre-level positioning. Particularly suitable for this kind of low-populated area with unstable RTK communication, SmartLink fill helped Noviy Proyekt specialists to retain the positioning accuracy even if the RTK data link was lost.
Based on the presented useful information, experts optimised their workflows and reduced the required time and costs to achieve quality work most effectively.