How do we use LiDAR and laser scanner?

Por Mario Carretero

The process: from scanning/LiDAR to 3D

LiDAR technology enables to compile more and more precise information of the elements required for the characterization of the existing cartography, crucial for the design of Hammond Bridge in Atlanta.

For the lifting of the existing roads and shoulders we used a terrestrial mobile LiDAR, with a significantly lower cost compared with the aerial version. We obtained accurate information on the surface of existing pavements, basis of this project, which mainly consists on the reconstruction of roads and widening of existing roadways running along the main axis of the highway.

For areas that required accurate precision and needed a significant quantity of data and couldn’t be compiled by a mobile LiDAR, we used a multistation scanner that enabled us to obtain information from the existing bridges and other cartographic elements that required deeper detail.

With the combination of these two methods for field data collection and processing these data with Bentley and Leica software, we obtained a 3D model of the element that can be used for many other purposes such as design, measuring or control.

The first task is to place the basis of the equipment so that we obtain full visibility of all the components that we intend to scan. In order to do that, we assign site coordinates in the locations- these are generated from our primary surveyed bases. The scanning of the different elements matching the structure is obtained with a 3cm range (maximum distance between each laser scan) . With this we obtain a point cloud composed of more than 3 million points that define very accurately the elements.

For scanning we used Leica Multistation MS50

Mobile LiDAR  was calibrated to obtain data with a 0.05’ (15 mm) precision in elevation, and for the border of the pavement, shoulder, divider and lanes with precision in X and Y of 25 mm, which covered  que GDOT (Georgia Department of Transportation)’s requirements for this works.

In order to obtain the vertical precision we had to establish and level control points prior to the measurements with LiDAR.  These control points were assigned topographical coordinates along the path at approximately 150m.

The result after processing calculations with the ground data was a Digital Terrain Model (DTM), from which the required details of the cartography for this type of reconstruction projects is obtained.

Herramientas utilizadas

After incorporating the systems of coordinates and units, raw data is imported from ScanStation, obtaining a first 3D view of the whole construction works.

After verifying that control point coordinates are correct and that all required areas have been covered by a point, we export a file typeTXT or PTS.

In Microstation we create a new 3D drawing where we import point clouds, visualizing in 3D the previously created design (DGN).

Herramientas utilizadas

With transversal sections we trace the outlines of the elements that will define the model. This is done with Bentley’s software Descartes, which creates sections on the point clouds wherever we indicate. Thus, we obtain the lines of components and elements we want to model.

After transforming the scanned elements to 3D, we incorporate as reference the DTM generated by LiDAR  (with information on the lane and dividers) to have a complete model combining the two, and, most important of all, a model that allows us to make the right measurements between them.

With the modelling module from Microstation, we create solids for the different elements we want in the 3D model.

Once all the solids are created and the LiDAR  grid has been incorporated, we obtain the 3D model based on the different textures the software provides.

With the visualization module Microstation/Openroads we can see different types of presentations and options.

In which areas are these technologies most useful?

Measurements

The main use of this technology is in measurements, for volumes, surfaces, distances, for the design and construction of the projects.

It is also useful for calculating volumes of the scanned materials, such as stockpiles, demolition surfaces for an existing bridge and its future phases of construction, or to define construction phases taking into account the gauges measurements and lanes widths.

Conflicts and interferences

Conflicts with affected services or any other possible interferences in the design,- for example, in this Project there was an interference with a tunnel- can be previously analyzed, quickly detected and guarantees a fast solution prior to any action.

Tunnel vs. roadway

Deformation surveying

Allows to survey deformations in walls of reinforced earth, both in vertical and horizontal, providing a basis for comparison between the initial surfaces and the recurring surveys, and controlling the differences between the three coordinates in an efficient and fast way.

As Builts

With laser scanner and its geometrical registration, we obtain a visualization of how the final state of the infrastructure would look like, so we can check it adjusts to the original design and/or quickly identify any difference that would surpass geometrical tolerances when compared to the 3D model design. We can generate a 3D document to register all the scanned elements for future use for as builts.

Construction progress

Combining several “as builts” from the different elements that are processed, we create a visual document quite fast, where we can see in 3D how  the project advances/evolves plus all the necessary documents and information for the creation of a completion site plan

Minimum Health & Safety Risks

Using these technologies we can carry out distance tacheometric surveys in areas where access is dangerous or restricted, such as rock slopes, structures in height, deep drainage, for example, and we can obtain a significant amount of relevant information for a more precise and accurate definition, reducing the risk of accidents.

Volumes

Laser scanner is also valuable to make fast, precise and reliable dimensional calculations for data collection and to calculate big volumes of material stockpiles in processing plants or moving temporary stockpiles in the working areas.

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