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Ground Extraction with Topography-Based Sampling in Trimble RealWorks

One of the difficulties with scanning projects is separating the features you want from those you do not. Moving objects such as vehicles or people can create inconsistencies across your project area. Removing these unwanted points while keeping those of importance is accomplished using an array of tools. This week's tip will focus on extracting just the ground. In the example below, we will go over some of Trimble® RealWorksTM various ground extraction sampling techniques which aim to perform this task.

The project cloud for this example contains numerous buildings, trees, utility lines, vehicles, and noise. The project area is focused around the central building, its adjoining property, and the roadways surrounding it.

The project property is fairly clean, but the roadways contain artifacts from moving vehicles. To clean these sections of the cloud we segment the roads from the rest of the project.

Segmenting the roads allows Trimble RealWorks Ground Extraction and Topography-Based Sampling tools to be used without the risk of removing wanted points.

The next step is to use Ground Extraction located under the Sampling tool found in Production mode. Click the Show Outliers (red) icon and then Extract. Tip: it may help to render your point cloud in white or gray scale so the red will stick out.

Trimble® RealWorksTM will show points above the ground plane in red which can be removed or kept based on the user’s needs. In this case, the outliers are removed.

Clicking the Create button will generate a Ground Extraction point cloud in the WorkSpace menu. Depending on the consistency and flatness of the ground extracted, further refinement may be necessary.

In this case, remnants of the vehicle tires are present. To remove them, the Topography-Based Sampling tool found in the Sampling drop-down menu is used.

Using the Topography-Based Sampling tool requires inputs for Resolution and Vertical Filtering Tolerance. Resolution works on a grid system using the user defined value. The larger the resolution parameter, the coarser the result. By default the Vertical Filtering Tolerance will be at the maximum setting, selecting no points. Moving the slider to the left lowers the tolerance, allowing selection of points. Continue moving the slider until all erroneous points are selected and click Erase Unwanted. This step may require multiple resolution inputs to allow more refined results. When satisfied with the results, click Create.

The process used here took minutes to complete and gave a clean point cloud ready to move towards project completion. Let us know what applications you found for these tools, and as always, keep on scanning.


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About the Author

Jeff Turgeon is the Technical Product Manager for 3D Laser Scanning Software within Trimble's Geospatial Sector. He is the Product Manager for Trimble RealWorks and Trimble Perspective, and the Vertical Lead for the scanning module within Trimble Business Center. He is a Canadian geomatics Engineer In Training with previous experience working for a British Columbia Land Surveyor. He was drawn to the geospatial industry for its development and use of technology, complex problem solving, global application, and focus on furthering efficiency and sustainability goals across an array of industries.

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