Throughout the last decade, LiDAR-based SLAM scanners like the Nexys Pro have become more widely used across the geospatial industry for quickly and easily capturing feature-rich 3D point clouds that can be transformed into accurate digital models for geospatial software like Trimble Business Center (TBC). Thanks to advances in the robustness of SLAM algorithms and the addition of GNSS sensors like the Trimble DA2, capturing a feature-rich and highly accurate 3D model of a large area of interest is possible after a simple 15 - 20 minute mapping mission with a Nexys scanning payload.
We recently took a trip to visit our channel partners in upstate PA, Keystone Precision Solutions, to capture a 3D point cloud and transform that into a digital elevation model (DEM) within Trimble Business Center. We did this using the Nexys Pro mounted to a backpack with theTrimble DA2 RTK accessory to further increase the final map accuracy and automate the georeferencing process to accurately position the final point cloud in the appropriate GNSS coordinate frame.
While working with Brent Fico, a Customer Solutions Engineer at Keystone, we identified a bunch of great tips for generating a digital elevation model with Nexys and Trimble Business Center we wanted to share with Nexys users and other geospatial professionals exploring SLAM scanning for their own mapping workflows. Here's a quick video of our trip:
Below we're going to walk through how to quickly capture a 3D model of an area of interest with Nexys and render it into a digital elevation model or digital terrain model using Trimble Business Center.
Once we arrived at the retention pond site we wanted to map, we assembled the Nexys Pro and Trimble DA2 RTK accessory onto the backpack to prepare for our DEM scan. Following our best practices for SLAM mapping we took a pre-survey look at the pond to map where we were planning to walk, checking for any obstructions or obstacles. Then once we have our path planned we can power on the Nexys Pro and Trimble DA2, and ensure that Nexys is receiving the Trimble DA2 RTK data via satellite indicators on the ExynView software. Now we're ready for mapping!
Nexys Pro mounted to a backpack with the Trimble DA2 accessory
Be sure to remain still when initializing the Nexys Pro, we find it best to put the backpack on the ground to start or find an object to lean on for stability. Be sure to keep the Nexys sensors unobstructed and not jostle the platform too aggressively or it will compromise the final point cloud. We've found it best to use the backpack in conjunction with the tablet to ensure operators are capturing the required dataset before they leave the site.
To create a point cloud file for Trimble Business Center we're going to process the exlog (.ex) captured by Nexys on the supplied tablet via ExynView. First, operators will need to transfer the corresponding exlog file from the Nexys to the tablet, then open up the Post Processing suite in ExynView. There they can load up a log to begin processing the capture and georeferencing the final point cloud into the appropriate coordinate frame. Be sure to "use GNSS measurements" when processing the log to include the Trimble DA2 RTK data stream.
During georeferencing, ExynView will ask the operator to define the coordinate reference system (CRS) before the final export. Using the Trimble DA2 RTK accessory with Nexys Pro automates high-accuracy georeferencing and drift-correction, delivering fast, accurate, precise scans while reducing or eliminating pre-surveyed ground control points. Combining the power of ExynAI's SLAM and the Trimble DA2 RTK ensures accurate, reliable, and robust point clouds captured in the field.
Once complete, be sure to export the final georeferenced point cloud into an LAS file. Then we're ready to move into Trimble Business Center.
Once you have processed, georeferenced, and exported your LAS file it's time to open up Trimble Business Center and start a new metric project. Once that project is created, go into the Project Settings and select the appropriate CRS.
Adding the EPSG coordinate system into the project settings
With all our settings ready to go, you can drag and drop the corresponding LAS file into Trimble Business Center and the software will ingest and align the point cloud to the appropriate coordinate frame. Operators can toggle on background maps inside TBC in order to QA/QC the data has been georeferenced properly once loaded into TBC.
Once the point cloud is properly loaded into TBC we're going to ask the software to attempt to classify some of the elements in the cloud -- high and low vegetation, buildings, dividers, poles, signs, and power lines, etc. For a digital elevation model, this step isn't 100% necessary but will be useful in helping to separate out our ground plane for analysis.
Point cloud elements identified by Trimble Business Center (vegetation, power lines, etc.)
Once it's completed classifying you can turn off all other elements except for your ground and then check for your high and low points to create your break lines.
To create our final digital elevation model, we're going to have TBC compute tens of thousands of individual points into triangles to create a mesh that will accurately represent the ground contours of the area we just scanned. To make that topography as accurate as possible we need to identify our breaklines and create a 3D polyline.
Breaklines are used to force triangle edges along a given line. These can be used to enhance the detail of the mesh, or to ensure that the ground model topography is represented more accurately by the mesh. For example, they can help to pick out the tops of embankments or the bottom of channels.
Creating a breakline to help mesh the bottom of the retention pond
This is a good time to evaluate your point cloud for any cleaning to make identifying your break line area easier.
Up until this point we were working with the point cloud in its full resolution, which is often millions of points. To render our final surface model we need to downsample the point cloud enough to still maintain an accurate representation of the environment.
Zoomed in screenshot of a downsampled point cloud with CAD points for scan points
For a flatter surface like a concrete slab or structure, you can choose to simply downsample the cloud by 0.3 - 0.5 meters to render a surface. For a more complex/grassy structure like our retention pond we chose to create CAD points from scan points. Still downsampling by 0.3 - 0.5 meters. But now each point contains a northing, easting, and elevation and will make the final rendering more flexible for editing any grassy/wild sections.
With our downsampled point cloud, now we have to simply go to our Surfaces tab, select our points and render the surface using Create Surface. The Trimble Business Center software will compute the surface and we can edit any errant triangles by deleting any stray CAD nodes that might have slipped through our cleaning process. Once you've finished editing any elevation data, now it's time to render the contours and prepare the data for export.
Final surface model rendered with Trimble Business Center
Staying in the Surfaces tab, there are few ways you can choose to create your contours. The Quick Contour tool will quickly add lines to your map by providing a set interval. For a flatter concrete surface you might need to increase the amount of contour lines in order to convey the appropriate topographical information for your final map. For our retention pond, we used Create Contours to add labels to our contour lines for a more informative final map.
Adding contour lines to our meshed digital elevation model
With that, now we have a completed digital elevation model. Depending on your end user this can be exported to a sheet view or a page for a project manager or architect. This makes it easier to share around so everyone can get a lay of the land. (Sorry I'll stop) With an accurate mesh DEM, engineers can perform virtual tests on a new retention pond to prepare for any drainage or overflow issues. And if changes need to be made, another quick scan with Nexys and Trimble DA2 RTK can ensure accurate, actionable datasets every time.
Say goodbye to vibes-based measurements while on-site with the added Trimble DA2 RTK accessory. Rapid scanning with Nexys + DA2 can also take the guesswork out of backfilling and compacting a new topsoil layer for a new project. Simply capture a before and after scan of the same area, compute surfaces for both, then perform a surface comparison to confirm the work on-site matches the plans.
Or if we wanted to get even more hands on, we could export this surface as an accurate representation of an actual job site. In TBC we could export this surface as a TTM file for Trimble software, or a civil 3D land XML file for AutoDesk. Once complete, you could import the surface data into whichever system you're using on-site, then backfeed that information into machine control and use that to command a bulldozer that's helping to level one segment of the elevation.
Now you begin to see the true power of quickly capturing digital twins with Nexys on an active work site. The data can be quickly transformed into actionable models that can be accurately measured or fed into machine control to make real-world changes happen. This keeps work moving, keeps key stakeholders up-to-date, and most importantly keeps surveyors on-site safe and informed.
