Aerial image reveals the damage at Shaffer Road. The washed out culvert is visible to the right of the road.
four GCP at each site, placing two on each side of the washout. One point was set as close as safely possible to the washout, and the second on the road up to 400 feet away. OHM Advisors captured the location of the GCPs using Trimble R10 GNSS receivers with Trimble TSC3 controllers running Trimble Ac- cess software. Connected by NTRIP (Network Transmission via In- ternet Protocol) to the Michigan DOT real time GNSS network, the R10s produced precise positions for each GCP. The R10 delivered coordinates tied directly to the required Michigan state plane coordi- nate zone. “We can connect our R10 receivers to the MDOT network anywhere we have cellular coverage,” Lillibridge said. “That way we can be confident that everything we collect can be used in the future.” At each site the crews used GNSS to collect a handful of check points needed to verify the accuracy of the aerial data. With control in place, OHM employed a DJI Phantom 4 Pro to capture aerial imagery. Where vegetation prevented aerial work, they used a Trimble S7 total station to capture details of the washout. On average, the crews spent roughly one hour surveying at each site and completed 12 sites in two days. “Those were long days,” Lillibridge recalled. “Once we were actually on a location, things went quickly. It was the
logistics of getting between the sites that presented a problem. At some places we had to go miles out of our way to get back into the other side of the sites.” Over the course of the next week and a half, the county identified an additional six sites that had been compromised. OHM Advisors crews surveyed them as well. Integrating the Data At the end of each day Lillibridge downloaded each site’s photos (roughly 60 images) to Esri Drone2Map for ArcGIS software. The GNSS data for control and check points was exported in CSV format directly to the Esri software, which produced a georeferenced point cloud. Lillibridge then transferred the point cloud to Trimble Business Center software (TBC). “We’re familiar with TBC for point cloud ma- nipulation, so we were able to clean up the clouds and pull contours,” he said. “Then we exported all of that to Civil3D. Our work flow is such that no matter where the data was coming from, our CAD techni- cians would see the same thing and have the same thing to manipulate, whether it was from the drone software or TBC as GNSS points or total station points. Once it gets to the CAD techs it looks identical.” Because TBC supports all common industry data standards, transfers were accomplished via simple drag and drop of project files.
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csengineermag.com
may 2020
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