How Drones Save Time on Bridge Replacement Projects

Aerial data capture reduces cost, saves time, and improves communication when building infrastructure

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Pinto Creek Bridge

Built in 1949, the Pinto Creek Bridge is a historic crossing on Highway 60, an hour and a half east of Phoenix, Arizona at the foothills of the Tonto National Forest. Built as part of President Roosevelt’s public works initiative, the steel arch bridge sits atop a steep ravine with scenic views of the countryside.

With over 6 decades of use, however, the bridge has started to show its age: inspections and appraisals have returned poor ratings, and the bridge was considered “structurally deficient” in a 2014 review.

The concrete is starting to fall off the pillars, leaving the rebar in plain view.

Though the bridge is still open and in use, its decay led to the decision to replace it. Because there are no easy detours on this roadway, the replacement bridge will be built just north of the current bridge, and then the old Pinto Creek bridge will be demolished.

3D rendering of the new bridge across Pinto Creek

The project is currently in its early stages: the Arizona Department of Transportation has been conducting geotechnical surveys of surrounding ravine, ensuring that the land is suitable for new infrastructure. Any new construction in this area must first go through an exhaustive survey process in order to ensure its sustainability and effect on the surrounding ravine within the Tonto Forest. It’s vital that the bridge blends with its natural surroundings and has limited impact on the existing plants and rock outcroppings. The ravine is approximately 80 meters deep, so traditional survey methods would require at least a day to complete the site from top to bottom. This includes using climbing equipment, rappelling down the ravine and walking the rocky terrain by foot, which comes with its own set of risks, hazards, and inefficiencies.

The solution

Surveying the Pinto Creek — a large, environmentally sensitive site with rough terrain — presented the perfect use case for drones, which are able to quickly capture aerial data and help land surveyors stay out of harm’s way. 3DR went to Pinto Creek and used Site Scan to capture the full context of the bridge and its underlying topology.

Jeremiah Johnson, Solutions Architect at 3DR, was the pilot for the day. He took off from the west end of the bridge, using the Site Scan app to plan the flight over the approximately 7.5 acre site. Jeremiah set up the crosshatch flight, and picked the appropriate altitude and gimbal angle for a scan of this size.

Sample image from Site Scan Field iPad app, showing the crosshatch grid of the Pinto Creek Bridge.

In half an hour, we surveyed the entire area with a single flight, taking 358 high-res photos and collecting millions of precise datapoints.

In half an hour, we surveyed the entire site with a single crosshatch flight, taking 358 high-res photos and collecting millions of datapoints.

“Mapping this ravine using traditional methods would take, at the very least, a day to survey from the top to bottom,” Jeremiah said. “The bigger challenge, though, is in processing the data from this traditional survey: to get the same kind of accuracy as Site Scan, it would take approximately two weeks of processing time.”

Jeremiah, Solutions Architect at 3DR, piloted this flight. It took just half an hour to capture the whole site.

Key deliverables

What, exactly, does a single 30 minute flight get you? By the end of the day, we had fully processed 4 main deliverables: an orthomosaic, a digital elevation model, overlays, and a point cloud.

1. Orthomosaics

By processing the 358 captured images, we stitched together a detailed orthomosaic of the entire site, providing clear aerial data that can be manipulated in Autodesk tools like Civil 3D, Infraworks, and more.

Orthomosaic image of the Pinto Creek Bridge from a single crosshatch flight, 30 degree gimbal

2. Digital Elevation Model (DEM)

We captured data that could be used to create a digital elevation model, giving surveyors a clear, accurate view of different heights throughout the ravine.

DEM in ReCap 360 Pro

3. Overlays

By bringing the topographical survey data into Site Scan and overlaying it on the orthomosaic, we’re able to easily compare reality to the original surey plans, and uncover new insights about the location.

4. Point Cloud

Using ReCap 360 Pro, we then created a point cloud of the site, which can easily be edited, measured, and annotated throughout the design and engineering process.