Drone Surveying of an Intersection With a Bridge
The required task was to survey and draft a 3D CAD plan, with a measurement accuracy of better than two centimeters (0.06 ft), of a section of Route 75 South 1½ kilometers (1 mile) in length.
Original Effort Estimate
The original effort estimate for the surveying project, using conventional surveying techniques, was three days. This appraisal was based on two days in the field, using a Total Station device from several different stations, and one day in the office to draft the detailed CAD plan of the road based on the Total Station measurements and the sketch done in the field. A key challenge related to the survey was occupational safety hazards due to heavy traffic on the road during most of the day. As a result, the survey was planned to be carried out at night, or during a weekend, and would require police supervision of the work.
Surveying and Processing with DatuSurvey™ (DatuGram™3D)
The actual time to survey and prepare a 3D CAD model of the road was only 6 hours. The time spent in the field was 1-hour: 30 minutes to photograph the road, and an additional 30 minutes to measure several control points along the road. The road was photographed with a regular Sony NEX-7 camera (24-mega pixel resolution) with a 16-mm wide-angle lens. A total of 50 images of the section of the road were made from the road’s shoulders toward the center. To minimize foreground obstructions in the images and expedite fieldwork, the camera was mounted on a small quadcopter that flew at an elevation of 35 to 45 meters (120 to 150 feet) above ground.
It is important to point out that, by surveying the road with a camera and quadcopter, the field crew was able to avoid entering the busy road. This permitted the survey to be done during daytime while maintaining the strictest occupational safety standards. A total of 18 points were measured along the road using a prism-less Total Station device; 13 of these points were used as control points for the geo-referencing of the images. The
selected control points were high-contrast, well-defined objects such as corners of road signs, corners of white marks painted on the road, etc.
The actual time in the office to geo-reference the images and draft the 3D CAD plan of the intersection directly on the images was 6 hours.
A 3D CAD model of the road in DXF format was generated by drafting directly on the oblique images using DatuSurvey™ ( DatuGram™3D) software. In addition to the DXF file, a list of the measured points in PNT format was generated. This included the point names, codes, descriptions, coordinates, and their measurement accuracy in all axes. Overall, 680 new measurements were made from the images. The measurement accuracy of all points was better than 2 centimeters (0.06 ft) in both position and elevation.
Based on the DXF files that were exported from DatuSurvey™ ( DatuGram™3D) , a CAD model of the bridge and road was created in AutoCAD by combining the 3 DXF files from the UAV, pole and ground sections.
The survey in the field using a regular camera mounted on a small quadcopter and telescopic pole proved to be rapid, effective, accurate, and furthermore – safer, compared to surveying using conventional techniques. Overall, the surveyor saved significant field and office time –
from 3 days to 4 Hours in the field, and from 3 days to 2 days in the office.