Case study by SPH Engineering · Bathymetry · Transport infrastructure
Validating drone bathymetry accuracy for road and bridge planning
For the North Carolina Department of Transportation (NCDOT), researchers at Mississippi State University and Appalachian State University tested whether a drone carrying a single-beam echo sounder and SkyHub could map underwater terrain accurately enough for infrastructure work. At their validation site the answer was 6.59 cm RMSE.
The challenge
NCDOT wanted to know if drone-mounted sonar could give the accuracy needed for infrastructure planning. Boat-based multibeam needs trained hydrographic surveyors and struggles in shallow or debris-filled water. Topographic LiDAR cannot see through the water surface.
A drone echo sounder measures the distance from the sensor to the bottom. To turn that into a bottom elevation, the drone's height above the water must also be known precisely. Standard DJI drones use barometric altimeters, which can drift by several metres during a flight, and that error goes straight into the depth result.
The solution
Mississippi State flew a DJI M300 RTK with an EchoLogger ECT D052S dual-frequency echo sounder. Appalachian State flew a DJI M600 Pro with an ECT 400S single-frequency echo sounder. Both used the SkyHub onboard computer.
- Altitude control: the teams used a laser altimeter with a surface-tracking algorithm, so SkyHub held the drone at a constant height over the water. The report notes that a radar altimeter setup drifts only about 5 cm.
- Geotagged logging: SkyHub tagged every ping with the drone's GNSS position and saved CSV, NMEA 0183 and SEG-Y files. Logging started automatically when the sensor touched the water.
- Live monitoring: the ground station showed depth during flight, so operators could spot bad data at once.
Missions were planned in UgCS from a KML boundary using the Area Scan tool, with line spacings of 5, 10, 20 and 30 m tested. Most flights used continuous parallel lines. Data was cleaned in Eye4Software Hydromagic and interpolated in ArcGIS Pro.
The results
- Accuracy: at a catfish pond with LiDAR and GNSS ground truth, raw data gave 10.20 cm RMSE. After filtering at 2 standard deviations, which kept 95.45% of points, RMSE was 6.59 cm.
- Interpolation: of 10 methods tested, Topo to Raster performed best (R² 82.9%, RMSE 6.3 cm at 5 m line spacing).
- Storm damage: surveys of Rhodhiss Lake before and after Hurricane Helene showed maximum depth up 2.5 m, some areas eroded by over 5 m, and 8,424 m³ of erosion over a 200 m stretch.
- Bridge scour: at Huffman Bridge, thousands of sonar points replaced 12 plumb-bob readings, with RMSE of 0.53 m and R² of 0.90 against ground truth.
Recommended settings from the report
- Flight line spacing of 5–10 m, and depth sampling every 5–10 m along each line.
- Flight speed of 0.5–0.8 m/s.
- About 550 m of survey line per battery, with a 40% safety margin.
- Best suited to water 0.5–20 m deep. Error rises below 0.5 m, and as a single-beam system the surface resolution is limited to about 1–2 m.
Where this applies in India
The same workflow supports engineering decisions wherever roads and railways meet water:
- scour checks around bridge piers, culverts and abutments on highways and railways;
- flood and hydraulic modelling of rivers and small water bodies near road corridors;
- before-and-after surveys of rivers and reservoirs following floods or cyclones;
- repeat sediment monitoring for drainage and canal maintenance;
- applied research by universities and engineering institutes.
SPH Engineering's bathymetry training is also available through HobiTech.
The equipment, available from HobiTech
| Echo sounders | EchoLogger ECT D052S (Mississippi State) and EchoLogger ECT 400S (Appalachian State) |
| Onboard computer | SkyHub by SPH Engineering with the SkyHub echo sounder software |
| Altimeter | Laser altimeter used in this project; a radar altimeter is also supported |
| Flight planning | UgCS - Drone Flight Planning Software |
| Drones | DJI M300 RTK and DJI M600 Pro were used in this project. Ask us for current compatible drones. |
| Processing | Eye4Software Hydromagic and ArcGIS Pro (used in the study) |
Case study by SPH Engineering, published January 2026, based on Pricope, Bashit, Yu and Shu, "Demonstrating the Capabilities of UAS Topobathymetric Sonar Mapping in Support of DOT Project Planning, Monitoring and Modeling", NCDOT Research Project 2024-32, Final Report (April 2025). This page summarises it in our own words; the research was carried out by Mississippi State University and Appalachian State University, not by HobiTech. Read the original case study on SPH Engineering ↗
Need engineering-grade depth data?
Tell us the water bodies, depth range and accuracy you need. We will specify the echo sounder, altimeter, SkyHub and UgCS setup, quoted in rupees with a GST invoice.