SLAM Technology Empowers Precise 3D Surveying of Ganjiang River's Bridge Clearance

The Ganjiang River, the mother river of Jiangxi, carries the poetic beauty and prosperity of the ancient Yuzhang region, bearing witness to Nanchang's transformation from an ancient city to a modern metropolis. Today, the rapid evolution of digital surveying technologies, such as handheld SLAM laser radars, has uncovered immense potential in their application to navigation surveying, offering high efficiency, precision, and contactless 3D space scanning capabilities.

Recently, the Tianbaonet technical team successfully completed a 3D scanning survey of the cross-river bridges along the Ganjiang River’s Nanchang section, utilizing the Lixel L2Pro handheld laser radar combined with a dual-head multi-beam depth sounder system in a collaborative operation. This time, we allowed SLAM technology to "sail through the waves," providing precise measurements of the navigation clearance for iconic bridges such as the Hero Bridge, Bayi Bridge, Ganjiang Bridge, and Tiehe Bridge, ensuring the safety of ship navigation.

The Ganjiang River stretches about 766 kilometers with a basin area of 83,500 square kilometers, making it the largest river in Jiangxi and an important tributary of the Yangtze River. After the completion of the Ganjiang Nanchang Hub in 2024, the low water level will stabilize at around 15.5 meters, allowing for the passage of 3,000-ton ships year-round, making it a true "golden waterway." The river passes through the city, branching into the lake, and is lined with numerous cross-river bridges. Precise measurement of the lowest points of each bridge's navigation opening (the vertical height from the water surface to the bottom of the bridge) is essential for ensuring safe ship passage and avoiding bridge collisions.

Navigation surveying is a foundational and strategic task to address the uncertainties in navigation and ensure the safety of waterway traffic. Traditional methods rely on manual bridge climbs, rope measurements of heights, and single-point depth soundings, which are not only inefficient and imprecise but also pose safety risks due to the involvement of people working in water and at heights. GPS signal weakness in bridge openings and shallow areas further complicates traditional surveying. This project covered four core cross-river bridges along the Ganjiang River in Nanchang, aiming to obtain precise data on the navigation clearance for safe ship operations.

Given the dense bridge network, complex water conditions, and weak GNSS signal areas along the Ganjiang River in Nanchang, the Tianbaonet technical team employed a collaborative solution of the Lixel L2Pro handheld laser radar and a dual-head multi-beam depth sounder system for both land and water surveys. The Lixel L2Pro was used for collecting 3D point clouds of near-shore, shallow water, bridge openings, and bridge bottoms, while the multi-beam depth sounder was deployed underwater to achieve comprehensive, integrated surveying coverage.

The core surveying device for this project was the Lixel L2Pro handheld laser radar scanner, which is designed for complex water environments and weak signal scenarios. It has four core advantages:

• Centimeter-level Accuracy + Strong Resistance to Weak Signals: Integrating multiple sensors with the Multi-SLAM algorithm, it can continue to acquire absolute coordinates even in GNSS signal-lost or weak coverage scenarios.
• Real-Time Mapping + Million-Point Cloud Density: The single scan can generate a point cloud density of up to 1 million points per square meter, and the high-frequency collection allows for real-time transmission of true-color point clouds with absolute coordinates that can be verified on-site.
• Lightweight and Easy to Operate: The lightweight design enables single-person operation without the need for large equipment or complex setup. A single person can scan multiple bridges in a single day.
• Upgraded Safety for Boat-Based Operations: To address the risks of working at the bow of the boat, the system is equipped with a special boat mount and an intelligent electric suction cup. The suction cup's status is monitored in real time with abnormal alarms. The scanner is mounted at the bow, powered externally, and operated from the cockpit, ensuring safe operations without exposing personnel to risk, even during long, extended navigation surveys.

Boat-Based Operation Diagram

• Composite Surveying: The combination of the Lixel L2Pro handheld scanner on the boat and the multi-beam depth sounder underwater provides synchronized water and land data collection, seamlessly integrating the data.
• Safety and Efficiency: The non-contact measurement avoids risks associated with bridge climbing, working in water, or at heights. The efficiency is improved by over 5 times compared to traditional methods, with a single person performing multiple bridge scans in a day.
• Precision and Reliability: The system is equipped with a surveying-grade RTK system, which ensures the point cloud data includes absolute coordinates, meeting the standards for navigation surveying.
• Full-Angle Coverage: With a point cloud density of 1 million points, it accurately captures the complex structures of bridge undersides, shore terrain, and navigation cross-section details, providing a complete 3D representation of the bridge.

• Field Data Collection: The Lixel L2Pro is used to scan the bridge navigation cross-sections from the boat, simultaneously capturing point clouds of the bridge undersides, piers, and near-shore areas. RTK is used to obtain precise coordinates, while the multi-beam depth sounder collects underwater terrain data.
• Post-Processing and Quality Control: Point clouds are registered, denoised, cropped, and stitched together to generate high-precision true-color 3D point cloud models.
• Deliverable Mapping: The data is processed in TRW and CAD platforms to extract features, vectorize, and generate bridge elevation, plan, and cross-section views, creating a 1:1 replica of the bridge structure.
• Precision Control: Using RTK-collected national 2000 coordinates during scanning, the data is transformed into the local coordinate system of Nanchang and aligned with local elevation benchmarks, meeting the requirements for navigation bridge surveying, structural inspection, and redesign.

During the Ganjiang River navigation surveying of the bridges, 3D point cloud data was successfully collected for the Hero Bridge, Bayi Bridge, Ganjiang Bridge, and Tiehe Bridge. Elevation drawings for each bridge were also produced. These results not only provide accurate data for measuring navigation clearance heights but also serve as valuable references for future structural inspections and redesigns.

Bridge Point Cloud Models:

Bayi Bridge

Ganjiang Bridge

Hero Bridge

Tiehe Bridge

Bridge Elevation Drawings:

Tiehe Bridge Imported to CAD with front view switch 

intensity render mode to create elevation drawing

1:1 Elevation Drawing

SLAM Technology in River Navigation: A Lightweight, High-Efficiency, High-Accuracy Innovation

The application of Multi-SLAM technology in river navigation surveying is a revolutionary practice that achieves lightweight, efficient, and highly accurate surveying. With RTK absolute positioning and centimeter-level point cloud accuracy, navigation bridge surveying has evolved from "single-point estimation" to "comprehensive 3D precision perception." This provides a solid digital foundation for smart navigation, safe passage, and bridge lifecycle management.

From the poetic Ganjiang River with its "setting sun and solitary wild goose flying together" to the "digital twin" empowered smart waterway, Tianbaonet continues to use SLAM technology to craft high-precision 3D digital maps for cross-river bridges. Moving forward, we will continue to lead the frontier of surveying technology, offering lightweight and high-precision 3D solutions for more water conservancy, transportation, and municipal projects.