Aerial Surveying of Highly Undulating Terrain

How to Address Point Cloud Density Variations?

When conducting large-scale aerial survey operations outdoors, we often encounter terrain with large elevation changes. When performing fixed-altitude aerial surveys using lightweight short-range LiDAR systems mounted on unmanned aerial vehicles (UAVs), the effective measurement range is typically less than 300 meters. When the elevation difference is too large, insufficient measurement range will lead to sparse or even missing point cloud density in areas with relatively low elevations. Inconsistencies in overall point cloud resolution and density parameters can affect reconstruction results during data post-processing.

To address this issue, we can consider using a long-range LiDAR system. However, the high price of these systems often prevents many geospatial surveyors from adopting them.

Is there a more cost-effective solution? Yes! We can achieve it by utilizing the terrain-following function during aerial survey operations with UAVs equipped with short-range LiDAR systems. By consistently flying the UAV at a relative height above the ground that corresponds to the terrain variations, we can overcome this issue. This is the terrain-following functionality in aerial surveying.

Currently, there are two commonly used terrain-following methods on the market :

One method is to import digital surface model (DSM) files into flight planning software to generate variable altitude flight paths. DSM files can be downloaded for free from geospatial websites, but they tend to be lower resolution and less timely.

Another is using image-based real-time terrain following. However, visual sensors are highly sensitive to lighting conditions, and the drone is often unable to carry additional payloads, making it impossible to achieve more precise LiDAR point cloud data collection.

Both methods have significant limitations. Is there a more convenient and efficient solution for terrain following?

GVI's Autonomous Terrain-following Function Enables Direct

Terrain-following Flight without DSM

In response to the above problems, GreenValley International LiAir X series products incorporate a newly developed autonomous terrain following flight function specially developed for terrain surveying and mapping.

When used with the GreenValley APP and the DJI M300 RTK or M350 RTK drone, the system does not need to rely on DSM information specific to the measurement area. Instead, it leverages real-time processing of LiDAR point cloud data to generate local DSMs for terrain assessment. By considering the relative height of the drone to the ground, historical terrain data, and flight trajectory, the flight height is dynamically adjusted in real-time. This enables real-time terrain following flights without relying on pre-existing DSM data.

This feature greatly reduces data preparation time in advance and can be deployed and used immediately.

Core Functionality Overview

Mode selection: GreenValley supports both DSM-based terrain following and Rreal-Time terrain following, allowing users to choose the appropriate mode based on their specific needs.

a. Real-time terrain following mode:

This mode does not require DSM files. It uses real-time point cloud data to dynamically adjust the drone's altitude by analyzing the terrain ahead, enabling variable-height flight.

b. DSM terrain following mode:

This mode supports both local DSM file imports and online DSM downloads. The online download feature allows for automatic retrieval of DSM files for the survey area, eliminating the need for post-processing or searching for resources. This greatly improves user-friendliness and operational efficiency, saving time.

GreenValley App supports real-time 3D point cloud visualization, allowing users to view the integrity of the scanned terrain in real time while flying along the terrain. This feature allows users to instantly understand the status of field operations and is suitable for emergency investigations, disaster response and other time-sensitive scenarios.

GreenValley App supports waypoint resumption, allowing users to resume terrain-following flights directly from the last completed waypoint in case the flight was interrupted due to insufficient battery or other reasons. This feature maximizes operational efficiency by minimizing downtime and optimizing the completion of survey missions.

In addition to the drone's built-in obstacle avoidance features, the system can assess the surrounding environment using real-time point cloud data to detect obstacles in front of and below the drone.

a. Forward detection: The system continuously scans the point cloud data within a 20-30m range ahead of the drone. If an obstacle is detected, the drone will immediately hover and halt terrain following.

b. Altitude difference detection: The system calculates the height difference between the drone’s current position and the highest point of the terrain below. If the difference is less than 20m, the drone will hover and stop terrain following.

Function Testing

Terrain-following Success Rate of 95%

Margin of Error less than 10%

Currently, the terrain following function of LiAir X series products has been tested and used in various places. It has been applied in diverse scenarios including forests, mines, and more. The total number of flights exceeded 800, and the terrain following success rate reached an impressive 95%. During terrain following flight, the system maintains a constant altitude above the ground based on the input flight altitude, with an error rate of less than 10%.