Guide to the use of Terrain Follow during aerial surveys with DJI drones. Find out how Terrain Follow can enhance drone mapping operations, while also helping to collect more accurate data, and discover the best workflows in Terrain Follow mode.
What are the benefits of DJI Terrain Follow mode for data acquisition?
Using Terrain Follow mode during drone flights offers several advantages. These include:
1: Adaptation to the terrain
Terrain Follow allows the drone to maintain a constant altitude above the ground, even if the ground level is uneven.
This is particularly useful in areas with variable terrain, such as hilly or mountainous regions.
By adapting to the terrain, the drone can capture images or data at a more constant and accurate altitude, resulting in better quality mapping results.
2: Smooth flight paths
When mapping an area, it is vital to ensure that the drone follows a smooth and consistent flight path.
Terrain Follow helps achieve this by automatically adjusting the drone’s altitude based on the ground level. This results in a smoother image or data by reducing distortions and inconsistencies caused by different altitudes.
3: Simplified mission planning
Mission planning is easier and more efficient thanks to the Terrain Follow mode.
Instead of manually setting and adjusting the altitude of the drone during flight, simply set the desired height above the ground and let the drone take care of the rest.
This simplification saves time and reduces the chances of human error during mission planning and execution. This can also improve flight safety.
4: Improved data accuracy
Maintaining a constant altitude above the ground helps improve data accuracy in mapping applications, including achieving a more constant ground sample distance (GSD). For more information on GSD, read our guide “Ground Sampling Distance and Impact on Drone Surveys”.
When the drone flies at a constant height, the distance between the camera/sensor and the ground remains relatively constant, allowing for more precise measurement and data collection.
This is particularly important when generating 3D models, orthomosaic maps or performing topographic surveys.
Follow the terrain with DJI drones
When surveying with DJI Enterprise drones, you can choose from three Terrain Follow options.
1. Terrain Follow in real time
2. Download a DSM from the Internet
3. Import your own DSM
DSM stands for Digital Surface Model and provides a basis for Terrain Follow missions. Read our blog Drones and DEM vs DTM vs DSM to learn more about DSMs.
1: Terrain Following in real time
The simplest method is real-time Terrain Following. However, at this stage, this option is only available on DJI Mavic 3 Enterprise Series drones.
This feature uses obstacle avoidance sensors to accurately understand the drone’s altitude above the ground. No internet connection is required.
To activate it, select ‘on’ under the Terrain Follow option…
…and choose Follow Live.
The benefits of using real-time Terrain Follow are:
A pre-flight site DSM is not required.
You don’t need internet connectivity to get DSM.
There are a couple of key points about Real-time Terrain Follow to make.
Firstly, Real-time Terrain Follow currently supports flights from 80 to 200 meters.
If you fly more than 200m above the ground, we recommend using the new in-app feature to download the global geoid file.
For flights below 80 meters, it is recommended to import a high precision DSM file which can be generated by DJI Terra after a flight.
|AGL (Above Ground Level)||Mode|
|<30m||High-precision DSM file|
|30-200m||Real-time Terrain Follow|
|>200m||In-app Feature and Global Geoid File|
Secondly, division systems cannot recognize very thin objects, such as power lines, so the drone cannot follow such objects.
Also, this method is not suitable for flights over water, as a live terrain feature cannot generate an accurate DSM.
It is also not suitable when the slope of the land is more than 75 degrees and it is not suitable when the light is insufficient.
2: Download from the Internet
The second option for Terrain Follow is to use the Internet download option when importing a DSM.
This method fetches the DSM from the Internet to use instead of requesting the site’s own DSM.
For this method, activate the Terrain Follow function and select DSM Follow…
…then select the DSM file, tap the plus button on the top right, then select “download from internet” and get the DSM from the cloud.
In this example, the source for the DSM was ASTER GDEM V3 with a resolution of 30 meters.
The advantages of this method are:
- A pre-flight site DSM is not required.
- Fly much lower than real-time Terrain Follow.
However, some disadvantages of this method are:
- Low DSM resolution at 30 meters.
- Inaccuracy and outdated DSM.
3: Traditional import
The third Terrain Follow option is the traditional import: Import a DSM into the DJI Pilot 2 app.
For this to happen, the site must have been previously discovered and the surface model of the site must be loaded onto a MicroSD card and inserted into the controller.
Then, activate the Terrain Follow function and select the DSM Follow.
…then select the DSM file and tap the plus button on the top right…
…then choose a file for the DSM, then select the tiff format DSM file from the microSD card and see the flight plan adjust to changes in altitude.
If you use this method, it is important that the DSM file has a maximum size of 20 MB and a spatial resolution of less than 10 meters.
Other considerations are:
- Use the WGS 84 coordinate reference system or the EPSG 4326 code;
- The vertical datum for elevation data must be ellipsoidal. This mode is more tedious, but it is the most accurate Terrain Follow method, as it allows for lower flights than real-time Terrain Follow.
Another note: If your goal is a Waypoint mission instead of a mapping mission, you can plan the Waypoint mission on DJI FlightHub 2 and adjust the waypoints to the terrain to maintain the same elevation above the ground.
DJI Terrain Follow: Summary
Terrain Follow can be an important feature for drone surveying operations, especially for site mapping projects that have large elevation variations.
In these cases, planning a flight based solely on height above the home position can cause inaccurate or insufficient overlap, result in varying levels of GSD, and potentially cause the drone to fly into the ground or an object.
Therefore, depending on the mission at hand, Terrain Follow can become a crucial function in increasing the accuracy and security of data collection, which in turn helps create more robust and reliable digital assets.