L1 Drone LiDAR Data: How To Post Process Point Clouds In DJI Terra

DJI Terra is a powerful drone software solution which enables surveyors to post-process their LiDAR and photogrammetry data from the DJI Zenmuse L1 sensor, which is compatible with the DJI M300 RTK drone.

This step-by-step tutorial will guide you through the steps of uploading your data into Terra, how to conduct initial analysis, and then upload your files to third-party software, like TerraSolid, to conduct further and deeper observations.

To find out the best flight parameters for the L1, to capture the best data possible for inputting into DJI Terra, read our guide to flying the L1. 

Step-by-Step Guide: Post-processing L1 LiDAR Data In DJI Terra

Before you start point cloud post-processing, ensure the software for DJI Terra is at least version 3.0.0.

Then insert the SD card of the Zenmuse L1 into the computer and copy the data acquired by the L1 to the local computer.

You're ready to begin:

1. Launch DJI Terra and create a LiDAR point cloud processing mission...
   
   
   
   ...then edit the mission name and save it.
   
   
   
   
2. Select and import the raw data into DJI Terra software. The data acquired each time will be stored in a folder named in this format: DJI, Year, Month, Date, Hour, Minute, Underscore, Sequence Number, Underscore, Custom Name.
   
   For example: DJI_202109071740_014_demo2
   
   
3. Import the jpeg images as well if you would like to process true colour point clouds.
   
   
   
    
4. To determine the reconstruction resolution, select the Point Cloud Density. Select between High, Medium and Low.
   
   
   
   • High: Original 100% point cloud data.
   • Medium: 1/4 original point cloud data.
   • Low: 6.25% original point cloud data.
   
   
5. Click Output Coordinate System Settings and select the appropriate output coordinate system and height datum.
   
   
   
   
   
6. Set Point Cloud Effective Distance, where data contained in the area greater than the effective distance will be considered as noise and will be filtered out after post-processing.
   
   
   
   
   
7. Click parameters and enable the Optimise Point Cloud Accuracy option to ensure greater accuracy and consistency of point cloud scan at different moments.
   
   
   
   The Point Cloud Accuracy Optimisation function can improve the relative accuracy of L1 modeling.
   
   However, please note that this function is a 'paid for' option, requiring the support of the professional version and above.
   
   
8. Click Reconstruction Output and select the formats of the point cloud you would like to have after post-processing.
   
   
   
   
9. Click the Start Processing button to start post-processing.
   
   
10. Once the point cloud processing is completed, the overall point cloud results will display on the left side of the software.
    
    
    
    
11. Switch different point cloud colouring modes by selecting between RGB...
    
    
    
    ...intensity...
    
    
    
    ...height...
    
    
    
    ....and echoes.
    
    
    
    
12. Click Annotation and Measurement to conduct measurements.
    
    This includes the coordinates of any point...
    
    
    
    ...the distance between two points...
    
    
    
    and the area...
    
    
    
    ...or volume of any multiple points in the point cloud results.
    
    
    
    
13. Return to the mission library, select the mission name, and click the folder icon to open the Mission Results folder.
    
    The point clouds in .las format are stored under terra_las folder and the files with .out are trajectory files of INS and IMU which can be imported into third-party software for further analysis. This includes packages such as TerraSolid. 

Post Processing With DJI Terra - Hints And Tips

The above sets out a step by step guide to post-processing in DJI Terra.

But here's some more advice to help you get the most from the L1-DJI Terra combination.

Supported Data Volume 

Each 4G available memory supports 1G Lidar original files. For example, a computer with 64G available memory can support 16G lidar source files.

If the data size exceeds this limit, it is recommended to divide multiple tasks, or not open point cloud accuracy optimisation, or use accuracy.

DJI Terra also has a Cluster licence, enabling multiple computers to conduct computations simultaneously, significantly improving efficiency.

DJI Terra Activation Code

Currently, each L1 unit is gifted with an activation code that can be bound to the DJI Terra electricity version for 6 months, which can be bound to 1 device. It does not support unbinding.

Using RTK Base Station Data

When using DJI Terra for L1 data processing, the base station data - such as the D-RTK 2 base station - must be available for the solution to be successful.

When the network signal is poor or the remote control image transmission signal is poor, you can use a third-party RTK base station or PPK data to assist L1 for data post-processing.

The steps to use D-RTK2 base station are as follows:

1. Confirm the time period (local time) of the task according to the point cloud data file directory stored in the microSD card.
2. Search for RTCM files in .DAT format stored by D-RTK 2 mobile stations or third-party RTK base stations in the same time period.
3. Rename the corresponding RTCM file to a file with the suffix of .RTB and copy it to the post-processing folder of the point cloud.
4. If there are multiple .DAT files, the official version of DJI Maps 3.0.0 already supports automatic merging of multiple base station files.

Note: The time in the RTCM file stored in the D-RTK 2 mobile station is in UTC format.

As an aside, third party base station file format supports: OEM, UBX, Rinex, RTCM.

In this case, rename the base station observation file name to:

• Oem format: DJI_YYYYMMDDHHMM_XXX.oem
• Ubx format: DJI_YYYYMMDDHHMM_XXX.ubx
• Rinexformat: DJI_YYYYMMDDHHMM_XXX.obs
• Rtcm format: DJI_YYYYMMDDHHMM_XXX.rtcm 

DJI Terra Post Processing - Troubleshooting

Having issues with post processing? If so, here's some hints and tips to help you overcome potential issues.

LiDAR point cloud POS data error

Possible Causes:

The RTK-related files in the input path (RTK base station file (.RTB), RTK main antenna file (.RTK), RTK secondary antenna file (.RTS), lever arm data file (.RTL)) record poor data, which may be The RTK signal is bad, or there is no RTK base station data.

The most common cause is an abnormal interruption of the RTK during flight. At this time, you need to use the PPK solution, or start the RTK and then collect the data again.

The load is fixed or hovering during the acquisition process. Currently, it does not support post-processing of collected data on the ground.

Raw data missing or file path error

Possible Causes:

The input path lacks the files needed for laser point cloud post-processing (including IMU data file (.IMU), RTK base station file (.RTB), RTK main antenna file (.RTK), RTK secondary antenna file (.RTS), pole Arm data files (.RTL), laser point cloud files (.LDR), radar and IMU calibration files (.CLI)), the most common is that RTK is not opened during flight, resulting in missing RTK files.

At this time, you need to use the PPK scheme or re-collect data, and you must turn on RTK when collecting.

The name or suffix of the file in the input path is incorrect. Check the file name and suffix according to the guide, especially when using third-party base station data, pay special attention to the naming and suffix. 

LiDAR point cloud raw data error

Possible Causes:

The content of the laser point cloud file (.LDR) in the input path is incorrect, which may be caused by copying incorrectly.

The acquisition time of the laser point cloud file (.LDR) in the input path and other input files (including IMU data file (.IMU), RTK base station file (.RTB), RTK main antenna file (.RTK), RTK secondary antenna file ( .RTS), the acquisition time of the lever arm data file (.RTL)) does not correspond, and there is no overlap. The collected data and file copy should be checked for errors.

LiDAR point cloud raw data error

Possible Causes:

Some input files in the input path (including IMU data file (.IMU), RTK base station file (.RTB), RTK main antenna file (.RTK), RTK secondary antenna file (.RTS), pole arm data file (.RTL) )) The content is incorrect or the data is missing
IMU data file (.IMU) and RTK data (RTK base station file (.RTB), RTK main antenna file (.RTK), RTK secondary antenna file (.RTS), lever arm data file (.RTL)) have no time overlap, Mismatch.

Check the data source at this time. Generally, you need to perform data collection again.