For the purpose of point cloud data processing, L1 needs to have centimeter-accurate positioning data with either RTK or PPK. The RTK FIX statis needs to be maintained throughout the duration of the L1 flight mission. If RTK connection stability cannot be guaranteed, PPK could also be used. The detailed workflows of each method are described below.
1.1 Ntrip (Custom Network RTK) Solution
In the RTK settings page of the DJI Pilot App, you can choose Custom Network RTK, and enter the NTRIP account information to connect to the Ntrip service. (Note: The remote controller must be connected to the internet through WiFI or the 4G dongle). If the Ntrip service is connected and the RTK status is FIX throughout the flight, the base station file will be automatically saved in the results file.
1.2 D-RTK 2 Base Station Solution
Set up D-RTK at a known point. Switch the D-RTK 2 to Mode 5. In the RTK settings page of the DJI Pilot App, select D-RTK 2 as RTK Service Type. Then link the aircraft to the D-RTK 2. In the RTK settings page, go to the Advanced Settings (Pin: 123456 by default). Modify the D-RTK 2's coordinates to the coordinates of the known point.
When high absolute accuracy us needed, you must set up the D-RTK 2 at a known point instead of using its self-convergent GNSS coordinates. This is because the static convergent coordinates of the D-RTK 2 have an error in the meter-level. If the D-RTK 2 position is not corrected with known coordinates, the absolute accurate of the result cannot be guaranteed and the point cloud data from multiple flights might not be consistent.
Note when setting up the D-RTK 2 at a known point it is important to add the rod height. When post processing, the default output will be WGS84 unless otherwise selected.
After setting up the D-RTK 2 base station, you can connect the aircraft to the D-RTK 2 for the flight mission.
Alternatively, you can use the PPK method where no real-time connection between the drone and the D-RTK 2 is required. With the PPK method, the distance between the D-RTK 2 and the aircraft is recommended to be withing 10km. Select "None" in the RTK service type. Turn off the RTK Positioning switch to go to GNSS flight mode. After data collection for the mission is completed, ise a Type C cable to connect the D-RTK 2 to a PC. Copy the base station file with a suffix .DAT for the corresponding time slot and paste the file in the same folder as L1's result file. In this way, DJI Terra will use the D-RTK 2 data for PPK processing to calculate the accurate POS data.
1.3 Third-party RTK Solution
A third-party RTK base station device can also be used for PPK. In this case, no connection is required between the drone and the RTK base station. The distance between the third-party RTK base and the aircraft is recommended to be within 10km. The distance between the third-party RTK base and the aircraft is recommended to be within 10km. After the flight is completed, search for the base station file for the corresponding time slot and rename the file following the suffix rules below. Then, copy the bast station file to the same directory as the LiDAR files. If the third-party RTK base station has not coordinates input for the known point, you can open the .O base station file in text form and modify the "APPPO POSITION XYZ" value to be the corrected coordinate of the RTK base in the ECEF coordinate system.
The L1 supports the following base station protocols and versions. (Note: The name of the renamed file DJI_YYYYMMDDHHMM_XXX should be consistent with the name of the .RTK file in the point cloud data directory). If there is a .RTB file in the same directory, it needs to be deleted.
Note: Base station files are necessary for L1 data processing. If neither Ntrip nor PPK files exist, L1 data cannot be processed. If the RTK is disconnected during the flight, the mission will be automatically paused to ensure data validity.
2. IMU Calibration
IMU calibration is a prerequisite for LiDAR accuracy. It is a key factor that impacts the final point cloud accuracy. For the L1, please note that IMU calibration is required before, during (every 100s of the flight mission), and after data collection to ensure the inertial navigation system accuracy.
Below are two ways of IMU calibrations that can be used during manual/mission (flight route) flight.
2.1 Calibration Flight Button in Manual Flight
Fly the drone to a proper altitude, switch to the camera screen, and click on "Calibration Flight" button. M300 RTK will automatically fly forward and backward 3 times to calibrate the IMU system at current altitude. Make sure there are not obstacles in the 30m range ahead of the drone.
2.2 IMU Calibration in Mapping/Oblique/Linear Flight Missions
In mapping/oblique flight missions, you can find the toggle "IMU Calibration". When turned on, acceleration and deceleration calibration flights will be inserted to the flight route at the starting point, the ending point, turning points, and along the route (every 100s). the calibration routes are shown in yellow in the figure below.
3. RGB Camera Recalibration
When the L1 has regular colorization issues, such as multiple lines appearing in the point cloud with ghosting effect, the user will need to recalibrate the internal and external parameters of the RGB camera of L1 following the below steps.
Note: This is a premium feature included in JI Terra Pro and more advanced versions.
3.1 Calibration data collection
- Create a 2D Mapping mission in DJI Pilot App and draw an area of around 200m*200m; The area should have vertical structures like buildings;
- Camera type: Zenmuse L1 - LiDAR Mapping;
- Flight Route Altitude: 100 m;
- Speed: 10 m/s;
- Enable the "Elevation Optimization" option;
- In Payload Settings, select "Single Reutrn", "240 kHz" and "Repetitive Scan", and enable the "RGB Coloring" option;
- Save the mission;
- In L1 camera settings, disable the camera dewarping;
- Execute the flight mission and obtain the raw data files collected by L1 and check whether the images are clear and sharp, if not they cannot be used for calibration.
3.2 Use DJI Terra Software to Generate the Calibration Files
- Use DJI Terra version 3.1.0 or above to create a new "LiDAR Point Cloud Processing" reconstruction mission;
- Import the dataset collected by defining the dataset directory and changing the "Scenarios" option to "Zenmuse L1 Calibration";
- Start processing in Terra;
- When the reconstruction is completed, please check if the point cloud model colorization is ok or not. If there are still multiple layers or ghosting effects, please repeat step 1) and 2). If the result is ok, you can proceed to use the calibration files from this mission;
- The calibration files with suffix ".tar" can be found under the PROJECT/lidars/terra_L1_cali" directly as shown below:
3.3 Run the Calibration files in L1
- Store the calibration file under the root directory of a microSD card and insert into the L1, connect L1 to a M300 RTK and power on the M300 RTK. Wait for about 5 minutes to complete the calibration. There is no App notification during the calibration currently.
- To confirm the calibration process is completed: remove the microSD card from the L1, open the log file with suffix ".txt", the calibration process is successful if the log file shows "all succeed".
3.4 How to restore factory parameter?
In case you need to restore the factory parameters of the L1 sensor, please follow the following steps:
- Create a new .txt text file, and name it as reset_cali_user. Open the file and write the SN serial number of the L1 that needs to be reset, with the format of "SN number: XXXXXXXXXXXXXX" ( find the SN serial number in the .CLI file that's saved on the microSD card, or view in the device version information in Pilot app by going to the three dots in the right in manual flight, the three dots sub menu, then About).
- Copy the .txt text file to the root directory of the microSD card, insert the microSD card into the L1 that needs to be calibrated, install the L1 onto the M300 RTK, and power on the aircraft, wait about 5 minutes to complete calibration.
- Record a set of point cloud data, remove the microSD card from the L1. Connect it to a computer and check the .txt format log file. If it displays all succeeded, the reset is successful. Users can also check whether the time parameter of the .CLI file is restored to the factory time.
4. Flight Mission Planning
A flight mission of the survey area can be pre-planned with the DJI Pilot App to enable automatic data capture of a polygon area or a strip area. Detailed parameter recommendations for typical scenarios are as below:
4.1 Topographic Surveying
This is a typical scenario of LiDAR to generate topographic maps, contour lines, etc. It may also be applicable to forest inventory monitoring by measuring the tree heights and density of the canopy.
Create a "Mapping Mission". You can import the KML file of the target area or manually draw the area on the map. The recommended parameter settings for topographic surveying are as follows:
How to enable Terrain Follow Mission?
DJI L1 supports terrain-follow flight for waypoint, mapping, and corridor flight plans. With the DJI Pilot app to perform a precise terrain follow flight, enable terrain follow in the mapping mission and import the DSM file including the altitude information.
To prepare files for terrain follow missions:
The DSM files of the measurement area can be obtained through the following two methods:
- Collect the 2D image data of the mapping area and perform a 2D reconstruction through DJI Terra. When processing, make sure to select Fruit Tree mode and use WGS84 as the coordinate system. After processing with DJI Terra, a .tif file will be generated and it can be imported using a microSD to the remote controller.
- Download the terrain mapping data from a geobrowser and import it to the microSD card of the remote controller.
Make sure the DSM file is a geographic coordinate system file, and not a projected coordinate system file. Otherwise, the imported file may not be recognized that the resolution of the imported file should be no more than 10 meters.
- Enable Terrain Follow and IMU Calibration in Mapping mission
- Tap "Select DSM File", tap +, select and import the file from the microSD card
(1) Prepare the DSM file of the survey area
For a Terrain follow mission, a DSM file including the elevation information of the terrain needs to be imported. Please note that the DSM file used for Terrain Follow Mission must use the WGS84 coordinate system, rather than project coordinates. The file should not exceed 20 MB in size, with a preferred resolution of less than 10 meters.
The DSM file of the test area can be obtained in either of the two methods below:
- Collect the 2D data of the target area and perform 2D reconstruction in DJI Terra with Fruit Tree mode. A "gsddsm.tif" file will be generated in the "map" folder of the mission files. It can be imported to DJI Pilot App for terrain follow.
- You can also download the terrain elevation data from an open database such as SRTM.
(2) Import the DSM file to DJI Pilot App
Terrain Follow and IMU calibration flight options can be enabled at the same time.
You can put the DSM file in a SD card and insert it to the RC. Then click on "Select DSM File" and select the DSM file in the SD card.
you can see the area covered by the DSM file on the map now. Then you can plan the survey area within this area.
Terrain Follow is recommended when the elevation difference of the survey area is greater than 100 meters.
4.2 Riverbank/Road Surveying
When the survey area is narrow strip of land such as roads, railways, riverbanks, etc., Linear Flight Mode can be used for L1 mission planning.
L1 can be used to collect point cloud data of transmission lines and the power pylons. As there are undulations in the power line corridor, the recommended workflow is to use the "Live Mission Record" function to create a waypoint flight mission so that we can fly with variable heights.
(1) In the DJI Pilot App, go to Flight Route - Creat a Route - Waypoint - Live Mission Rec. Note that you need to record in RTK mode.
(2) Fly the M300 RTK 10-20m above the power pylon and record the current point by pressing the C1 button.
(3) Repeat this action for all structures within the mission area.
(4) Save the mission.
(5) Edit the mission parameters according to the tables below:
5. Checkpoint Setup
In topographic surveying missions, checkpoints can be used to verify the point cloud accuracy. The output result from the L1 is 3D point cloud data in LAS format. Unlike the 3D models obtained with visual cameras, the point cloud contains no structural information, so its checkpoints differ from those in visible light surveying. If you use normal photogrammetry markers and mark the checkpoints in RGB mode, there will be an extra error due to the colorization. Please note the GCP management feature is not supported for L1 data in Terra.
Two types of checkpoints are recommended for LiDAR, as shown below:
5.1 3D Checkpoints
L1 point clouds can be displayed in reflectivity mode. Therefore, if the checkpoints are marked with significant differences in reflectivity, they can be clearly recognized in the point cloud model. Highly reflective paints are recommended to be used for checkpoint markings. The checkpoint size should be greater than 1m x 1m. When measuring the checkpoint in DJI Terra, you can use the RGB display mode to roughly locate the checkpoint and then switch to reflectivity display mode to accurately measure the coordinate of the center of the checkpoint.
In addition, if there are objects with sharp reflectivity differences in the survey area such as a zebra crossing, they can be also used as checkpoints.
5.2 Elevation Checkpoints
When you use third-party point cloud analysis software such as TerraSolid, LiDAR360 or Point Cloud Automata, they may provide the feature to output an elevation accuracy report. Usually, the elevation accuracy check feature is based on the assumptions that the horizontal accuracy of the LiDAR system can be neglected, and the points near the checkpoint are all on the same elevation plan.
Therefore, it is recommended to set up the elevation checkpoint on a flat surface of a least 1m x 1m in size. The checkpoints should be evenly distributed in the survey area and have differences in elevation, rather than all being located on the same plane. To test the penetrability of LiDAR, it is recommended to set a certain number of checkpoints under trees.
6. Field Data Collection
6.1 Power on L1 for Warmup
Before recording data, you need to attach the L1 to the single-downward gimbal of M300 RTK and power it on to warm up the L1. The warmuo takes about 3-5 minutes. Wait until a "Payload IMU warmed up" prompt appears.
6.2 Flight Mission Execution
If the IMU Calibration feature is enabled in the mission settings, the drone will automatically perform IMU calibration by performing a shuttle flight. No data will be collected during the calibration.
During the flight, you can switch between Camera, LiDAR, and SBS (side by side) view by pressing on the left side option. If the flight mission is paused and then resumed, the drone will automatically perform IMU calibration at the break point.
6.3 Manual Data Collection
when you need to collect L1 data of building facades, powerlines, or other complex structures, manual flight is also an option.
L1 Camera Parameters
You can set L1 parameters in "MENU". Recommended parameter values for manual flight are: "Non-repetitive scan", "Dual-echo", "240 kHz Sample rate", "RGB coloring" on, and 3s as the shooing interval. Make sure that the Ntrip is properly connected or an RTK base station has been set up before the flight.
RGB coloring means that the L1's visible-light camera takes photos concurrently to collect LiDAR point cloud data. We recommend always enabling this option except during nighttime operations.
Point Cloud Record
Fly the drone near the target area and adjust the gimbal to a proper angle for data collection. then click on the "liDAR" option on the left side of the viewer to go to the point cloud screen and click the point cloud recording button
to start recording. During manual flight, we recommend a speed of 5-12 m/s and a distance from the subject of 50-100 meters. During manual flight, once the aircraft flies at a constant speed for 100s, you need to pause the point cloud recording, perform the IMU calibration and then resume the point cloud recording.
Note: Do not collect data when the drone is static on the ground or hovering in the air, otherwise DJI Terra may encounter reconstruction errors in data post-processing.
7. Data Storage
In manual flight mode, the collected point cloud data and RGB images (if RGB coloring enabled) are saved in the micro SD card of L1 at DCIM/DJI_YYYMMDDHHMM_serial number_XXX. You can search for the data by time.
When a flight mission is completed, the collected data are saved in the DCIM folder of the micro SD card and named after mission name.
The LiDAR files should include files with suffixes of CLC (LiDAR camera calibration data)l CLI (LiDAR IMU calibration data), CMI (visual calibration data), IMU (inertial navigation data), LDR (LiDAR point cloud raw data), MNF (visual data, which is currently omitted with no impact), RTB (RTK base station data), RTK (RTK main antenna data), RTS (RTK sub-antenna data), and RTL (rod arm data). If model coloring is enabled, there will also be JPG files (photo data).
If the .RTB file is missing, it is because the RTK was not connected or was disconnected during the flight, Please see Chapter 1 of this article for the detailed requirements for RTK/PPK setup.