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DJI L1 IMU Calibration question
1155 18 1-18 23:45
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Panph
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Hi to everyone,
I'm an experienced pilot and surveyor but new to Enterprise product, in specific  Matrice 300 + P1 + L1 for surveying.
I've been making photogrammetry for last 6 years, so using P1 with right sorftwares is not a problem, but surveying with L1 (my first lidar solution) it's a whole new world.
I've followed a course and lot of tutorials and workflow videos about L1, I've just made 2 works with it with great results. Now that i'm studying deeply the L1 and all it's settings and funcionality, i have few question, specially on IMU Calibration during flight.

-In a flat (or nearly flat) area, i use a Mapping mission with IMU Calibration turned ON, so, as i've understand, the imu calibrates at every turn in flight path.
But i live in North of Italy, where nothing is flat, so i need to use for almost every survey the Terrain Following funcion, that disable the automatic IMU Calibration (at least with actual firmware version).
For every flight, i manually calibrate the imu at the beginning and when finishing the flight, but what during the mission? The frequent stops (or deceleration and then acceleration) due to height changes in flight path could calibrate the imu during flight? do i have to stop the mission every 100sec, make a calibration and then restart?

-Another question is regard an hypothetic single building survey, where i need to get the facades as better as possible. In this case i would use a manual flight instead of a mapping area (or an oblique mission with -45° gimbal?) and what movement i have to do with the drone? as i understand, it's not recommended to orbit the building as for a photogrammetry flight, but make a single or double up and down parallel to the facade with L1 pitch slightly negative? Passing from a facade to another, it's recommended to pause the Lidar acquisition, make a calibration and then restart the point cloud acquisition? Any tips for this type of surveying are accepted

Thanks
1-18 23:45
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tutos
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Very good question, because the calibration button is grayed out and disabled in terrain following mode (or any mission mode). DJI, how is supposed to do the calibration with terrain following mode?

Jose
1-19 06:49
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Panph
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tutos Posted at 1-19 06:49
Very good question, because the calibration button is grayed out and disabled in terrain following mode (or any mission mode). DJI, how is supposed to do the calibration with terrain following mode?

Jose

In the course i've done before working with the L1, they said to make a calibration before and after the mission flight, but they said nothing about during the flight. It's not so hard working with big areas to deal with 20 or more mins of flight, which teorically needs at least 1 calibration every 100sec of recording.
In the class they said that the calibration is a sequence of fast movement of the drone, which moves the L1 gimbal and keeps the IMU "in alert". The degradation of the data is during flights with more than 100sec linear flight without "shakings".
My question is that the simple height variation during flights, that force the drone to slow down and accelerate, is sufficient to shake enough the gimbal and so keeping the imu calibrated.
Another unofficial tip i found is to pause and resume the mission, to force the drone for a fast stop and a restart, this should recalibrate the IMU.
Anyone has a confirmation of this procedure?
1-19 07:23
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aebrilievi
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Great questions; I also wait and hope for an answer, official or not...
1-20 09:31
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fansb2377630
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This is a very good question, sounds like the exact same issues and questions we have. We have been using the terrain follow but the calibration has always been an issue. DJI never is a huge help when I have reached out to them. So we are conducting a series of 3 test tomorrow. The flight is 3,100 feet long and maybe only 200 feet wide.  There is about 110ft elevation change over the course of the 3,000 feet. However, I will conduct 3 flights:

- 1x flight that is normal with Auto Calibration on at a constant AGL
-1x flight that is terrain follow and will have no 100s calibrations minus start and end.
-1x flight that is terrain follow and will have calibrations every 100s, turns at end of leg, and at the start and end.

I will also set several GCP for checking. Hopefully I can get results by the end of the week. Hopefully the no 100s calibration flight will show the same results as the one that does have the 100s calibration.  If the only thing a calibration does in between legs is "wake it up" then the change in elevation and sudden turns on the end of the legs should do the trick.
1-31 11:33
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aebrilievi
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fansb2377630 Posted at 1-31 11:33
This is a very good question, sounds like the exact same issues and questions we have. We have been using the terrain follow but the calibration has always been an issue. DJI never is a huge help when I have reached out to them. So we are conducting a series of 3 test tomorrow. The flight is 3,100 feet long and maybe only 200 feet wide.  There is about 110ft elevation change over the course of the 3,000 feet. However, I will conduct 3 flights:

- 1x flight that is normal with Auto Calibration on at a constant AGL


Great, I'll wait for your test results
2-1 01:13
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myrnostrum
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fansb2377630 Posted at 1-31 11:33
This is a very good question, sounds like the exact same issues and questions we have. We have been using the terrain follow but the calibration has always been an issue. DJI never is a huge help when I have reached out to them. So we are conducting a series of 3 test tomorrow. The flight is 3,100 feet long and maybe only 200 feet wide.  There is about 110ft elevation change over the course of the 3,000 feet. However, I will conduct 3 flights:

- 1x flight that is normal with Auto Calibration on at a constant AGL

We're thinking through the same issues as well. How did the results look from your tests?
3-4 19:44
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Panph
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Just one observation: this week i've done a 12ha survey @60m with terrain follow. The difference between the highest and the lowest point were about 150m, 12min flight @4m/s speed, non repetitive scan path.
I need to admit i forgot to calibrate the imu before the mapping mission, i've done only at the end.This was the area, were 6 paths parallel to countours, from top to bottom, 45% overlap.


With 6 GCP in the area, i've found errors around 2 to 4cm in XYZ. So pretty great results!! Maybe just the Terrain Follow "shakes" during the flight are enough to calibrate the IMU.

Peterploner_low.jpg
3-6 00:03
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sandy06
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Hello!

I have a L1 recently and have now seen many videos on data acquisition. Terrain follow is essential for me and my missions. However, this calibration flights before and after the flight missions is never mentioned. While flying, the calibration option is not selectable. So how do I do these calibration flights before and after? Thanks for your help!
3-22 00:45
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Panph
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sandy06 Posted at 3-22 00:45
Hello!

I have a L1 recently and have now seen many videos on data acquisition. Terrain follow is essential for me and my missions. However, this calibration flights before and after the flight missions is never mentioned. While flying, the calibration option is not selectable. So how do I do these calibration flights before and after? Thanks for your help!

I've followed a training course where i bought the Matrice, the instructor told me this workflow.

Calibrating before starting the task is essential for precision. Furthermore, is compulsory if you want to optimize the point cloud in DJI Terra with RINEX file, to increase precision. Last survey i made with terrain follow, i forgot to calibrate before the flying, and when importing the data in DJI Terra (and optimizing with RINEX file in folder) i had an error that told me it can't process the data due to not initial calibration. I had to process without RINEX and match the paths in TerraMatch.

The workflow is pretty simple:
-aircraft on the ground, create (or open) a mission with terrain follow without IMU Calibration and leave it in background.
- Take off  MANUALLY and get closer to start point manually (to reduce the time from calibration to the task).
- CALIBRATE the imu with the proper button while in L1 screen
- Start the mission in map view
- When finished, BEFORE landing, CALIBRATE again with proper button in L1 screen.
- Land.

With this workflow i've had good results in terms of data, and that's what i've been told to do in the L1 training course.
3-22 01:35
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fans3cf3ca7b
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Hello,
We used this weekend the Terrain follow  mode with the L1 on large areas.
We noticed that the Matrice stops and restarts at each photo and that it is impossible to take the photos in continuous mode of the Matrice movement. Maybe this is deliberate on the part of DJI to calibrate the IMU at each stop and restart. DJI can you explain us the procedure to calibrate the IMU of the L1 for missions with Terrain Follow.
Thank's
JLB

3-22 01:38
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Panph
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fans3cf3ca7b Posted at 3-22 01:38
Hello,
We used this weekend the Terrain follow  mode with the L1 on large areas.
We noticed that the Matrice stops and restarts at each photo and that it is impossible to take the photos in continuous mode of the Matrice movement. Maybe this is deliberate on the part of DJI to calibrate the IMU at each stop and restart. DJI can you explain us the procedure to calibrate the IMU of the L1 for missions with Terrain Follow.

Yes, you're right. The IMU calibration is simply some "shakes" of the L1 gimbal to force the IMU to work and not remain in fixed position.

The "start and stops" in the Terrain Follow mission i think is for this reason. As i told previously, i've done TF mission with Control Points on the ground and with these "shakes" i had no issues with precision.
3-22 01:42
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fans3cf3ca7b
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Grazie Panph per questa informazione, abbiamo appena coperto più di 100ha in montagna questo WE ed eravamo in dubbio su quando eseguire la calibrazione IMU. Vi terrò informati sulla precisione che otterremo.
3-22 01:47
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Panph
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fans3cf3ca7b Posted at 3-22 01:47
Grazie Panph per questa informazione, abbiamo appena coperto più di 100ha in montagna questo WE ed eravamo in dubbio su quando eseguire la calibrazione IMU. Vi terrò informati sulla precisione che otterremo.

Ottimo, tienici informati!
io settimana prossima ne ho 200ha di montagna a strampiombo sul lago di Garda... Essendo la parete troppo verticale non posso neanche lavorare in automatico con il Terrain Follow ma mi tocca lavorare in manuale addirittura.... Avrò GCP presi con il rover solo sulla parte bassa (dislivello totale 900m), sulla parte alta, dato che la P1 mi da sempre precisioni inferiori ai 4cm, faccio una passata sulla cima con la P1 e uso qualche punto di quello come GCP.
Speriamo!

ENG
Great, keep us informed!
Next week I have 200ha of mountain rocks over Lake Garda ... Since the wall is too vertical I can't even work automatically with the Terrain Follow but I have to work manually even .... I will have GCPs taken with the rover only on the lower part (total difference in altitude 900m), on the upper part, since the P1 always gives me accuracies of less than 4cm, I make a pass on the top with the P1 and use some points of that as GCP.
Let's hope!

3-22 02:57
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fans3cf3ca7b
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900m di muri!!... incredibile... non ho il P1 , per le ortofoto le faccio con il P4 RTK. Abbiamo presto una missione nelle Alpi tra Digne les bains e Nizza dove dovremo anche prendere delle pareti verticali (ponti).
Sapete come viene gestito il RTH in modalità terrain follow. Cosa succede se la matrice decolla ai piedi della montagna, l'RTH è impostato a 120m di altezza e perdiamo il segnale radio con la matrice in cima 500m più in alto del decollo. Se avete la risposta, sarei interessato.
Cordialmente, JL

ENG
900m of walls!! unbelievable... I don't have the P1 :-( , for the ortho I do them with the P4 RTK. We have soon a mission in the Alps between Digne les bains and Nice where we will also have to take vertical walls (bridges).
Do you know how the RTH is managed in terrain follow mode. What happens if the matrix takes off at the bottom of the mountain, that the RTH is set to 120m height and that we lose the radio signal with the matrix at the top 500m higher than the takeoff. If you have the answer I am interested. Sincerely, JL
3-22 05:53
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Panph
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fans3cf3ca7b Posted at 3-22 05:53
900m di muri!!... incredibile... non ho il P1  , per le ortofoto le faccio con il P4 RTK. Abbiamo presto una missione nelle Alpi tra Digne les bains e Nizza dove dovremo anche prendere delle pareti verticali (ponti).
Sapete come viene gestito il RTH in modalità terrain follow. Cosa succede se la matrice decolla ai piedi della montagna, l'RTH è impostato a 120m di altezza e perdiamo il segnale radio con la matrice in cima 500m più in alto del decollo. Se avete la risposta, sarei interessato.
Cordialmente, JL

L'altitudine di RTH deve sempre essere impostata sull'ostacolo più alto nell'area di volo, indipendentemente dal terrain follow!! mi spiego: tu hai impostato 120m di rth; sei per via del terrain follow a 120m AGL ma 400 ATL. Il matrice segue il terreno e va in una depressione, e perdi il contatto radio. Rth impostato a 120m e il drone rimane li bloccato, se trova un ostacolo superiore a 120m da quando perdi il segnale radio.
Quando perde il segnale e sei ad una quota superiore a quella dell'rth, lui mantiene quella quota per tornare in direzione di casa.

Quindi io imposterei la quota rth a 500m, quindi anche se vai in una depressione e perde il segnale, lui sale fino a 500m prima di tornare a casa, e anche prima di raggiungere i 500m riprenderai sicuramente il contatto radio per riprendere il controllo.

Diverso l'advanced rth ad esempio del Mavic3, che invece segue il percorso fatto per arrivare li, e lo fa al contrario. Se fosse implementato anche sul matrice non sarebbe male.


The RTH altitude must always be set to the highest obstacle in the flight area, regardless of the terrain follow !! let me explain: you have set 120m of rth; you are due to the terrain follow at 120m AGL but 400 ATL. The matrice follows the ground and goes into a depression, and you lose radio contact. Rth set to 120m and the drone gets stuck there, if it finds an obstacle higher than 120m since you lose the radio signal.
When he loses his signal and you are at an altitude above the RTH, he maintains that altitude to return home.

So I would set the rth altitude to 500m, so even if you go into a depression and lose the signal, he climbs up to 500m before returning home, and even before he reaches 500m you will definitely regain radio contact to regain control.

The advanced rth for example of the Mavic3 is different, which instead follows the path taken to get there, and does it in reverse. If it were also implemented on the matrice it would not be bad.
3-22 08:55
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fans3cf3ca7b
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Grazie Panph per queste informazioni dettagliate.
Cordiali saluti,
JL
3-23 02:22
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Tehnic STS
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Hello,

Do you think the lack of a correct IMU calibration can give me errors from 1 to 2 meters ?
3-31 13:08
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Panph
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Tehnic STS Posted at 3-31 13:08
Hello,

Do you think the lack of a correct IMU calibration can give me errors from 1 to 2 meters ?

Yes, it is possible, it depends even of the rtk status while scanning.
In some cases (it happened to me) DJI Terra without calibration cannot even process the data.
3-31 23:26
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