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How does Mini measure altitude...
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djiuser_f1qEQatHsKbC
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...and what happens if you fly it DOWN from the top of a mountain?

I thought I read somewhere that altitude is measured from the point of takeoff - so if that's the case, launching it from a mountain into a valley will give you a negative number for the altitude eh? Any ramifications to doing this?
2020-2-4
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frankymusik
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... it is exactly like that.

It's just the relative altitude, as the difference to the altitude from the starting point.

2020-2-4
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DJI Stephen
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Hello and good day. Thank you for reaching out and for the inquiry. With regards to this matter the DJI Mavic Mini's altitude measured using a barometer device on the drone. The barometer is zeroed at launch, so the display simply shows the drone's altitude above the point at which you launched. In addition, the altitude is calculated by an on board barometer that detects pressure and measures the difference from when it was launched to where it is now. Thank you.
2020-2-4
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120ccpm
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DJI Stephen Posted at 2-4 09:18
Hello and good day. Thank you for reaching out and for the inquiry. With regards to this matter the DJI Mavic Mini's altitude measured using a barometer device on the drone. The barometer is zeroed at launch, so the display simply shows the drone's altitude above the point at which you launched. In addition, the altitude is calculated by an on board barometer that detects pressure and measures the difference from when it was launched to where it is now. Thank you.

Hi Stephen, some of us were discussing this topic just recently. In the flight logs, the MM also records the actual elevation (as in height above see level), both current and at takeoff. Do you know if that comes from the GPS or from the barometer?
2020-2-4
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120ccpm Posted at 2-4 11:01
Hi Stephen, some of us were discussing this topic just recently. In the flight logs, the MM also records the actual elevation (as in height above see level), both current and at takeoff. Do you know if that comes from the GPS or from the barometer?

Hello and good day 120ccpm. Thank you for reaching out and for the inquiry. Let me please forward this inquiry to the designated DJI department for us to be able to give out the best answer and explanation with regards to this inquiry. Thank you.
2020-2-4
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120ccpm
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DJI Stephen Posted at 2-4 11:18
Hello and good day 120ccpm. Thank you for reaching out and for the inquiry. Let me please forward this inquiry to the designated DJI department for us to be able to give out the best answer and explanation with regards to this inquiry. Thank you.

Hi Stephen, did you get any response back from the engineers?
2020-2-25
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120ccpm Posted at 2-4 11:01
Hi Stephen, some of us were discussing this topic just recently. In the flight logs, the MM also records the actual elevation (as in height above see level), both current and at takeoff. Do you know if that comes from the GPS or from the barometer?

It can only come from either of two places, the GPS or mapping information (probably Google). The barometer cannot know anything relative to sea level as it has no idea what that is. And in the case of GPS, it can also be wildly innacurate. If the information is derived from mapping data, then it can only do that if there is an internet connection available during the flight.
2020-2-25
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Labroides
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120ccpm Posted at 2-25 15:03
Hi Stephen, did you get any response back from the engineers?

The relative height comes from the barometric sensor in all DJI drones.
DJI do use GPS for the Absolute height , but only for the Mavic 2 using recent firmware.
For all others, DJI still use barometer data converted to a nominal ASL height assuming standard atmospheric conditions.
The result is often +/- 200 feet or more.
It gets confusing because DJI label the field GPS data even though it was actually derived from barometer data.
2020-2-25
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120ccpm Posted at 2-4 11:01
Hi Stephen, some of us were discussing this topic just recently. In the flight logs, the MM also records the actual elevation (as in height above see level), both current and at takeoff. Do you know if that comes from the GPS or from the barometer?

Absolute elevation cannot be measured by a barometer, unless this is reset in a known position/altitude (provided, for instance, by GPS)
2020-2-26
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Labroides
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emaneg Posted at 2-26 03:17
Absolute elevation cannot be measured by a barometer, unless this is reset in a known position/altitude (provided, for instance, by GPS)


Absolute elevation cannot be measured
(accurately) by a barometer
And that's why the Absolute Altitude figure is so wrong.
See the post above.
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Labroides Posted at 2-26 03:48
Absolute elevation cannot be measured (accurately) by a barometer
And that's why the Absolute Altitude figure is so wrong.
See the post above.

Labroides, I've been using since long time ago a barometer/altimeter (and my grand father used the same meter well before me) and I had very good readings in mountain trekking, when GPS was just a dream. Of course I had to adjust the outer ring when I was in a place of known altitude asl.
Readings could be affected by sudden weather changes, but in the MM flight time span I hardly believe this can occur.

Hence I think that if the MM barometer is internally set to the local absolute altitude asl by means of GPS when the homepoint is set, it can give a fairly good indication of absolute altitude, provided that the bird barometer is termally compensated and that its aerodynamics is not perturbating too much the air pressure arounf the sensor.



2020-2-26
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emaneg Posted at 2-26 09:34
Labroides, I've been using since long time ago a barometer/altimeter (and my grand father used the same meter well before me) and I had very good readings in mountain trekking, when GPS was just a dream. Of course I had to adjust the outer ring when I was in a place of known altitude asl.
Readings could be affected by sudden weather changes, but in the MM flight time span I hardly believe this can occur.

Readings could be affected by sudden weather changes, but in the MM flight time span I hardly believe this can occur.
...

provided that the bird barometer is termally compensated and that its  aerodynamics is not perturbating too much the air pressure arounf the  sensor.
These are easily confirmed by observing that the height shown in the app does not change due to wind or flying fast and the height of your launch spot does not vary much between the beginning and end of a flight


Hence I think that if the MM barometer is internally set to the local absolute altitude asl by means of GPS when the homepoint is set, it can give a fairly good indication of absolute altitude.
But it is not corrected at all and the Absolute Altitude as this example (showing an error of approx 121 metres or 400 ft) demonstrates.






Altitudes.jpg
2020-2-26
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120ccpm
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Labroides Posted at 2-25 15:30
The relative height comes from the barometric sensor in all DJI drones.
DJI do use GPS for the Absolute height , but only for the Mavic 2 using recent firmware.
For all others, DJI still use barometer data converted to a nominal ASL height assuming standard atmospheric conditions.

How do you know for sure that the MM uses the barometer for altitude?
2020-2-26
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120ccpm Posted at 2-26 16:43
How do you know for sure that the MM uses the barometer for altitude?

Because every other DJI drone does and altitude data from the Mini doesn't seem to be any different from the others.
2020-2-26
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GPS uses triangulation Its more accurate in horizontal, Its Your barometric altimeter and downward positioning sensors are what determine your distance from the ground.

This is certainly the way it is in all other mavics .
2020-2-26
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Niknik
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It will measure negative altitude as simple as that
2020-2-26
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120ccpm
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GPS uses triangulation Its more accurate in horizontal, Its Your barometric altimeter and downward positioning sensors are what determine your distance from the ground.
I know GPS is not very accurate to measure altitude, but the same goes for a barometric altimeter that is not adjusted for sea-level reference pressure.
As for "distance from the ground", I don't see how the barometer has anything to do with that. I recall reading somewhere that the P3S (no VPS) was using the barometer to detect when the AC was about to touch the ground, by sensing a sudden difference in pressure. Are you referring to this? For sure, the barometer is used to provide accurate measure of the height above take-off point, but that's not AGL.
2020-2-26
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Labroides Posted at 2-26 17:18
Because every other DJI drone does and altitude data from the Mini doesn't seem to be any different from the others.

You also said the M2 switched from barometer to GPS with a simple firmware update, so how do you know the MM didn't use GPS from the beginning?
2020-2-26
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120ccpm Posted at 2-26 18:27
You also said the M2 switched from barometer to GPS with a simple firmware update, so how do you know the MM didn't use GPS from the beginning?

Because GPS altitude is very innacurate, and DJI would know that, so why would they use GPS?

I know GPS is not very accurate to measure altitude, but the same goes for a barometric altimeter that is not adjusted for sea-level reference pressure.

Why is sea-level important? A good barometer can quite accurately measure the elevation of the aircraft above the lift-off point, and that is more important to the aircraft and flier.


2020-2-26
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120ccpm Posted at 2-26 18:27
You also said the M2 switched from barometer to GPS with a simple firmware update, so how do you know the MM didn't use GPS from the beginning?

You also said the M2 switched from barometer to  GPS with a simple firmware update, so how do you know the MM didn't use  GPS from the beginning?

The Mavic 2 still uses it's  barometric sensor for all flight purposes and the altitude display you  see on your screen.  What different about the M2 is that DJI changed the  data source for Absolute Altitude (which 99% of users don't know  anything about).
Until it was changed for the M2, all DJI drones got  it from barometric data and converted that to a very inaccurate  approximation of height above sea level.
DJI confused anyone that investigated it by labeling it as GPS data.
After  many complaints from people using their drones for mapping, DJI changed  it to use GPS for the M2 recently but have not fixed it for other  drones yet.

But that has nothing to do with the altitude you see in the app.
That still comes from barometric sensors for all DJI drones.

Rather  than quizzing me on how I know the Mini doesn't use GPS, it would make  more sense if you were to tell me why you think they might have used a  system completely different from all their other drones.
2020-2-26
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More internet myths and urban lore...
GPS does not use Triangulation.  Triangulation measures angles.  GPS does not use angles.  GPS receivers have non-directional attennas.





GPS positioning uses distance.  Involving common intersection point  of spheres radius distance from GPS satellites  transmissions or Trilateration.


2020-2-26
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I think it’s fair to say , I came to this thread to answer the question, how does mini measure it’s altitude, it’s also clear I answered it correctly and it was my main reason for posting.
But as usual we have other trolls who come to the thread, NOT to answer the question asked by the OP, but to troll others who are indeed giving Correct and relevant answers,  now the same troll who has been trolling me for a long time now is continually being put in his place, but still always trying to steer the threads in a manner nothing to do with topic being discussed. It seems strange that someone would go to all the bother of posting the information he got from wiki or elsewhere  which has nothing to do with this thread.

So I’m letting him know he’s totally off topic, he has not tried or even referred to to the OP’s question, and has basically posted what amounts to trash, I don’t think I need deal with him further as usual he’s way out of his comfort zone here.

As far as I have learned it's actually the earth that's in the way of accurate measurement by gps really don't think it matters whether it's trilateration or triangulation . And I never mentioned gps measured anything to do with height measurements in drones, so maybe others will now see this is a targeted exercise.


2020-2-27
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hallmark007
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Back on topic, found this article I had from sometime back .





5E136020-20F1-4DC8-BCD8-2568808AEE4F.jpeg
BCA2E81E-AEDB-46BD-A004-0565C4C54DAD.jpeg
2020-2-27
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djiuser_f1qEQatHsKbC
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hallmark007 Posted at 2-27 03:25
Back on topic, found this article I had from sometime back .

Thanks all, a lot of good info here, and some not so good, but anyway...

Back to my original question, and perhaps I didn't spell out my concern in enough detail. It seems that the Mini measures height from where it was launched, so that if you launch from a hill into a valley it will show a negative altitude. I don't see much problem with that, except... how do you know how high you are from the valley floor? Do you just have to eyeball it from the monitor?
2020-2-27
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djiuser_f1qEQatHsKbC Posted at 2-27 03:54
Thanks all, a lot of good info here, and some not so good, but anyway...

Back to my original question, and perhaps I didn't spell out my concern in enough detail. It seems that the Mini measures height from where it was launched, so that if you launch from a hill into a valley it will show a negative altitude. I don't see much problem with that, except... how do you know how high you are from the valley floor? Do you just have to eyeball it from the monitor?

You could look over the cliff ;+), all references are to ATO (at take off) and need to be to initiate a good Rth . One thing I don’t advise is to fly into a canyons or gorges if you cannot see the drone, risk of loosing gps.
2020-2-27
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HedgeTrimmer
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"But as usual we have other trolls who come to the thread, NOT to answer the question asked by the OP, but to troll others who are indeed giving Correct and relevant answers, "

Correct and releavant answers?   Try, Wrong and Hypocrite!   

GPS uses distance, not angles.  (see GPS.gov link and quote below)  

As always it is okay for DJI Troll god to go off topic, but don't anyone else comment on his off topic lead.


"A lie told often enough becomes the truth." ~ Vladimir Lenin



GPS.gov - Activity: How to find a position using GPS
   Orbiting the Earth are a number of Global Positioning System (GPS)  satellites that can help determine your location on the planet. The  concepts behind GPS positioning are very simple, but the application and  implementation require amazing precision.
GPS positioning works on two basic mathematical concepts. The first is  called trilateration, which literally means positioning from three  distances. The second concept is the relationship between distance  traveled, rate (speed) of travel and amount of time spent traveling, or:[size=110%]  Distance = Rate × Time
[size=110%]

The first concept, trilateration, is the focus of this activity. It  centers around finding your position on the Earth by knowing the  location of orbiting GPS satellites and the distance from those  satellites to your location on the planet. However, there is no way to  actually take a yardstick, tape measure, etc., and measure the distance  from your location up to the satellites. So how can we use trilateration  if we can't physically measure the distances? The answer lies in the  second concept, relating distance, rate and time. The trick lies in the  fact that GPS satellites are always sending out radio signals.


2020-2-27
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hallmark007
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HEDGETRIMMER HAS JUST THRASHED ANOTHER THREAD . TOTALLY OFF TOPIC AS USUAL .
HIS TOTAL OFF THE WALL COMMENTS , NOTHING TO DO WITH THIS THREAD, HES JUST A NASTY TROLL .
2020-2-27
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djiuser_f1qEQatHsKbC Posted at 2-27 03:54
Thanks all, a lot of good info here, and some not so good, but anyway...

Back to my original question, and perhaps I didn't spell out my concern in enough detail. It seems that the Mini measures height from where it was launched, so that if you launch from a hill into a valley it will show a negative altitude. I don't see much problem with that, except... how do you know how high you are from the valley floor? Do you just have to eyeball it from the monitor?

"except... how do you know how high you are from the valley floor? Do you just have to eyeball it from the monitor?"

The answer is yes, you will have to gauge it from the picture on your monitor, as the aircraft cannot tell you how high it is above the valley floor.
2020-2-27
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120ccpm
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Geebax Posted at 2-26 18:44
Because GPS altitude is very innacurate, and DJI would know that, so why would they use GPS?

I know GPS is not very accurate to measure altitude, but the same goes for a barometric altimeter that is not adjusted for sea-level reference pressure.

Why is sea-level important? A good barometer can quite accurately measure the elevation of the aircraft above the lift-off point, and that is more important to the aircraft and flier.

Height above take-off point is calculated through the barometer, there is no question about that. But the MM also tracks actual altitude (as in height above sea level) in the logs, and that's what my question to DJI was about: does that come from the GPS or from the barometer? Just a curiosity.
2020-2-27
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120ccpm Posted at 2-27 14:17
Why is sea-level important? A good barometer can quite accurately measure the elevation of the aircraft above the lift-off point, and that is more important to the aircraft and flier.

Height above take-off point is calculated through the barometer, there is no question about that. But the MM also tracks actual altitude (as in height above sea level) in the logs, and that's what my question to DJI was about: does that come from the GPS or from the barometer? Just a curiosity.

DJI are hopeless at getting back to you with any technical information.
I've answered your query and explained it in detail above.

As for the argument about whether triangulation is the correct word, it's a very trivial point.
Technically it's trilateration, but the difference is beyond most people's understanding and certainly makes no difference at this level.
To argue over it is being unnecessarily pedantic.

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120ccpm
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Labroides Posted at 2-26 19:00
You also said the M2 switched from barometer to  GPS with a simple firmware update, so how do you know the MM didn't use  GPS from the beginning?

The Mavic 2 still uses it's  barometric sensor for all flight purposes and the altitude display you  see on your screen.  What different about the M2 is that DJI changed the  data source for Absolute Altitude (which 99% of users don't know  anything about).

Rather  than quizzing me on how I know the Mini doesn't use GPS, it would make  more sense if you were to tell me why you think they might have used a  system completely different from all their other drones.

Not quizzing, simply asking. I have absolutely no opinion about it, both altitude values (GPS and barometric) are available to the AC, my question was simply how do you know which one the MM sticks in the logs. In particular, considering you said DJI recently changed that in the M2 with a simple firmware update.
2020-2-27
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120ccpm Posted at 2-27 14:31
Rather  than quizzing me on how I know the Mini doesn't use GPS, it would make  more sense if you were to tell me why you think they might have used a  system completely different from all their other drones.

Not quizzing, simply asking. I have absolutely no opinion about it, both altitude values (GPS and barometric) are available to the AC, my question was simply how do you know which one the MM sticks in the logs. In particular, considering you said DJI recently changed that in the M2 with a simple firmware update.


My question was purely about altitude (ASL), only visible in the logs.

That the (very inaccurate) Absolute Altitude comes from barometer data is demonstrated in the picture I posted in post: https://forum.dji.com/forum.php? ... =208432&pid=2094545
Testing at the same site and the same relative height with different atmospheric pressure has confirmed this.
If GPS was the source of the data, the error would be much less.
2020-2-27
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120ccpm
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Labroides Posted at 2-27 14:38
My question was purely about altitude (ASL), only visible in the logs.
That the (very inaccurate) Absolute Altitude comes from barometer data is demonstrated in the picture I posted in post: https://forum.dji.com/forum.php? ... =208432&pid=2094545
Testing at the same site and the same relative height with different atmospheric pressure has confirmed this.

Understood. You wonder why DJI is sticking to barometric altitude, if GPS is consistently more accurate.  In any case, like you said, the vast majority of the users doesn't even know altitude is being recorded. I came across it by chance, when I was playing with DashWare and the MM logs.
2020-2-27
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djiuser_f1qEQatHsKbC Posted at 2-27 03:54
Thanks all, a lot of good info here, and some not so good, but anyway...

Back to my original question, and perhaps I didn't spell out my concern in enough detail. It seems that the Mini measures height from where it was launched, so that if you launch from a hill into a valley it will show a negative altitude. I don't see much problem with that, except... how do you know how high you are from the valley floor? Do you just have to eyeball it from the monitor?

Sorry you have to wade through some blunt and almost unfriendly replies to get your info and answers.  To answer your primary concern, yes, as you descend into a valley your altitude will show a negative number. And it can be extremely hard to gauge how high you are above trees or water when your visual line of sight to your drone is way below and you're looking down on your drone from above.
My advice is to use the screen as much as you can in this situation;  keep tilting the camera up to the horizontal and then you can see ahead and gauge from the screen how high above the valley floor you still are.  


Cheers,
Ian

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hallmark007 Posted at 2-27 03:01
I think it’s fair to say , I came to this thread to answer the question, how does mini measure it’s altitude, it’s also clear I answered it correctly and it was my main reason for posting.
But as usual we have other trolls who come to the thread, NOT to answer the question asked by the OP, but to troll others who are indeed giving Correct and relevant answers,  now the same troll who has been trolling me for a long time now is continually being put in his place, but still always trying to steer the threads in a manner nothing to do with topic being discussed. It seems strange that someone would go to all the bother of posting the information he got from wiki or elsewhere  which has nothing to do with this thread.


Yeah I was reading through trying to learn something and this thread was interesting and useful. Then I read the post you are referring to and I thought... oh dear! There is always one! Talking of tools, is there an app for the Mac for flight data and or for flying or at least viewing a live flight?
2020-5-20
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Yep, GPS does use delay times to determine position. The receiver gets the sat's geo position and also measures the time difference between other sats. It is mature technology now. I used to work on LORAN systems which did the exact same thing but from fixed transmitter positions on the Earth's surface. Once processors became fast enough to do the complex calculations required by sats moving around, LORAN quickly became obsolete.
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frankymusik Posted at 2-4 08:45
... it is exactly like that.

It's just the relative altitude, as the difference to the altitude from the starting point.

Seemsmy Mavic Mini, once it senses it is close to or below the balcony I took off from, it goes into a landing mode. Just like when flying or hovering too close to the ground.
2020-11-6
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Seems my Mavic Mini, once it senses it is close to or below the balcony I took off from, it goes into a landing mode. Just like when flying or hovering too close to the ground.
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Hi all , I am new to drones and have a doubt, I have a Mavic  mini 2  and as per local government guidelines can’t be flown above 15 meters altitude and at 15 meters we get a warning that maximum altitude reached and can’t go beyond that. If I fly the drone from a hill upto 15 meters and at same altitude move it into the valley will the altitude is taken as above permissible altitude and what will happen to the drone? Will it plunge downwards into the valley to maintain the altitude? Thank you in advance and hoping for a reply
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Hi all , I am new to drones and have a doubt, I have a Mavic  mini 2  and as per local government guidelines can’t be flown above 15 meters altitude and at 15 meters we get a warning that maximum altitude reached and can’t go beyond that. If I fly the drone from a hill upto 15 meters and at same altitude move it into the valley will the altitude is taken as above permissible altitude and what will happen to the drone? Will it plunge downwards into the valley to maintain the altitude? Thank you in advance and hoping for a reply
1-9 22:36
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