How does the Mavic measure it’s height?

[Mossiback]

... I have a watch with an altimeter and that shows pressure changes just walking upstairs in the house !

I have an altimeter I use for hiking that shows a resolution of two feet.

 

[Vegasrobbi]

I use to pack open cell foam around the baro sensor on my APM boards. Spinning the quad in the wind would cause it to rise and fall with changing pressures.

 

[Storyline]

I have an altimeter I use for hiking that shows a resolution of two feet.

They are amazingly accurate. Presumably they are not an aneroid type which have a chamber that physically alters so I wonder how they work .....j

o/t There is something attractive about barometers. I have always fancied one of those that come in a wooden case and draw a pressure line on a paper roll on clockwork drum. Or alternatively a large column of mercury - can't remember average pressure in inches but it is a bit under three feet iirc so a lot of mercury. Both styles do not come cheap

 

[jwblair]

In theory it should be quite accurate. In practice I discovered that the displayed altitude drifted by several feet during flight See my post in another thread:
mavic pro negative altitude on landing

 

[Storyline]

In theory it should be quite accurate. In practice I discovered that the displayed altitude drifted by several feet during flight See my post in another thread:
mavic pro negative altitude on landing

.....
In my calibration tests I discovered that the displayed altitude during a flight could have an error of up to 8 feet and that the GPS altitude stored in the picture's metadata was totally useless. This has nothing to do with changing barometric pressure, which was constant during the tests. We observed displayed altitude errors in both the Mavic Pro and Phantom 4.

I've attached my calibration data below.
John

Aloha!
Interesting idea and a few thoughts strike me.

I wonder if DJI use the same type of barometer in both of those aircraft and it is not a particularly accurate model.

Secondly, although you say that atmospheric pressure remained constant during your tests are you sure that the barometer you were using to confirm that was accurate enough ? The comments above about how tiny changes of altitude can be measured suggest that it only needs a tiny change in pressure to make a difference in the report - a bit of a circular argument there ?

Thirdly, I assume you were measuring the Surface Level Pressure but maybe there were changes of pressure in the air column that were also atmospheric rather than just being caused by a change in altitude (see post about vertical wind shear above)

I cannot think of a way around your problem* but it would be useful to know just how accurate these type of digital barometers can be - eg my watch (medium quality Suunto) might be displaying spurious readings as could Mossiback's altimeter. *I suppose you could have a boat with your reference distance displayed on board that you would have to include in each shot but it would make your surveying a lot more time consuming.

I agree about GPS altitude data being near useless - did you see the post on this or another thread explaining why - in order to triangulate an accurate altitude your GPS set would need to receive signals from satellites that are obscured by the earth itself !

Good luck with your research - sounds like a bit of a dream job studying marine wildlife in Hawaii

Paul

 

[drekko]

I googled my question and found this post. It was pretty technical so Ill ask in simple terms and if possible give me a simple answer
Lets say I climb a mountain or building, take off my Mavic 2 Pro from there. It is already elevated 100 meters

Do I gain another 100 meters of max height distance?

 

[msinger]

Do I gain another 100 meters of max height distance?

When RTH is initiated, your Mavic will ascend from the current altitude displayed at the bottom of DJI GO up to the RTH Altitude. If it's already above the RTH Altitude, then it will return home at its current altitude.

 

[Mossiback]

...Lets say I climb a mountain or building, take off my Mavic 2 Pro from there. It is already elevated 100 meters

Do I gain another 100 meters of max height distance?

It does not matter what your elevation is when you launch. The Mavic starts at zero height. That may be why Go 4 shows height instead of elevation.

 

[Marko9219]

No its not. Its more accurate in horizontal. GPS uses triangulation. Your barometric altimeter and downward positioning sensors are what determine your distance from the ground.

Actually, no, it doesn't use triangulation. Google "trilateration".

 

[GraemeD]

I was trying to confirm how a Mavic drone determines altitude on the DJI GO4 app and came across this thread. This was a really useful discussion and answered all my questions.

As a former holder of a PPL (aircraft), I wanted to contribute by saying that all aircraft use barometric pressure to determine their altitude at any time, and yet it is very accurate. Part of standard flight procedure (at takeoff, throughout the flight and prior to landing) is to regularly calibrate the altimeter due to any pressure changes from weather and conditions. This is typically done by setting a dial on the altimeter to values of what is known as either QNH or QFE. IF you set the dial so that altitude is 0 whilst on the ground, you are setting the QFE, your alitmeter then always reads the height above this point. QNH is the reading you would need to set to get 0 on the instrument if you were at sea level. Use this and your altimeter will show height above sea level.

ATC, airport information broadcasts, or advisory services will always provide updates as required. And knowing QFE when landing is obviously very important (especially if using flying using an ILS approach)

Sounds like drones recalibrate to 0 (QFE) at point of launch, but don't make any changes after this.

Which leads to two important points. Regulations (at least here in oz) say you can't fly above 400ft from the ground, So if you launch from high ground, go straight up before moving, then fly over ground much lower down, you could quickly be in breach of the regulation.

Conversely, if you launch from low ground, but want to land somewhere much higher, don't rely on the indicated height and make sure your RTH altitude accounts for the variation, or you could be in trouble.

 

[gnirtS]

Correct. Mavic (all the DJI drones i think) use a QFE that is set at takeoff and not readjusted. Its also highly likely the sensor isnt temperature compensated and drifts somewhat during flight.
You do get the last 30ft or so displayed as a proper height above ground using the vision (or ultrasonic) systems on the drones though.

 

[Paulo Lobo]

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