How does the Mavic measure it’s height?

[tcope]

The Phantoms altitude calculation is apparently based on GPS. Any idea why it's implemented like this?

It is not GPS based. It is based on a reading from a barometer as well.

 

[MaxHam]

It is not GPS based. It is based on a reading from a barometer as well.

Did you read under the link I provided?
That guy is long and well known as provider of solid information. Almost like Msinger ;-)

This might also point in the direction of Phantoms using GPS, not sure though, I'm no tech specialist:
dshean/sfm_tools

 

[msinger]

The Phantoms altitude calculation is apparently based on GPS

It's not based on GPS. It uses the internal barometer (like all other DJI drones).

 

[tcope]

Did you read under the link I provided?
That guy is long and well known as provider of solid information. Almost like Msinger ;-)

It appears you misunderstand his post. Phantoms use a barometer, not GPS for altitude.

 

[MaxHam]

It appears you misunderstand his post. Phantoms use a barometer, not GPS for altitude.

I think it's pretty clear what he says. Maybe he is just wrong?
"Your Phantom uses its barometer for all flight functions but since back in the P3 days, DJI uses GPS for altitude in Exif data."

 

[msinger]

Are you asking about the drone's altitude or Exif data? Those are two entirely different things.

 

[Meta4]

I think it's pretty clear what he says. Maybe he is just wrong?
"Your Phantom uses its barometer for all flight functions but since back in the P3 days, DJI uses GPS for altitude in Exif data."

What he said was pretty clear. And you are the one that is wrong.
Your Phantom uses its barometer to provide altitude data for all flight functions.
But the altitude data displayed in the exif info of all still images comes from GPS.

 

[Storyline]

Really interesting thread

Little by little I am understanding more and more about my Mavic. I realised the altitude on the controller was relative to the take off point and must be measured using a barometer as when you descend to land you get a near zero figure again and I know GPS altitude is not very accurate. I have also got a watch which has an altimeter (barometer) and this is so accurate (relatively) you can see it change just walking upstairs ! I had never known why GPS was so poor and now the explanation of the earth being in the way of the satellites that would be needed to make a 3D triangulation makes perfect sense !

As regards the accuracy of GPS receivers and the post above about the very accurate marine GPS this may be correct but I have always thought that the apparent amazing accuracy we get on our marine GPS was due to the averaging of many readings combined with slow speed of movement. This is very much a gut feeling and I was reading the other day of a micro GPS receiver mounted on a tiny breakout pcb. The description said it was the most accurate civil GPS receiver in the world and it just cost 25 dollars ! No idea if this is correct but the info came from a specialist electronic component site for model makers and not a dodgy auction site description !

 

[MaxHam]

Are you asking about the drone's altitude or Exif data? Those are two entirely different things.

Actually both ;-)
I was assuming that the altitude shown in exif is the same which is shown on my display.
Need to check this next time in the air when I take a photo.

I understand that the drones can only keep their altitude by the use of a barometer.
From that DJI thread I concluded if exif shows GPS readings that altitude was measured by GPS.
Guess I'm wrong. But why would not the correct (Barometer) altitude be written into the exif?

 

[MaxHam]

What he said was pretty clear. And you are the one that is wrong.
Your Phantom uses its barometer to provide altitude data for all flight functions.
But the altitude data displayed in the exif info of all still images comes from GPS.

I get it now. When he said "DJI uses GPS for altitude in Exif data" I concluded that GPS is used to measure altitude.
So I hope what I see on my display is Barometer altitude ;-)

 

[Meta4]

I was assuming that the altitude shown in exif is the same which is shown on my display.
Need to check this next time in the air when I take a photo.

Here's an example for you:

GPS altitude can be out by +/- 400 feet and it swings over short time periods.

But why would not the correct (Barometer) altitude be written into the exif?

It's still there in a hidden corner of the exif info as shown above.
Until about three years ago DJI showed the barometer altitude in exif info but they never gave an explanation for the change
It might be a standard?

 

[Lake_Flyer]

One question:
The Phantoms altitude calculation is apparently based on GPS. Any idea why it's implemeneted like this?
If Mavic uses Barometer that's good to hear.
EXIF issues / Altitude confusion?

Every type of Phantom since the P1 (every DJI drone actually) use a barometer for maintaining altitude while flying. It has nothing to do with the EXIF GPS data stored with your footage. And it shows how inaccurate GPS data can be.

 

[Storyline]

Here's an example for you:

GPS altitude can be out by +/- 400 feet and it swings over short time periods.

It's still there in a hidden corner of the exif info as shown above.
Until about three years ago DJI showed the barometer altitude in exif info but they never gave an explanation for the change
It might be a standard?

I have never really checked the EXIF of my Mavic photos but just had a look at one and found that it looks like Litchi does write the barometric data .....

</dc:description>
<drone-dji:AbsoluteAltitude>+18.02</drone-dji:AbsoluteAltitude>
<drone-dji:RelativeAltitude>+17.70</drone-dji:RelativeAltitude>
<drone-dji:GimbalRollDegree>+0.00</drone-dji:GimbalRollDegree>
<drone-dji:GimbalYawDegree>+133.20</drone-dji:GimbalYawDegree>
<drone-dji:GimbalPitchDegree>-38.50</drone-dji:GimbalPitchDegree>
<drone-dji:FlightRollDegree>+2.80</drone-dji:FlightRollDegree>
<drone-dji:FlightYawDegree>+133.30</drone-dji:FlightYawDegree>
<drone-dji:FlightPitchDegree>-0.30</drone-dji:FlightPitchDegree>
<drone-dji:CalibratedFocalLength>3051.613037</drone-dji:CalibratedFocalLength>
<drone-dji:CalibratedOpticalCenterX>2000.000000</drone-dji:CalibratedOpticalCenterX>
<drone-dji:CalibratedOpticalCenterY>1125.000000</drone-dji:CalibratedOpticalCenterY>
<crs:Version>7.0</crs:Version>

I should have said that this is different to the GPS altitude included further down the list

 

[Meta4]

I have never really checked the EXIF of my Mavic photos but just had a look at one and found that it looks like Litchi does write the barometric data .....

It doesn't matter whether you fly with Litchi or Go, the exif info will be the same, just like RTH and all the other things that are set in your firmware.
What you've shown above comes from the XMS data.

 

[Storyline]

It doesn't matter whether you fly with Litchi or Go, the exif info will be the same, just like RTH and all the other things that are set in your firmware.
What you've shown above comes from the XMS data.

Got it - my fault as I did not read your previous post properly - I did not see the Absolute & Relative altitudes at the top of your photo but just read the sentence that said that up until 3 years ago DJI included barometer data in EXIF. Then when I saw I had the Abs & Rel altitudes (which I assume is barometric info) in my Photoshop metadata I thought it would also be in the EXIF.

I had forgotten how it is a bit more difficult to just show pure EXIF as most programs/websites/utilities just mix up everything and call it metadata. I ended up downloading a standalone windows executable just to be sure I got only EXIF. That brought back memories of pre GUI days ! Anyway, it is a remarkably small amount of info and I was disappointed that it does not include a shutter actuation count or info about video hours etc so you can't tell how much the camera has been used. Maybe the info is available somewhere else.

 

[Vegasrobbi]

A little side note. The on board baro is used for altitude calculations and VPS is used when close to the ground to terrain follow. VPS is not effective at higher speeds or in high winds when flying at a high angle. High winds can affect the baro reading causing altitude fluctuations.

Many drones can use an off board baro for altitude control like during follow me. This way the device baro controls the altitude of the drone in relation to the subject being followed rather than the on board baro.

Litchi use to offer this option in Follow me I don't know if they do for the M2.

 

[Storyline]

A little side note. The on board baro is used for altitude calculations and VPS is used when close to the ground to terrain follow. VPS is not effective at higher speeds or in high winds when flying at a high angle. High winds can affect the baro reading causing altitude fluctuations.

Many drones can use an off board baro for altitude control like during follow me. This way the device baro controls the altitude of the drone in relation to the subject being followed rather than the on board baro.

Litchi use to offer this option in Follow me I don't know if they do for the M2.

I am not disputing what you say but I wonder how high winds can effect the barometer reading ?

I have never used Follow Me in Litchi but I can see how using the difference between the on board barometer and a barometer on the subject must make it much more accurate. However both barometers must have the same calibration unless they are both zeroed at the start of the process when you know the target is on the screen.

I have been so wrapped up learning how to get the best out of the Mavic's camera that I have not really tried any of the intelligent flight modes but I must do because I find all this stuff about the sensors fascinating.

 

[Meta4]

I am not disputing what you say but I wonder how high winds can effect the barometer reading ?

Not enough that you'd notice.
You're not going to see a lot of significant changes in air pressure over the short time of a drone flight.

 

[Vegasrobbi]

Not enough that you'd notice.
You're not going to see a lot of significant changes in air pressure over the short time of a drone flight.

You'd be surprised. Here's a P4 drifting 20 feet vertically in strong winds. The object of dynamic altitude is to maintain altitude in relation to the subject. Advanced follow drones like Halo require an altitude calibration between the device and drone each flight. Halo can follow up a steep trail at 30+ mph without VPS.

 

[Storyline]

The things you end up researching when you own a drone !

The changes in air pressure over small vertical distances you are both discussing could be caused by wind shear. Normally wind shear is horizontal (the sort that produces those dramatic shots of aircraft landing crabwise) but more rarely, there is also vertical wind shear. I am guessing but maybe this is a kind of laminar air column with layers of air at different pressures. If so, this could cause the discrepancies that affect the flight. Even small changes in height produce quite marked air pressure differences. I have remarked on another thread how I have a watch with an altimeter and that shows pressure changes just walking upstairs in the house !