Altitude accuracy at low level?

[KiterTodd]

I was flying the Mavic this weekend (0.400) and doing some relatively low passes up and down a beach. Over water a bit and over sand a bit. I was not scrapping the water surface like some of you do, but still framed a nice shot at 11' up and ran it at full speed (non-Sport).

After a couple passes I thought it didn't seem to me that it was 11' high. So I stood up, let it hover near me, and found it to be flying at about 5.5'

I was flying on a flat beach and if anything, the Mavic was flying down the slope from where I was sitting a bit so it was probably more like 2' below take-off altitude (so should have said 3.5').

All other flight characteristics were normal. Flew straight and level. Hovered fine. And before the flight I re-calibrated the compass.

Is this low height accuracy normal? I don't think I've noticed this before so I honestly can't say if this is a new thing or not, but I also don't want to fly it in the drink some time when I think I'm getting a shot 4' off the water!

Thanks for any tips.

 

[Tinmania]

Did you check the log first to verify there was absolutely no down movement on the left stick? Also, what did the altitude say in go4?

I have seen my MP ascend a little every now and then, but not descend, and not as much as you indicated. There was one time I "swore" it flew backwards on its own, but that was on my 3rd or 4th flight with the MP and in checking the log, yep, it was me who commanded it backwards (when I wanted to just lower the gimble).



Mike

 

[KiterTodd]

Did you check the log first to verify there was absolutely no down movement on the left stick? Also, what did the altitude say in go4?
...

That's a good idea, I hadn't checked the log post flight. I just assumed that the remote display and the Go4 display would mirror each other.

The log shows 6.6' when the display was showing 11'.
I suspect that is only a foot or two higher than actual. If it's a foot, I'm not concerned.

Will check this again the next time I fly.

There was no down movement on the stick. I had hands off controls at this time.

 

[laurens23]

the Mavic has 2 ways to determine altitude.

a barometric altimeter and a sonar.
I think it is the barometric altitude that is displayed and used for holding altitude. The barometric altimeter is set to 0 on takeoff and measures the altitude from takeoff location. Going uphill on the beach does not change the (displayed) altitude. The barometric altitude can easily change due to changes in weather conditions. I have taken off and landed at the same location 8 minutes later with the barometric height at -27ft. That was an extreme case, but 1-2ft changes over a few minutes are not unexpected.

The sonar can very accurately detect and keep a specific height above the terrain. This is used in the terrain follow mode. The sonar is also used for/during (detecting) landing.

 

[KiterTodd]

the Mavic has 2 ways to determine altitude.

a barometric altimeter and a sonar.
I think it is the barometric altitude that is displayed and used for holding altitude. The barometric altimeter is set to 0 on takeoff and measures the altitude from takeoff location. Going uphill on the beach does not change the (displayed) altitude. The barometric altitude can easily change due to changes in weather conditions. I have taken off and landed at the same location 8 minutes later with the barometric height at -27ft. That was an extreme case, but 1-2ft changes over a few minutes are not unexpected.

The sonar can very accurately detect and keep a specific height above the terrain. This is used in the terrain follow mode. The sonar is also used for/during (detecting) landing.

Thanks. Good info.
What is the Go4 App showing, GPS altitude?

I'm not sure your comment regarding going uphill not changing the altitude makes sense. I have certainly flown up and down hills and the altitude is always immediately relative to the take-off position. ?

 

[tcope]

As mentioned, altitude is above the take off point, not where the Mavic is currently located. Take off to 10' and fly over a cliff. You may now be at 100' but still only 10' above the take off point.

 

[KiterTodd]

As mentioned, altitude is above the take off point, not where the Mavic is currently located. Take off to 10' and fly over a cliff. You may now be at 100' but still only 10' above the take off point.

Ahh, okay, we're say the same thing, then.

 

[laurens23]

Thanks. Good info.
What is the Go4 App showing, GPS altitude?

the apps shows the barometric altitude.
As far I know GPS altitude is not used by the drone. GPS altitude is very inaccurate (it is more of a coincidental gimmick and not what GPS was designed for) There is however a max takeoff altitude shown in the Flight record overview of the DJI Go 4 app. I don't know if they get this from GPS or for an online topographic map.

 

[genesimmons]

the mav must use gps altitude for something, in my photo app it shows the gps altitude the image was taken at,i have to subtract my home town elevation to get proper altitude.