Negative attitude?

[Aerohead]

The other day while doing a low, high speed run in sport mode over salt water I could swear the altitude had a negative number (and nothing to smoke that day). Question: Anyone know if the Mavics is below sea level will it show a minus altitude or is it based on height of the floor? Need to know if I was just space’in. (That particular day it was a -3.2 tide.)

 

[MacPap]

Altitude for the copter is measured with zero being the point of take off.

 

[pixl]

Altitude shown is always altitude AGL at the home point. If you started from shore, you would have been above water level, and if you dipped below the home point altitude as you got closer to the water you would be in the negatives.

A word of caution... flying close to water can mess with the ground sensors and cause erratic behavior. It's also very hard to judge distance above water when flying VLOS as the drone gets farther away from you. As tempting as it is, skimming the water's surface is filled with peril.

 

[Aerohead]

Altitude for the copter is measured with zero being the point of take off.

 

[Aerohead]

Thank you, that makes sense. Kinda freaked me out since the down sensors were turned off, I'm on my boat, flying 3' over saltwater, doing 39mph, then see a negative -8 alt. Flyin fun.

 

[Chip]

Altitude shown is always altitude AGL at the home point. If you started from shore, you would have been above water level, and if you dipped below the home point altitude as you got closer to the water you would be in the negatives.

A word of caution... flying close to water can mess with the ground sensors and cause erratic behavior. It's also very hard to judge distance above water when flying VLOS as the drone gets farther away from you. As tempting as it is, skimming the water's surface is filled with peril.

In this specific situation, going negative, is one risk triggering auto land? If so, can risk be totally eliminated by switching off altitude sensor and "safe landing protection?" I have practiced skimming over ground at 6 feet and never seen sudden loss of altitude with or without sensors but realize the water is a much bigger problem if something goes wrong.

 

[pixl]

In this specific situation, going negative, is one risk triggering auto land? If so, can risk be totally eliminated by switching off altitude sensor and "safe landing protection?" I have practiced skimming over ground at 6 feet and never seen sudden loss of altitude with or without sensors but realize the water is a much bigger problem if something goes wrong.

 

[pixl]

I'll preface this by saying that my experience is largely with the Inspire, but I believe some of it is also true for the Mavic.

The specs are that vertical stability is maintained to +/- 1 meter. Since we know that the sensors can trigger autoland OR push it higher than you want to fly, turning them off and skimming along at 3' high is inside the margin of error for the Mavic to actually touch the water. As it gets closer the propwash will kick up some spray which can also effect flight.

On glassy-still water with no wind, this is less likely to be a factor, but in those conditions it's impossible to "see" the water via FPV, and as the drone gets further away, height is impossible to judge flying VLOS.

My method for getting close is to first note the water level by hovering close by to within a foot of the surface (this will usually be a negative number), add 4' to that number and call that my absolute "floor". If there are wind and waves I'll adjust accordingly to a higher floor.

It may not be the most daring flight level, but in 300 hours on the inspire, I never lost it, and i did almost half my flying over water, much of it beyond VLOS. With the Inspire Pro, you're always conscious that a crash on water is a $4000 loss, hence the caution.

I'm a little more daring with the Mavic; it's one of the things I like about it. Less to lose, plus flying with even greater abandon, having refresh

 

[Aerohead]

In this specific situation, going negative, is one risk triggering auto land? If so, can risk be totally eliminated by switching off altitude sensor and "safe landing protection?" I have practiced skimming over ground at 6 feet and never seen sudden loss of altitude with or without sensors but realize the water is a much bigger problem if something goes wrong.

 

[Aerohead]

I get a lot of practice and know my limits (as he cartwheels it in). I fly over water the majority of the time for the space, noise factor, and boating. Believe me, I know I'm only allowed one mistake, but hey, it adds to the challenge.

 

[pixl]

I get a lot of practice and know my limits (as he cartwheels it in). I fly over water the majority of the time for the space, noise factor, and boating. Believe me, I know I'm only allowed one mistake, but hey, it adds to the challenge.

Yup. It's a thrill.

 

[dck22]

Oh you brave souls. I'm just starting my adventure with the MP and I believe it will be a long time before I venture over water deeper than a puddle. Although now that I think about it, I have insurance and DJI Refresh so if it drowns or is lost at sea I would be covered.

 

[MacIak]

As all the above: Negative altitude is normal in your case. I almost buried my Mav in the sea because of skimming the water. 1 week later it was buried there anyway, different cause. Just add a bit altitude, it's brain gets messed up a bit because of the water. Also your live-feed could suffer minor delay. Losing a mav feels pretty shite. Only when it's gone you miss that particular one.

 

[msinger]

In this specific situation, going negative, is one risk triggering auto land?

Forced Landing mode will only be initiated if the Landing Protection setting is enabled, the downward sensors detect the ground is near (or water in this case), and the throttle is in the full down position.

 

[gnirtS]

Mavic altitude is (i) relative to take off point and (ii) isn't overly accurate. Its quite normal for it to drift during a flight.

Its possible the electronics aren't temperature compensated onboard which would explain part of it.

Altitude is only used for RTH and a convenience display for the pilot. Nobody should be flying blindly relying on an altitude when low anyway - its a recipe for disaster. Even without low quality electronics lots of things can change the air pressure during a flight (and the barometric altimeter is only a pressure sensor).
Autoland is triggered by the VPS not the barometric altitude. They're separate, unlinked systems.