Negative altitude and inaccurate RTH

null · Aug 23, 2017

I finally got to fly my Mavic for the first time yesterday and I absolutely love the thing.

I just noticed a couple of odd things:

The RTH function always landed the drone at least a metre or two from the take-off point. I did what I needed to do to accurately record the take-off point (ascend to about 10 m and allow it to hover for a bit). I tried both the automatic take-off with the option to record the home point accurately enabled and taking off manually and ascending to about 10 m myself. I checked that I was definitely recording the GPS home point at the aircraft's location and not at my location. I'd already ordered a landing pad to assist with this, but it's still on its way. Instead, I used a sheet of plastic and marked a large, bold 'X' on it to give the VPS something to lock on to. The plastic was a sort of drab green colour, so I don't know if perhaps there just wasn't enough contrast against the grass, but I would have thought that the big 'X' would have been enough.

On the final flight, I noticed that the altitude was being reported as a negative value. I know that this will happen if you have taken off and are flying lower than your take-off altitude, but I definitely wasn't doing that. I was in a very flat park and probably at an actual height of about 2 m above the take off point/ground, but the app was reporting an altitude of about -1 m or so. Is this normal and/or is there something wrong with the drone?

Also, I got home and realised that almost all of the videos (I just left it recording most of the time I was flying) were blurry. I didn't use the C1 button to focus at all as it was in the AFC mode. Should I have been using C1 to focus, regardless? I've attached a couple of screen grabs from the video. The first is the view prior to take-off, and the grass is perfectly in focus. The second is from during the flight and the image is clearly not well focussed.

QuadKid · Aug 23, 2017

Yes you should always center your shot and hit focus either by a preset function such as C1, C2, or the 5 position toggle button.
As far as setting the home point, once the AC is connected and running you should be able to swipe the app screen to the left and it should give you the option to set home point to the AC or set home point to the controller, setting it to the controller (Me) should allow you to walk around to a different location ( many people use this feature to fly from boats ect...) and the AC will return to wherever the controller is located.

TrayBoz · Aug 23, 2017

We will not tolerate your negative attitude around here.

1) The barometer/altimeter built into the mavic is not always that accurate, so I wouldn't be to concerned by the altitude being shown is off by +/- 10 feet or so.

2) Even with AutoFocus turned on, you have to manually Tap-To-Focus on the screen where/when you want to focus. You will have to do this continuously as you fly, as your distance to your subject(s) is constantly changing.

If you have Tap-To-Focus assigned to C1 (like many of us PILOTS do) then it will auto focus to the center of the frame each time you hit C1. I've just gotten in the habit of hitting my C1 button every few minutes while I'm flying, or any time I know I'm about to shoot a potentially cool scene. (I always film every flight in it's entirety, so I'm recording from launch to landing).

3) If you have the Precision Landing setting turned on, and use it properly, your RTH function should land you within a few inches of where you launched from. Without Precision Landing, expect it to be within several feet, in my experience.

Happy Flying/Filming!

null · Aug 23, 2017

Thanks for the tips, guys. I'll be sure to focus from now on. Does anyone now why you need to do that, though? Isn't AFC supposed to be autofocus continuous, the idea being that it just keeps focussing itself on whatever it's pointing at? Is it just buggy in the Mavic?

As for the RTH, I'm quite sure that I followed the proper procedure. Precision landing was turned on and I allowed it to ascend high enough to record the home point with the vision system. I tried the automatic take-off that does this and automatically ascends to about 6 m if you choose that option at take-off and I also did it manually, ascending to about 10 m, as the manual suggests. In both cases, I allowed it to hover in place at that height for some time before moving away. Each time I tried the RTH, it was a good metre or two (say, 4 to 6 feet) from the take-off point. I've seen the videos where people have demonstrated this, and it's always much, much closer. The results I was getting would be at about the limits of the accuracy I'd expect with just GPS, assuming good GPS signal (I had 15 ±2 satellites every time I checked).

Oconnb910 · Aug 23, 2017

null said:
Thanks for the tips, guys. I'll be sure to focus from now on. Does anyone now why you need to do that, though? Isn't AFC supposed to be autofocus continuous, the idea being that it just keeps focussing itself on whatever it's pointing at? Is it just buggy in the Mavic?

As for the RTH, I'm quite sure that I followed the proper procedure. Precision landing was turned on and I allowed it to ascend high enough to record the home point with the vision system. I tried the automatic take-off that does this and automatically ascends to about 6 m if you choose that option at take-off and I also did it manually, ascending to about 10 m, as the manual suggests. In both cases, I allowed it to hover in place at that height for some time before moving away. Each time I tried the RTH, it was a good metre or two (say, 4 to 6 feet) from the take-off point. I've seen the videos where people have demonstrated this, and it's always much, much closer. The results I was getting would be at about the limits of the accuracy I'd expect with just GPS, assuming good GPS signal (I had 15 ±2 satellites every time I checked).

Are you powering on the mavic at its take-off position or powering it on and moving it to its position?

null · Aug 23, 2017

Oconnb910 said:
Are you powering on the mavic at its take-off position or powering it on and moving it to its position?

Mostly powering it on right at the take-off position. I also went in to the menu and set the home point to the aircraft location.

gnirtS · Aug 23, 2017

"auto take off" seems to climb to 6m for precision whereas the manual states it should be 10m. Im guessing thats more a typo than a glitch though.
You need to go straight up, the landing area needs to have decent contrast or patterning on it for the cameras to help. No flying forward or sideways until up.
I generally get upset if it misses the landing point by more than 3-4 inches its that good. The one time i have had it miss by a foot or so was when i zoom climbed up to 30m rather than a slow ascent. Not sure if related or coincidence but maybe that. Only other incident i can recall it got hit by a wind gust whilst about 4ft high so went backwards a foot or so. Sure enough when it landed it tried to land over that drifted position not the take off one.

My general procedures is power drone on where it'll take off, then connect RC, load app, wipe my SD card and cache, check my settings. This should have given it long enough for a good GPS lock, if not wait a bit more. Then CSC start motors, check everything is good, check satellites are a good number. Then i manually set the home point and verify it on the map along with RC distance indicator etc. Then its a slow climb to 10m, stop there, check everything is ok then fly off. Doing that ive found precision landing works all the time. (My landing "pad" is a towel thrown on the floor or sometimes my house door mat, nothing special).