So on one of my first flights (beginner mode) I had some really weird behavior of the GPS.  I took off in the middle of our office courtyard which has a big round bulls-eye of brick surrounded by grass in the middle of it.  The courtyard is about 27 M wide between two 5 story buildings and over 100m long.  The take off point was exactly here.  https://goo.gl/maps/cwrtJkwuCyM2

Before takeoff I got a GPS lock on 14 satellites, 4-5 bars, but I couldn't see the screen too well because of bright sunlight, so I missed that the home point wasn't on the bulls-eye but actually on the corner of the building roof to the right.   I also don't think I got up to 10M altitude to get a precision landing fix before moving around. (I know, stupid)  Anyway, I was standing to the left of the bulls-eye.

The mavic was flying fine, I was flying it up and down that green area in the center courtyard.  I gave the RTH button a try and the mavic flew up to 30 meters like I expected to and then started heading for the building on the right.  It seemed to be trying to go up higher than 30 meters but got stopped by the altitude limit, I canceled RTH and flew it back down to a landing on the bulls-eye.

So I'm looking at the flight recorder trying to see what went wrong and I'm seeing some really weird stuff.  The blue dot (me?) is drifting all over the place, as much as 30m from where I am actually standing.

Most of the mavic flight recording shows it off by about 11m to the right, flying right next to the building on the right.   When I canceled the RTH the track was at the edge of the building roof.  But the weird thing is that the track ended smack on the bulls eye when I landed it manually.  I'm just guessing but it looks like the fix was off 15m or so on takeoff and through most of the flight, but when I did RTH and the Mavic flew up to 30 meters it got a better fix so that when I landed the map was right on target.

This is making me ask a few questions.

1. Is there a way to get a better indication of GPS quality in the app, like how stable the fix is when you are on the ground or an estimate of error?
2. Is the Mavic doing anything like GPS averaging when on the ground to try and get a more accurate home fix?
3. Could we get a warning if the GPS fix does an unusual jump while in the air?  Mine probably changed by a good 15m and the Mavic wouldn't have seen acceleration on the IMU to support that big a change.
4. Is there any way to tell when the Mavic has gotten a good "precision landing" fix with the vision system?  Maybe after it acquires an image of where it wants to land, it could show it?
5. Is there any way to take a look through the vision system cameras so you can take a quick look at what's below without moving the main camera?

If you upload the log from the flight using the instructions found here and post a link to it we may be able to help figure out what happened.

I will get the log up soon.  I think I did figure out at least part of the problem though.  I went out into the courtyard today with just my phone and found that the area between the two buildings has really bad GPS problems.  The phone reported a good lock with the estimated error being very small.   However even when I was standing still the fix kept wandering around by as much as 20m.  When I walked out of that courtyard area towards the front of the third building, the fix stabilized and stayed put, bang on target on the map.

So I'm thinking that even though the phone (or Mavic) can see 14 satellites with good signal strength and says it has a low estimated error, something about all the multipath reflections between the buildings is making the fix unstable.

It seemed like the Mavic itself was dealing with the unstable fix ok (not jiggling around or anything), the problem was that the initial home point was way off and I didn't spot it.

So I guess the moral here is to pay close attention to the home point on the map, and don't think everything is ok just because you have a good GPS lock.