Ground Elevation data in incorrect

It is better to take your data quickly than it is to have it span multiple days. Doing it over this much time will have moving shadows and affect the accuracy of the reconstruction. You can turn off the slowdown in the Settings. 

The vision sensor warning is something you need to handle in the DJI app. It won't affect the data that is collected but can cause the flight to get stopped prematurely. 

The "Over Flight Maximum Radius" warning comes up with you have a value set in the DJI app. We recommend not having a value there or having it set to 10 km or something like that. 

There is some curvature to your model, most likely for the reasons noted above.

The GCP process does not change the shape of the model. It aligns things to the lat and long points and then attempts to minimize the error vertically. 

Taking out curvature is hard to do over such a large area. That is why we don't do it. You need a lot of super accurate GCPs spread fairly evenly throughout the area to do it properly. Most people are not prepared to provide GCP data of the quality needed to do this correction so we don't offer it.

In this case you would have been better off separating the area into northern and southern regions and then merging the results. There were be less curvature that way and the results would likely turn out more accurately. 

Yes, I have already re-calibrated the sensors (after the flight) just wanted to make sure it wasn't the cause. 

I tried turning the maximum range off in DJI go, but that only bought me one flight, I then set it to the maximum allowed (I think it was 8000M) and that bought me another flight, i then read to just have DJI-go open would solve it, and I tried that, it seamed to work. 

We usually try to fly on bright days, and can fly the site in 2-3 hrs, however due to FAA restrictions on flying over people we are limited to weekend flights and it has rained nearly every weekend this summer. It was overcast with only a couple brief periods of sunshine(thus no shadows). We have now purchased 2 more batteries and a faster charger to reduce the time spent waiting for batteries to charge which eventually caused us to postpone to the 2nd day.

The elevation data was disappointing, and would have been handy if it was reliable, however taking the time to do more GCP's is not justify-able, because the time spent doing that, we could just go shoot far more accurate GPS points on the points of interest.

Thank you for clarifying the cause, next time, we will attempt to fly all it in one day with better conditions and more batteries.