Aerial Survey How to Establish Site GCPs 

We used a survey grade RTK GPS with our own base station on a known coordinate, we just run it for 180 epochs( 3 minutes).

BAD!!!!

Phantom 4 pro makes GCP=1cm from 36,5m altitude above ground. Typical flight is on 70m AGL (above ground level). So GSD (ground sample distance) is 2cm. It gives 2cm horizonal and 4cm vertical accuracy. If you compare 2 flights it is root square of 2 multiplied by accuracy - so if point clouds from 2 different flights are no more than √2*2*GSD in this case 5,5cm from eachother it means the data is OK.