I get good results with my Phantom 4 Pro drone when mapping my heavily treed 7ha site by flying missions on overcast days, at 91.4 m' altitude with 80% front & side overlaps at 4 m per sec to minimize blurring in the direction of travel. This results in a 1/60 sec shutter speed or 6.7 cm camera travel while the shutter is open. The mission takes 18 minutes on one battery and generates about 200 photos for uploading. I review all the photos before uploading and remove any with noticeable blur. This typically removes all photos at the 9 turns in the mission which are blurred due to the rapid rotation.
You can see some of my results in my post today (10 Jan 2018) titled: "Automatic Tree Detection and Characterization with DroneDeploy".
Your site is 14X larger than mine. So it would require 14 battery swaps if you used my settings. A challenge.
What are the key setting for your mission? Altitude, sun or overcast conditions, speed, overlaps?