My company is having a party on the roof of the adjacent 6 stony parking garage. The precast concrete garage is in a mid-sized city with two towers under construction to the west, a river to the south, and the remaining blocks are low-rise approximately as tall as the garage. I flew it successfully once but now I question the possibility of flying over the garage again. Other than compass interference or maybe the Vision Positioning System going inactive, are there other concerns I should consider. I do not plan on flying over the party goers, I just want to get a good company photo as it will be a Halloween party. The GPS signals seem strong at the location has good sky visibility. But there will probably be enough metal I will have to be prepared to fly P-ATTI. As for prelfight; if you are of the opinion it is Safe to Fly, would you avoid compass calibration on location and fly only if the sensors are in range? My thoughts are I can calibrate the compass and IMU at a nearly safe location such as an empty field a couple of blocks away. Thank you in advance for your help.