I fly mostly in the city of Philadelphia and was just curious if others who fly in big/bigger cities have tips or notes about flying in the cities as opposed to suburban rural areas. For me I found after trial and error (painful) that I have to calibrate my compass every time I take off in the city (even if only a few blocks apart) because of so much interference and even when everything looks fine then take off and hover very low for a little bit to make sure everything is in fact working properly. There were instances when my P3A would say everything is great for takeoff, compass calibrated and as soon as I take off starts flying all sorts of whack. Basically compass numbers should be around 1500 +/- 100 for my P3P to work well in the city, I've had it acting weird at around 1250 and 1800s. Anything you guys have to share?