I'll be flying at a small concrete materials site next week in a congested urban area. Every inch of the site is concrete, which I am sure is rebar reinforced to handle their heavy equipment traffic (dump trucks, cement mixers, etc.). I'm considering bringing a short 2 ft wooden step ladder to launch the drone from, but I am unsure as to how far above the rebar the drone needs to be in order for the compass to not be affected by it. Any opinions or experience with this?