I tried launcing my P4 this weekend from atop a parking Garage and struck out due magnetic interferance of the compass.  I assume this was from the metal rebar in the contrete.  I moved to a parking lot on the ground and all was good.  I noticed that if I held it up a few feet off the garage roof all turned green.

Are there any reccomdations or Do's & Don'ts around launching from the top of parking garages?  if you had a wooden platform that's elevated is that enough to be safe or are the other risks I should be aware of?

I have the same experience trying to launch from the top of a picnic table laterin the day only to realize it had metal legs under it.

That has been my experience as well. I try to avoid large concrete structures for the most part.

i use a little table if I am flying from a parking deck. I fly construction sites and you have to be very mindful of stacks of rebar and precast pieces etc. There is a lot of metal on construction sites!

seems like carrying a small pocket compass with you and observing it for deflections might be useful in these situations.  

Kneepuck Posted at 2016-3-29 07:39
seems like carrying a small pocket compass with you and observing it for deflections might be useful ...

You could always try using your moral compass, but in my case that would inevitably lead me astray.