One of the reasons I quit the hobby in about '87 was that every time I went detecting on public property I had to talk nicely to a cop, politely turn off my detector and immediately leave. That bummed me into submission and abandonment back then. Hopefully I've come back more educated, and definitely devoted to "permissions".
I'm a little confused. If you were getting "scrammed" and "ushered away" by cops when md'ing public property, then ... why ? Under what law or rule were they doing this ? Was there a law that said "no md'ing" ? If so, then who can give you "permission" then to detect at places where there's a specific prohibition ? For example: Shoplifting is not allowed. Right ? Because there's a law against it. Right ? Ok, how successful do you think I'd be if I went in to the police dept. and asked "hello, can I please have permission to shoplift" ?
So I'm trying to figure out why you were getting booted. Perhaps it's because someone thought you were about to leave holes. Ie.: the dreaded "alter" and "deface" verbiage. Ok, so if you think permission will head-this-off-at-the-pass (and your name-dropping and paper in your back-pocket will deflect all busy-bodies), then how do you phrase it when you show up asking at city halls ? Eg.: "Hi, can I dig holes in the park please?". Or are you more ... uh ... discreet and say "Hi can I metal detect?" (since mention of "holes" and "dig" are sure-fire permission killers. Eh ?)
Ok, so assuming you've minced words and only said "metal detecting" rather than "dig" and "holes", ok: What do you do if the following happens: You get accosted . You proudly whip out your permission. The person who's griping (cop or gardener or whomever) simply gets on his cell-phone, calls down to city hall and says: " But he's tearing the place up" (which isn't true, of course). Then guess what happens to your permission ?
I don't disagree that it's a more comforting feeling to be carrying around "permission" (as if it were needed) for public parks. And ... sure .... it might deflect some busy-bodies. Granted. But I'd just be worried that if I'm showing up at city halls asking "can I metal detect", that I could get a "no", before I even start. There are SCORES of examples of this, if you wish to see the links. People who went in asking, and fetched "no's", where no rule or law *specifically* said such a thing. At places where, quite frankly, md'ing was never a problem before.
Do you see how you could be shooting yourself in your own foot ? And worse yet: Guess what that pencil-pusher desk-jockey will think, the next time he passes a park and sees an md'r ? (whom he might never have given a 2nd glance to before). He'll remember the earlier inquiry, and start booting others !! I've seen this happen before. It becomes a self-fulfilling vicious loop.