T o d d P a t t i s t wrote:
Mxsmanic wrote:

What regulations determine the absolute lowest altitude you can fly
above the ground in the U.S.?

It requires a minimum of:
"An altitude of 500 feet above the surface, except over open
water or sparsely populated areas. In those cases, the
aircraft may not be operated closer than 500 feet to any
person, vessel, vehicle, or structure."

"Congested areas" have higher minimums.

In Canada it's the same, with the proviso attached "except
when taking off or landing." It would be impossible to land or take off
at most airports if we had to stay 500' away from any person,
structure, vehicle or vessel.
Low flying kills people. There are unmarked wires, big birds,
unmarked or unlighted towers of all sorts. An engine failure at low
altitude means no options but pretty much straight ahead into whatever
is there. We've experienced several birdstrikes near the ground, and
just west of here is a 100' tower that must be under the minimum for
lighting, and it blends in really well with the ground. Every so often
someone snags a powerline they didn't see.

Dan