I had the impression that the helicopter took off from the
construction
site across the street, about 500 feet away, though I did not see it
actually take off. The construction is building a road through a
vacant
field. In fact, most of the land in that direction for several miles
is
farmland or vacant fields.

These are the FAR's regarding minimum safe altitudes as found on AOPA
Online:
http://www.aopa.org/pilot/features/regrvw9701.html#119

See especially part (d) of 91.119:

"Except when necessary for takeoff or landing, no person may operate an
aircraft below the following altitudes:

(a) Anywhere. An altitude allowing, if a power unit fails, an emergency
landing without undue hazard to persons or property on the surface.

(b) Over congested areas. Over any congested area of a city, town, or
settlement, or over any open air assembly of persons, an altitude of
1,000 feet above the highest obstacle within a horizontal radius of
2,000 feet of the aircraft.

(c) Over other than congested areas. 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.

(d) Helicopters. Helicopters may be operated at less than the minimums
prescribed in paragraph (b) or (c) of this section if the operation is
conducted without hazard to persons or property on the surface. In
addition, each person operating a helicopter shall comply with any
routes or altitudes specifically prescribed for helicopters by the
Administrator. "

If the helicopter was taking off then he's legal. Also since you
described the area as mostly vacant fields, I believe that would
qualify as a sparsely populated area. Also since it's a helicopter they
are allowed to operate less than the minimums unless it's hazardous to
people and property on the ground (other than percieved altitude, was
the helicopter doing anything seemingly dangerous?).

IMHO it's nothing to be worried about. On a side note concerning
Walmart, you can't be without your camera for more than an hour or two
while you shop?

Patrick Pohler
PP-SEL (KOSU)