How low can you legally go?
In very sparse areas, you can fly as low as you want but you must be high enough to make a safe emergency landing. 91.119(a)
If there is any kind of human presence, including people or property, then you must be at least 500 feet away from them or 500 feet above. This is pretty low. Use the ground elevation and your altimeter to determine your height above the ground. 91.119(c)
In most places there are people and structures spread out, so 500 feet above should be considered the limit. The 500 feet away rule really only applies over open water and truly empty areas of land.
Finally, over a “congested” area you must fly 1000 feet above everything within 2000 feet horizontally of you. I like to imagine a wide cylinder around the plane with a 2000 foot radius and hanging 1000 feet below the plane. You cannot let anything get into this cylinder. 91.119(b)
The FAA has intentionally been completely vague about the meaning of the word “congested”. In fact, I’ve heard that a few people on the airport ramp might make up a congested area in an enforcement action.
The reality here is that if there is some incident involving others the FAA will consider those others part of a congested area if they possibly can. So if you are flying over the desert and you see a group of people hiking together you should probably just consider them part of a congested area to be on the safe side.
How low can you safely go?
In most cases, flying at the legal minimum height is not the safest option. With less altitude, there is a lot less time to deal with emergencies like the engine failing. Five people died in a helicopter crash recently, and if the tour operator had flown the aircraft higher they probably would have survived.
In most aircraft I don’t recommend spending time at the minimum altitude unless you happen to be flying right over a salt flat or other runway-like surface.
At 500 feet above the ground, even farmland will not give you too many landing options if the engine dies. Most of the world’s land has variation including plant life, ditches, creeks, hills and more that will get in your way so leave room to maneuver and glide.
§ 91.119 Minimum safe altitudes: General.
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.