Hello everyone reading this. I am in the planning phase for building a quadcopter based on Arduino. I need a way to measure distance from the ground. My first thought was to use an altimeter, but of course that measures distance from sea level, not from the ground. I need a way to measure the distance from the ground though, as recreational quadcopters are not allowed to fly higher than 400 feet. I thought about using the starting altitude and subtracting it from the current altitude, but that doesn't take into account things like rough terrain. So my question is, what would be the best way to measure such a long distance relatively accurately (I'm wanting something around +-2 feet, but more precise is welcome).
The reason I want it to be as accurate as I stated is cuz I want to implement an auto landing feature for when the drone's battery is low (to prevent sudden power loss at altitudes 50+ above the ground and obvious breakage of parts).
Delta_G:
The limit on quads is relative to sea level, just like altitude in planes. It would be ridiculous to base it on the ground beneath since that is subject to change with the terrain.
If you need something to find the ground when you get closer to it then use an ultrasonic sensor.
So, in other words, you're saying I wouldn't be able to even fly it where I'm at, as my altitude is roughly 1000 feet at my residence? Cuz I figured that it had to be distance from the ground, as I've seen people fly quads around here
Delta_G:
No, in other words I don't think I had that right. That's why I pulled it back. But I don't think you have to follow every hill and valley either. I just know that airplanes altimeters work based on sea level. I don't know how they are supposed to know ground level. Maybe they get it from a chart or something.
Either way, I haven't seen the FAA rigorously enforcing that to great precision. I don't think they're out there looking for a guy at 401 feet or anything.
If you want distance at that range you're pretty much stuck with laser range finder. That might be a bit heavy though.
The reason I was wanting to measure at that distance was to program the drone to not fly above a certain height, mainly during the testing of it (especially during the programming of the gyroscope/accelerometer as I've never done any programming with either sensor). But once it's fully functional, I wanted to keep it below 200 feet, and display a warning on a controller (I'm gonna build my own controller as well for this) once it exceeds that elevation. But if there isn't a way to do that without heavy components, I can just use a altimeter like I originally thought, and that would give me rough estimates and use the ultrasonic sensor for the auto landing. Thank you for your input though 
Delta_G:
No, in other words I don't think I had that right. That's why I pulled it back. But I don't think you have to follow every hill and valley either. I just know that airplanes altimeters work based on sea level. I don't know how they are supposed to know ground level. Maybe they get it from a chart or something.
Either way, I haven't seen the FAA rigorously enforcing that to great precision. I don't think they're out there looking for a guy at 401 feet or anything.
If you want distance at that range you're pretty much stuck with laser range finder. That might be a bit heavy though.
Actually the usual altimeter just works on the local air pressure. When the pilot is ready to take off, the/she just sets the altimeter to the altitude of the airport they are at. Then again when ready to land, they must get the air pressure on the ground at that airport and again adjust the altimeter. It is imperative to know the correct altitude of where they are landing, so they are not landing above the runway or below it!
Paul
killer13666:
I need a way to measure the distance from the ground.
Not sure. Maybe planes use radar (radio altimeter technique), but planes can carry heavy things and has adequate power reserves. Or ground radar is able to track the plane and send details to the plane.
Hi,
Use an altimeter, just set it readout to zero on the ground and make sure it doesn't go over 400feet in the air.
Whats the problem, you are not going to be in the air long enough for a substantial change in local air pressure to cause a change in local zero feet.
As has been said before, 401, 402, 405 feet not critical, the regs are to stop you guys becoming UBER pilots.
If you are going to fly over undulating land, then the 400feet limit will not undulate, it would probably consider zero feet to be average or highest undulation.
Tom... 
Altimiters use atmospheric air pressire, and there is an adjustment that you use to zero for the airport you take off from.
since the pressure changes with weather, radio broadcasts adivse pilots of local air pressire so they can make adjustments.
that will get you a general idea of your altitude. no one really cares if you are at 400 feet, 402 or 398 feet
Planes keep a vertical separation of 1,000 feet or more so if your altimeter is off by 450 feet and the other guy is off by 450 feet, you have some wiggle room. (above 40,000 feet they do 2,000 feet because altimeters are less accurate)
If you have GPS, your GPS can give you height based on telemetry of the satellites.
many of the high end quad copters zero out the starting point in 3 dimensions with GPS.
ground warning radar is to get an accurate distance to ground when weather is bad, rain or fog.
and it looks down. you would need a second one to look forward.
vertical things, buildings, the White Cliffs of Dover, power transmission poles, water towers, would not be sensed by ground warning radar. sloped things, hills and mountains will be detected.
The answer lies in your needs. take off and land automatically in the 3 dimensional spot is best with GPS,
distance to ground over short distances and varying locations would be best by ground detection radar.