Does someone know and can share what the difference is between the Altitude settings in Mission Planner for Relative, Absolute and Terrain. I know that the Terrain is for those planes equipped for terrain following, but what about the other 2 settings?  I have had some problems with my X-8 wing holding requested altitude and suspect this might be the problem. HELP and thanks!

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I don't know what confusion you are referring to. The thread is disscussing all three altitude types, including Terrain Following vs. Verify Height.

All of this is backwards and sideways from the rest of the aviation world in many ways.  The rest of the aviation world:

Absolute Altitude = AGL.  You altitude above the terrain below the aircraft. Always referred to as "AGL".
True Altitude = MSL.  Your altitude above mean sea level.  Always referred to as MSL.
Pressure altitude, density altitude, and indicated altitude are not really relevant for our purposes here and now, but are all circumstance variations of MSL.

There is no such thing as "relative altitude" in aviation. This presents an issue for us, since that's what we're using.  The altitude relative to the home altitude.  I would prefer to see the real aviation terms be used correctly, and call relative altitude exactly what it is.

I think many would like to be able to use AGL.  I think part of the problem is that sensors and data storage and computing headroom on most FCers don't make true AGL possible at this time.  It takes a lot of data room to pre-store much terrain or sensors that most multirotors don't have at this time.

It is much simpler for the FC by barometer to have a baseline form where it armed and then do it's best to provide a "relative altitude" from that start point.  Simple to execute and simple to understand even though it is not always the most useful.

Correct. You need to calibrate baro with the local air pressure before MSL would be valid before take off. Relative makes it much easier for most people to handle.

I agree.  Relative altitude is the most practical altitude for most of our applications.  Anyone that requires precise AGL and MSL altitudes would be happy to go through those additional efforts.  Relative as a standard basic altitude is fine for 90% of the operators.

I just want the terminology to be accurate and consistent. Arducopter uses calls altitude above sea level "absolute altitude", and that is wrong.  Absolute altitude in aviation is the altitude above ground.

I suggest the altitudes reported by arducopter should simply be referred to as: Relative, ASL, and AGL.

Using relative altitude is the best way to go for more than the vast majority of users.  All one needs to know is how hight above the ground I am on does the vehicle need to fly to not hit something.  Using MSL altitude has too many factors such as pressure system, ambient temperature compensation, and calibration and instrument placement errors which would be unique to every situation and vehicle.  

So after reading all this (which is kinda of funny) because I was just on a walk with my dog and we live way up on a hill and I was thinking in my head if I launched my multirotor (APM 2.6) from my house at 100ft and wanted to fly it over to the park which is quite a way down the hill, then I would be at probably 400ft from the ground. So I was trying to think what would be the best way to have it go down to 100ft over the park and then return without crashing into something. Is this even possible or do you just have to guess?

@Brad, we've made altitude sensors suitable for 'copters and fixed wings that can give accurate AGL beyond 330ft (SF10/C). I know that many commercial flight controllers aren't able to use this functionality yet, but with the current FAA reg's it's probably going to become critical for all UAS users to monitor AGL.

I think you mean to say "relative, MSL, and AGL".

I agree that "absolute altitude" is wrong, and has the potential to confuse someone with aviation knowledge.

That's a sweet device, I want one!

I'm still struggling to understand it 100%.

Let's assume the following:

I take off from home, altitude at ground level is 20 meter. First waypoint is altitude 100 meter, relative mode, but tickbox verify height not ticked. Will I now climb to 100 meter or 120 meter?

Second assumption:

I planning waypoint 2, 100 meter, waypoint 3, 100 meter, but there is a hill of 150 meter in between. I select relative, check the tick box verify height and I set TERRAIN_ENABLE to 1 and TERRAIN_FOLLOW to 1. I plan this mission without being connected to the plane. When I done planning my mission, I upload the mission to the APM using MAVLINK.

Will the plane try to over the hill or through the hill?

I have a few more scenarios, but these are the most relative.

I am a bit confused on how Mission Planner itself deals with verify height. If I generate my waypoints via Survery (Grid) then check verify height, all of my waypoint alt values are unchanged. However if verify height is checked before I generate the waypoints, each waypoint alt value is different (higher or lower relative to the initial height I set). So my question is when I write these values to the copter in the first scenario I just described do the values get changed to those reflected in the second scenario I described?

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