Hi all,

I'm flying an APM2.5 with version 2.68 of the code. I'm having an issue with altitude.

I boot up the auto pilot and wait for it to get GPS lock. Often times, the altitude says around 20m or so. My altitude above mean see level (which I think is what GPS uses) is about 20m so this could be correct. I then hit the "zero home altitude" button on the mission planner and my altitude does indeed move to 0. The problem is that when I go to fly in auto, the desired altitude of each waypoint is not what I've set it to in my mission. Often times I set all my waypoints to 80m. Instead of flying around at 80m, the autopilot will fly around at another altitude like 60m (The fact that 80-20=60 doesn't seem to be just a coincidence!). And it's not as though the autopilot is trying to get to 80m; it's desired altitude is indeed 60m. If I don't zero out the altitude when I boot up the APM, then the mission will indeed be executed at the correct indicated altitude (80m), but it wont be 80m with respect to the ground. It will be 80m with respect to whatever altitude it started up with (most times around 20m).

Has anyone else noticed this behavior?

Anyone know how the altitude is supposed to be handled? It seems to me that if you enter waypoint altitudes and you don't have the abs altitude button checked, then the desired altitude should be relative to what you started out as. And in the case of zeroing out the altitude while on the ground, then I can't see why it wouldn't fly at exactly the desired altitude of each waypoint...

Any help would be appreciated.

Also, I do not have absolute altitude checked in the flight planning tab.


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I have the same issue i think.
Will do some more testing, maybe its just mission planner messing things up.

So it safe to use the plane in all modes except maybe AUTO without bringing the computer to the field? I dont really like to always set altitude to zero and bring my PC and based on what Will says i dont have to?



Where is the "zero home altitude" button in mission planner ?





I never used any "zero home altitude" and my plane got the altitude right in auto both with absolute altitude and with over-the-start-location heights. I would have known otherwise, because my field is at 470 meters above sea level :)

So don't use that button will be my advice.

There is another thing to pay attention to: At least for me, the APM accepts and stores the home altitude from the GPS a little early, while it is still pretty bad accuracy. This error will stay, even after a minute more when the GPS has a much better fix. My solution is just to leave APM and  GPS running for a few minutes (perfect time to check out the rest of the aircraft) and then press the reset button (not cycle the power). Now APM will reboot and set its home on a better fix. If I have a computer with me I still check on the altimeter.



Thanks! That is some great information. Bad thing for me is that the APM is buried inside my plane, but I have a hatch so I can reach it from the side. Ill guess I have to redesign the resetbutton on the APM 2.5 case to get it a bit higher up.

I have a Nexus 7 setup with telemetry and Droidplanner and it would be cool just to use that in the field.



Hey, here it is:


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