Altitude Data and Arducopter

I have been struggling to get my Arducopter to set off to the first waypoint at the correct altitude and part of the problem is that I don' t know whether the various altitude values are absolute or relative to the home position. At the moment it behaves like a lovesick angel when I engage auto although it does set off in the right direction. 

On the Flight Data page, Status window, the alt and altoffsethome are both 0 when at home. I assume alt is relative. What is the altoffsethome?

On the Flight Planner screen I have the following altitude data values:

Default alt

Hold Default alt

Waypoint alt

Mouse location alt

Alt abs

I think they are with the exception of the last one all relative. Is that correct?

I am worried about the default altitude as that is very high (100m) but how do I change that and what does the Hold Default Alt checkbox do? When is the default used and how can I change it? 

The alt abs figure sometimes is zero and sometimes 130 which is what it should be. Where does this value come from? Should I manually set it to the local absolute altitude which is what I thought I should do but I also changed it to 0 to see if that worked? Can I use the mouse to give me the altitude from the map?

What is the minimum waypoint altitude I can set -  being a 'copter I would like to set it to between 2 and 5 metres - is that possible - does it navigate using GPS altitude which tends to be somewhat variable or pressure altitude?

Sorry so many questions!!

regards Peter



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  • Been there with your questions Peter!

    Wait until you get a 3D fix with the gps, it then shows actual altitude (AMSL). Then you can either use absolute height (height above mean sea level AMSL) or relative height (where the apm is when it gets a fix).

    Once you have a fix, then you can press the set home alt button on the main flight data screen and it will zero the altitude to where the apm is and pass that information with the lat & long to the flight planning screen (bottom right). Be careful, if you pc is a little slow, it can register two or more clicks on the set default alt button and toggle it back again without you noticing!! We wondered why our height was so low during one sortie with the fixed wing!

    To change the default RTL altitude and route altitude just change the default altitude figure from 100 to whatever you want, my quad is at 20 metres and our fixed wing is at 100 metres. We use absolute height for the fixed wing, so the planner adds another 52 metres on any waypoints and the default height for us as that is usually where we turn on the apm. Obviosuly that would change if we went to another location.

    We then write the waypoints even though there aren't any to record the changes to waypoint radius, loiter radius, default alt etc. It usually says it has set the parameters. I don't know if it automatically does that when we check or un check the boxes, but when we select read waypoints, it shows the changes we have made. If I remember correctly, I think you can change these parameters directly in the configuration screen on the left and then writing the changes.

    The quad I use relative above turn on height. Your accuracy for low heights will be around 1 to 2 metres (APM 1 and better with an APM 2), you can have a mix of pressure altitude and gps altitude depending on the settings you choose, or use a sonar sensor. Our fixed wing holds its heights very well with an apm 2 and is a little erratic with an apm 1, but not by much.

    The quad has a sonar sensor fitted and was very accurate down to centimetres, but when testing the other day, it was at about 3 to 4 metres when I selected loiter (maintain current height), It unfortunately had lost its height information and thought it was at zero, so powered off the motors and landed hard, one broken prop as it flipped over but that was all. I had since unplugged the sonar sensor and have yet to try it without it.

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