APM Planner - does it use absolute alt?

Please help me with APM planner.

I have APM with 2.012 fw + magnetometer installed. In mission planner (0.4.19) had set 8 WPs with default alt set to 100m.

First interresting point here is - I can't let home location alt set to 0, there is still nag message:

3690864929?profile=originalThen when I set some home alt e.g. 100m and don't have checked absolute alt and my elevation graph looks like this (abs. alt of this place is 180m by google maps):

3690864961?profile=originalBy this graph one can see the alt is still meant as absolute!

Questtion is - must i set the home alt? I thought the home position here is not relevant, because it is set during APM startup - on the GPS lock. Then the home alt should be always absolute (to locked GPS alt) and all WPS then relative to this altitude. Why must i set this home alt?

This weird planning i can see only in Planner MAVLINK in planner 1.0x it is simple.

Please can somebody clear this MAVLINK planning form me?

- what is here absolute alt checkbox

- why the home alt must be set

- is this home alt absolute or relitive to locked GPS alt

- what is the relation between home alt and WPs altitudes


Today i tried some flights with APM 2.012 and with no success. The only valuable result was with sabilization mode. Auto mode started descend to ground and withouth my intervention aircraft would hit the ground.Even the RTL descended to ground.

Therefore i suspect something wrong in planning. Please help me understand this MAVLINK planner.


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  • I need to flight plan a mission using Absolute Altitudes (MSL).

    Someone else will be providing me the coordinates, and I plan to paste them into the .txt file then load them into MP.

    What kind of altitude is used in the altitude column in the MP flight plan .txt file? I think it is relative alt to home?

    If I want MSL altitude, do I just check "Absolute Altitude" before I load in the waypoint?

    Do I enter MSL altitudes into the .txt file first?

    How does MP know the MSL altitude anyway? There is no way to enter a barometric altimeter setting?

  • Hmm I may be in the right area for a similar Q. I am using APM2.5 for aerial mapping. I'm very new at it so still surfing the learning curve.

    My questions is about clarifying Absolute Alt vs. Relevant Alt in terms of which one to use for aerial mapping over undulating terrain. Usually Relevant Alt is fine with my mapping but an area I was mapping yesterday was very undulating (variance of 200m over a distance of 200m).

    My concern was about getting pitch angles in my photos because I thought the APM would try to maintain my set Relevant Alt with the terrain/contours. My solution, which didn't work but that is another story, was to fly an absolute alt which would over come this issue. Because the absolute alt didn't work I would just like to know if my concern about the UAV trying to maintain to relevant alt is justified?

    Cheers in advance,


  • i have the same problem Auto mode started descend to ground and withouth my intervention aircraft would hit the ground.Even the RTL descended to ground.

    i´m using APM 2.012 and the latest mission planer(0.4.18). all readings from GPS (MediaTek MT3329 GPS) and preassure sensor are loocking fine.

    any ideas

  • Hi,

    Have you checked in CLI the readings from your GPS? I had a similar problem: using a NMEA GPS (the one from EagleTree systems) in APM 1.02, the altitude read from the GPS was 10x times higher, i.e. 100m instead of 10m. I have modified the following line in AP_GPS_NMEA.cpp:

    altitude = parsenumber(parseptr, 1) * 100; // altitude in decimeters * 100 = milimeters

    altitude = parsenumber(parseptr, 1) * 10; // altitude in decimeters * 100 = milimeters !modification

    and now the readings are correct.

    This problem is not shown in APM 2. Still I am wondering how the altitude is calculated when you use mix gps-airpressure  (#define ALTITUDE_MIX        0.5) with absolute altitude, since the gps will read sea-level and pressure sensor will read relative altitude?


  • I too am using the MAVlink stuff for the first time now after having breezed round with the normal stuff but if functionality hasn't changed you "Home" co-ords are for your benefit whilst planning as when you turn on the APM system the Home is overwritten with the GPS data for Home. So as long as you use the planner to do relative altitude then 0 will be your Home altitude (in my case GE data is WAY out).

    I actually asked this same question almost exactly the same time as there was a thread saying the planner was absolute so I wanted to check it had been fixed.

  • Developer

    Home alt is always absolute. and should be set to ground height... not flying height

    rtl height is defined as chris stated.

    if absolute alt is not picked then all wps are relative to home.

  • Yet another question - what altitude will APM try to keep when i switch the mode to RTL.

    For example plane is 200m high (from ground) and 400 away from home pos. Then i switch to RTL mode, what altitude APM try to keep? Is it the one set in mission planner?

    3692200116?profile=originalOr will it keep the actual altitude when the RTL mode was activated, no matter what is set for home alt?

  • 3D Robotics
    Home is set automatically at first GPS lock. You can change it manually if you want, but then you must also set an altitude.

    Defaults are all relative altitude.
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