AP_GPS: log altitude above ellipsoid instead of undulation #29904
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Ran into minor confusion over why I had to change the datatype to include |
Ryanf55
approved these changes
Apr 29, 2025
ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: ArduPilot#29199 . Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion. Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
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I've rebased this on master so hopefully it will pass CI now |
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this is included in 4.6.0-beta6 |
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ArduPilot's internal definition of undulation is reversed in sign compared to the accepted convention and this creates confusion: #29199 .
Instead of the internal definition, log the altitude above ellipsoid, which ArduPilot already consistently and correctly calculates and broadcasts over e.g. DroneCAN and MAVLink. Switching the quantity logged allows users to accommodate the problem in old logs (i.e. always flip GPA.Und sign to match the accepted convention) and reduces opportunity for future confusion.
Future work will clean up or replace the internal representation, but this should be a complete fix for the issue from a user perspective.
Tested using u-blox, u-blox over AP_Periph over DroneCAN, and NovAtel.
Geoid information using EGM96 from my (approximate) back yard showing an ellipsoid height of 60.57m, an undulation of -28.35m, and a mean sea level height of 88.92m:
Dataflash log from a u-blox NEO-M9N, values match well (receiver-calculated undulation is -30.07m):
Corresponding telemetry log:
