I’m using 2x holybro f9p gnss recievers on my copter for 2d position and yaw control.
I would like to use GPS for altitude also, while connected to RTK/NTRIP.
I want to pre-program my missions from home, as I have a precise terrain model of the area I will be flying.
My terrain model elevations are relative to a local vertical reference system.
The F9P module sends a MSL height to the autopilot, relative to the EGM96 10x10 geoid. I need to find a way to extract the correct vertical.
Fly at a known elevation and calculate the offset. Then use that offset when planning
Unfortunately, the elevation registered in ardupilot (when using the UBLOX F9P - not sure if true for all receivers) is the MSL elevation - this elevation is in reference to a geoid. I hear rumours that the F9P chip has a EGM96 10 degree grid as its geoid.
A geoid won’t have the same elevation offset when moving position 
The trouble is that there are many different types of geoid, depending on which reference system you/your customer chooses.
So I am looking for a way to navigate using the ellipsoid elevation, which is a great reference height because it is easily mathematically defined, and I can create my mission offsets relative to it.
There are Lua bindings for both terrain and GPS data. Perhaps those can be leveraged?
Interesting! I haven’t worked with Lua scripts yet, but it does seem like they have some very useful applications.
I’ll look into it.
After wrapping my brain around the problem a little better, I’m afraid scripting may not be the answer here.
@hendjosh, I know you were very interested in altitude reference frames during the dev conference. Can you perhaps help guide the solution here?
I think Tridge got this sorted by adding a parameter to configure the UBlox right?
Yes!
Tridge was very kind to implement the option!
I tested it today, and it worked.
Will be doing a test Monday, on a GPS fix-point where I know very precisely the height above ellipsoid at a coordinate point on terrain.
Hopefully the value used by ardupilot will be the same.
