Hi thought I would share a test I did with the compass.
I always wondered what the x,y,z,mod meant so here is what I found.
The x axis is through the aircraft left side to right side
The y axis is through the front to rear
The z axis is through the top to bottom

Assumes the P3 is flat and stationary
If the reading in the sensors of the Go app show:
y=0 the front is pointed north or south
with y=0 and x= (about) 500 (positive) the front is north
with y=0 and x= (about) -500(negative) the front is south

if x=0 the front is pointed east or west
with x=0 and y=(about) 500(positive)  the front is west
with x=0 and y=(about) -500(negative)  the front is east

The z azis is used by the math of the processor when the aircraft is tilted
because the numbers all change when tilted.

Hope this helps

Does it appear that this may be a reliable method to confirm the calibration of the AC electronic compass?  DId you reach any conclusion regarding the  value of MOD when the AC nose is pointed north.

Huey_930 Posted at 2016-1-17 23:30
Does it appear that this may be a reliable method to confirm the calibration of the AC electronic co ...

MOD value should be at 1450 +/- 100

Jkeller84 Posted at 2016-1-17 20:39
MOD value should be at 1450 +/- 100

Thanks for the info.  I'll see  if I get  roughly duplicate results after calibration and orienting to mag  north.  

Huey_930 Posted at 2016-1-17 23:30
Does it appear that this may be a reliable method to confirm the calibration of the AC electronic co ...

Mod value stays roughly the same regardless of direction the aircraft points.
I seem to always have the 1350 to 1500 mod reading like jkeller says.
I have checked the orientation several times while the P3 is level and it does seem accurate to magnetic north using a handheld compass to compare and compared to known northerly direction.
Not sure what the mod actually is but I think it'stands for modulas and it's number is the result of a formula using square of the x,y,z numbers.
Somebody that knows can chime in  on that.
I'll be doing further testing when the weather breaks.