Auto-Declination and verifying your Multi-Rotor's Compass Calibration.

After I properly and successfully calibrated the compass (magnetometer) in my Quanum Nova (APM-252 with single external compass) the other day ... I noticed that what it considered to be exact North was about 7 degrees off from what my analog hand-compass read. I was outside, with a GPS-Fix and Auto-Declination was on (which is best). I remember seeing something like this before but I was pretty sure the magnetometer (and whole quadcopter for that matter) was calibrated properly, so I thought I would investigate further.

I posted a message in the forums, and forum-member DKEmxr had this to say:
"Your hand compass will read Magnetic North. After calibration and auto declination correction the magnetometer will read close to True North. So it depends on where you are in the world as to how much this differs."

So, according to this article, Central Texas is about 7 degrees East Declination to Magnetic North (what hand-compass shows as exact North). Or, said another way, the hand-compass' Magnetic North minus 7 degrees = True North (what APM-based multi-rotor shows as exact North).

I also found this article.  For example, a 15 degree error, on a mile-long flight puts you a quarter-mile (440 yards) off-course or away from your desired destination. I think I will investigate buying a compass that takes declination into consideration. Until then, I will just subtract 7 degrees to measurements taken with my old hard-compass, or expect it to be 7 degrees off when comparing it to the Nova quadcopter or its software.

So, when the multi-rotor is facing its exact North at 0 Degrees, the non-declination-aware old-school hand-compass will read more like 7 Degrees (slightly East).

I'm pretty sure this applies to all other navigational systems on earth. I'm not 100% sure about DJI and other UAS, but it does also apply to PixHawk-based systems.