UPDATED


The following is follow up to previous Thread on Mavics compasses sensitive to magnetic interference at ground.

UPDATE: Not true for Phantom.  It's compass is lcoated never very bottom of land gear (aka Leg). Smaller drones like Mavic Pro, Mavic Air, and Spark; sit closer to ground than larger drones like Inspire or Phantom.  By nature of smaller drones, their compasses (three magnetic field strength senors - XYZ axis) sit closer to ground, that compasses of larger drones.  Thus making smaller drones more susceptible to sources of magnetic interference at or below surface.

To demonstrate this, per SparksBird suggestion, Mavic Pro was started out on ground, then raised using blocks of wood till two internal compasses showed ''Green'' (good to go).
To make this more real-world, instead of using 4"x20" strip of sheet metal, this time Mavic Pro  was placed over an area of concrete that had alternating Steel  rebar.

Before getting to pictures, I want to throw out a word of caution.  It is possible for Mavic Pro to show "Green" (good to go), when there is some magnetic interference detected.  One compass shows Yellow bar and other Green bar.  There was no way to capture this, being Sensor Status Window, causes Main Screen Status to go away.


Photos from second of three tested postions

0.00 inches off ground

Drone 0.00" off ground - Yellow & Red bars




Drone 2.50" off ground - Yellow & Yellow bars

The reason there are two 2.50" off ground pictures is bottom compass (#2) was toggling between Yellow & Green.

Drone 2.50" off ground - Yellow & Green bars




Drone 3.00" off ground - Green & Green bars



Photos from third of three tested postions


Drone 0.00" off ground - Yellow & Red bars



Drone 1.50" off ground - Yellow & Green bars

The reason there are two 1.50" off ground pictures is Top compass (#1) was toggling between Yellow & Green.

Drone 1.50" off ground - Green & Green bars




Drone 2.50" off ground - Green & Green bars


UPDATE: Seems I got bad info first time I looked at compass position of Phantom.  After doing some more looking around and research....  The Phantom compasses are not located at top as I had previously seen.  The Phantom compasses are located towards bottom of landing gear (aka legs).  Thus putting Phantom compasses very close to ground.  

Phantom 4 Pro - 234mm (9.2") overall height
Phantom - 190mm (7.48") overall height
Mavic Pro - 83mm (3.26") overall height
Mavic Air - 64mm (2.51") overall height
Spark  - 55mm (2.16") overall height


UPDATE: Compass located near very bottom of legs puts Phatom compass below Mavic Pro's compass.  Take worst case above of 3.00" off ground, add it to height of  Maivc Pro (3.26") and it is still lower than Phantom at 7.48" by 1.22".  As you can see, even 0.5" can make a difference.

Result of this testing strongly suggests we need to use extra precaution when flying smaller drones from areas where there could be a source of Magnetic Interference at or below ground surface.  Possibly a very strong source (underground power lines) down as deep as 1-meter.

Need to be careful on concrete surface as MA can detect rebar under very well. I commonly see compass interference when put MA on concrete. Need to move it around to find a place with less interference.

One of the reasons I bought the Mavic Air instead of the Pro is to launch and catch it from my hand. I HATE putting the drone on the ground. Not only does it get dirty but you never know what kind of interference it's getting from the ground/floor. I made a video demonstrating how I launch and catch my Spark with one hand, but I use the same techniques with my Mavic Air.

GDL Posted at 2018-6-11 20:30
Need to be careful on concrete surface as MA can detect rebar under very well. I commonly see compass interference when put MA on concrete. Need to move it around to find a place with less interference.

I would not fly drone off any surface, unless both Compass sensors show Green bars.
What does bother me is that GO-4 App can show "Green" (good to go) message even if there is magnetic interference.  The GO-4 App should "Yellow" (with some warning) message.

Notice in first picture, Labeled  ''Drone 0.00" off ground - Yellow & Red bars''; that Remote Control says - ''Ready to Go''.  
When R.C. should say ''Not Ready - M.I.''.