This is quite misleading and could get your drone into trouble.
You are confusing a simple compass error with a yaw error.
A compass error is confusingly named.
It would be more accurately called a compass warning.
That's when the compass detects a magnetic field much stronger than the earth's normal magnetic field and warns you of a problem, as you've found out with a manhole cover.
But a yaw error is much more dangerous as the magnetic field that causes it is no stronger than the compass warning threshold.
You get no warning but the magnetic field is not aligned with the earth's magnetic field.
Your compass is affected by it and the IMU initialises the the gyro sensors based on the incorrect compass reading.
If you launch and get out of the magnetic influence, your compass is now reading correctly but the gyro sensors are giving conflicting information.
The result is usually a fast, out of control drone making a curved flight and crashing.
This happens because the drone detects that it has moved from it's position and tries to correct, but the correction puts it further off course and it corrects and corrects etc etc.
We see yaw error incidents frequently in the forums and the most common cause is launching from reinforced concrete surfaces.
It can be caused by launching a few feet from a large steel object or somewhere that puts the compass sensor just an inch or two from a small steel bolt etc.
Here's what's inside a typical sidewalk:
Launching from reinforced concrete is like buying a lottery ticket.
The mesh might be deep enough that it doesn't cause a problem.
You might place the compass in your landing gear far enough from the steel to avoid issues.
Or you might put your drone where the mesh is closer to the surface and/or overlapped (centre of pic above) etc and get your compass sensor in just the right spot to end up with a yaw error.
The result of that is usually very bad.
Calibrating the compass cannot do anything to solve magnetic issues caused by something that's not part of the drone.
If your Phantom is hovering and flying properly, recalibrating the compass is completely unnecessary.
(I still haven't calibrated anything on my P4 pro and it flies perfectly after 4.5 years including international travel).
Compass calibration is probably the most misunderstood aspect of drone flying.
To get a good understanding of what it really does, read the start of this thread:
BACKGROUND The aircraft compass is often misunderstood, both in terms of function and purpose, but it is important to understand what the compass measures, why, and what happens to that information. At the most basic level the purpose of the compass is to measure the earth’s magnetic field in...
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