Compass/Heading issues

[Diatom]

Two issues with old Garmin 5212 / GHP 10:
1. Path oscillation (16 degrees peak-to-peak) on Autopilot. Issue resolved by recalibrating Autopilot unit.
2. Persistent 8 degree heading error on both GPS screen & radar screen. NOT cured by recalibration. "Whatever is directly in my path shows up at 11:59 on the screen where it should be at 12:00 - as if display imagery is rotated 8 degrees counterclockwise".
Crawl down to the dungeon to check the fluxgate: it's mounted on behind the helm heel-kick panel, at top. Mount is secure, does not freely rotate. No stray metal objects beyond cladding screws; wire bundles are 10" away along hull but at similar elevation.
Check readings dockside:
AP reads 323T / 307M
GPS (Garmin screen) reports heading 343M (does this even have meaning at rest? Not sure).
Physical compass atop helm reads 296 (obviously M)
Freshly-calibrated iPhone 14 reads 307M

Anyone see what's going on? Is a persistent 8 degree to port reporting error within tolerance?

/tmm

 

[Full_O_Fish]

Garmin screen may be set to use GPS as heading source rather than using the autopilot heading sensor. Should be able to check it under settings, preferred sources, heading.

 

[CaspersCruiser]

On your Garmin 5212 chartplotter, what is labeled "GPS Heading" is not a heading at all. It is the course your vessel is tracking over the water. If the vessel is at rest, the GPS Heading display is not valid. It is valid only if the vessel is moving.

Your vessel's heading is the direction the bow is pointing, whether at rest or moving. Underway, wind and/or current can cause your vessel to be heading one direction and tracking another. Unless the wind or current is very strong, those two should be within about 10 degrees of each other when moving at a normal cruise speed.

Check to see that all electronic displays of direction are set to the same reference, either magnetic or true.

 

[Diatom]

Thanks for advice CaspersCruiser & Full-o-Fish.
Finally found the heading source options in my old Garmin, choices are "Auto" or "GPS".
"GPS" default gives close agreement with autopilot when at speed.
"Auto" gives a serious discrepancy but I have no idea what it means, nor whether it's true/magnetic.
Slow/rest understandably generates discrepancies on "GPS" setting.

As for the persistent 8 degree offset, it's between visual vs. chartplotter (whether radar or GPS screens).
I think it's so simple as parallax error. Played around with it today and I fear I can chalk it up to inexperience - the boat is NOT headed precisely where it appears from the helm to be headed, and once corrected with a bit of bobbing & weaving and averaging, I think the screen is accurate.

/tmm