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After about a month of twiddleing, I've finally got all three compass heading measurements to agree.

The first problem was that the binnacle compass was off, on the 90 and 270 degree axis + or - 25 degrees, causing all other measurements to be off if the binnacle compass was used as reference (see graph #1).

I leaned how to adjust the magnet on the rod below the compass and adjust it to read true magnetic at all four points.

Then only after that was corrected, the autopilot heading sensor was calibrated with no problem and agreed with binnacle compass (see graph #2).

The last was the Airmar GPS magnetic heading sensor. After trying a couple of locations for least interference (I settled on the aft cabin closet, on the aft wall), it was a struggle to get the auto calibration to kick in.

The trick was to get a constant (and specific at 3 degrees per second) turning rate, wait 2 circles before it starts to calibrate. Then hang on for another two turns while it does the calibrations. Then, BINGO, it was calibrated! Finally.

Now all three heading measurements agree within a few degrees (see graph #3)

All this will make it more accurate when I ask the autopilot to steer to a position marked on the GPS map.

Also, the Garmin radar overlay function now shows the radar image correctly positioned over the GPS map at all compass headings. Yippee!

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