But you need zero wind. I had purchased the two large iron spheres typical of compasses on steel boats but the adjuster finally realized they were the reason for some very strange readings. Perhaps someone could enlighten me as to the purpose of the exercise above and beyond a standard compass swing that has served us well enough for a century? Ships not fitted with degaussing coils can be degaussed either by dragging a cable carrying around amps alongside the ship or, more efectively, by encircling the vessel with cable. It is intuitively obvious one could use the highly accurate GPS track for this. The farther you are from the waypoint the better, but a couple of miles provides sufficient accuracy.
We are talking about calibrating a compass in flight
Aligning a compass or HSI in flight, using GPS track?
Sure enough, 60 degrees of error through E-W, 0 degrees through N-S. Plus or minus 10 degrees should be fine. To counter this, the card of a boat compass usually has a small counter-weight attached to enable it to sit level. No; see my original post. I do claim to have done plenty of navigation without electronic fixing aids such as Decca or GPS.