Geessh! 15 degrees off from the
GPS? Well the
GPS adjusted to Magnetic will only have and show what it will be with variation. You still have to adjust for deviation.
Deviation changes as you change course, the variation does not, it changes as you change positions around the world.
you may or may not need compensating magnetics to adjust your
compass to the least deviation error possible. And then develop a Deviation card for all of the cardinal and inter-cardinal points. Then you can use the
compass as it should be used.
Have fun.
Notes: No magnetic
compass can be adjusted to show true north on all points of steering. Due to variation and deviation, so don't even bother to try to do so. It may show Magnetic North if variation and deviation cancel each other out. But then again read Bowditch, Dutton's or Chapman's on
compass adjusting. You really need to do this in order to understand what is happening. Then call in a professional
compass adjuster.
But a gyro-compass can and does show true north. Any errors on the Gyro can be removed by mechanical adjustments.
Plot everything on the chart in True degrees, then convert to magnetic for the headings to steer by.
I've swung vessels and have developed deviation cards for them. Have done some
compass adusting also. But then I hold a 1600 Ton Master / 2nd Mate Unlimited tonnage, Ocean license and have the knowledge on how to do this.