There are several reasons why the compasses indicate different headings.
- They are not both aligned to the vesselís longitudinal axis.
- They are experiencing different magnetic fields caused by the boat
- They are experiencing different magnetic fields caused by the pier.
- One or both are mechanically defective with the directive magnets and the card misaligned
- A combination of the above.
The check is when underway away from the pier. If the difference is constant on all headings, the issue is either 1 or 4. If the difference in heading reading varies with heading, the issue is 2 and may also have an element of 1 & 4. If everything is alright, the issue was 3.
Determining the error in you compass
is not difficult. If you need more help contact me.