The title might better be stated as:  Why don't my GPS and compass agree?  The answer is compass ERROR.

Compass errors are always present.  The first one we all need to know about is Variation.  This is the angular difference between the local meridian or True North and a great circle to the near magnetic pole, called Magnetic North.  This angle varies with location and can be found on any chart compass rose.  Use the angle listed in the center of the compass rose nearest your current location.  Applied to a true course or bearing, correction for variation will yield the magnetic course or bearing.  Most GPS and LORAN units can be set to show courses and bearings in True or Magnetic and will self-correct for location from data stored in the unit.  The navigator must always correct for variation while plotting.

Deviation is the error due to the installation and location of the compass on the vessel.  This deviation is addressed by dividing it into components for correction.  Misalignment of the compass mounting with the hull is called error "A".  When A is present the compass will exhibit error in the same direction on nearly all headings.  The mounting is slewed or shimmed to correct.  Deviation on East and West headings is called "B".  Large iron masses forward or aft of the compass cause this error and it may be corrected by means of the fore-and-aft built in corrector rod or with bench magnets.  Deviation on North and South is called "C" and may be corrected in the same manner as B, using the athwartship corrector rod.  Other sources of deviation exist on steel-hulled vessels but we need not concern ourselves with them on fiberglass or wood boats.

Island Marine Instrument Co. is available for compass adjusting.  The process will require about two hours underway  while the corrections are made.  It is important the compasses be in good condition prior to the adjust, i.e.: No bubble, not discolored, dial moving freely.  If problems are noted, you may contact us to arrange repairs prior to the job.  We adjust the compass to less than 2 degrees of error on most wood and fiberglass boats.  We are making appointments  now and the sheet is filling up!  Call us today to reserve your time slot.  Our website has additional information at http://www.islandmarineinst.com, e-mail Cdrcompass@islandmarineinst.com.  Phone/Fax (425) 258-4120. 

Bill Haimes