Jump to content

Compass 101 - basic info

Recommended Posts

After reading some of the recent topics it seems that this Compass 101 info below might be helpful. This is not a comprehensive write up, just a few basic concepts and gotchas that I have learned over time while boating.


A compass by itself points to the magnetic North. To get the true North direction, the compass reading must be corrected for two effects.

The first is magnetic deviation: the angular difference between magnetic North and the particular compass reading affected by nearby sources of magnetic or electromagnetic interference such as metals and electronics nearby. The correction for magnetic deviation is achieved by doing initial compass calibration after compass is installed in its permanent location on a boat. A compass might need to be (re)calibrated after any metals were re-positioned or any electronics were installed nearby. Customarily and unfortunately, neither of the calibrations is done on most pleasure boats.

The second is magnetic declination or variation: the angular difference between magnetic North at your local direction of the Earth's magnetic field and true North. The values to calculate magnetic declination or variation at your particular location are provided on paper charts and can be automatically calculated on electronic charts providing that your chart plotter is set to show the true North heading.

The above applies to either type of compass, analog with magnetic needle or electronic compass aka fluxgate compass aka heading sensor. Either device operates under the same principle and seeks “local” magnetic North, and is affected by magnetic deviation and declination. There is no practical difference in accuracy between them, the difference is in how they are calibrated and used.


The GPS can calculate the true North but GPS is not a heading sensor. A chart plotter without a heading sensor connected can calculate the “heading” only if its GPS unit is in motion. The calculated “heading” is course over ground (COG) and not the boat’s heading aka course to steer (CTS).



The electronic chart plotter can only synchronize radar view with chart orientation if its GPS is in motion. It all gets out of whack when GPS unit is still or is off unless an electronic heading sensor providing heading information is connected to that chart plotter. Relevant warning regarding heading sensor aka electronic compass from one of Garmin’s manuals:


Radar Overlay and Chart Data Alignment

When using the Radar Overlay, the chartplotter aligns radar data with chart data based on the boat heading, which is based by default on data from a magnetic heading sensor connected using a NMEA 0183 or NMEA 2000 network. If a heading sensor is not available, the boat heading is based on GPS tracking data.

GPS tracking data indicates the direction in which the boat is moving, not the direction in which the boat is pointing. If the boat is drifting backward or sideways due to a current or wind, the Radar Overlay may not perfectly align with the chart data. This situation should be avoided by using boat-heading data from an electronic compass.

If the boat heading is based on data from a magnetic heading sensor or an automatic pilot, the heading data could be compromised due to incorrect setup, mechanical malfunction, magnetic interference, or other factors. If the heading data is compromised, the Radar Overlay may not align perfectly with the chart data.


Last but not least, do not forget to set your chart plotter to a desired heading reference. Remember which reference you navigate in and be consistent in using the same reference across all your devices, analog or electronic. Example on setting heading reference from one of Garmin’s manuals:


Setting the Heading Reference

You can set the directional reference used in calculating heading information.

  1. From the Home screen, select Configure > Units > Heading.
  2. Complete an action:
  • Select True to set true north as the heading reference.
  • Select Grid to set grid north as the heading reference (000o).
  • Select User Mag Var to set the magnetic variation value manually, enter the magnetic variation, and select Done.


Finally, there is plenty of info out there that explains compass and GPS, and marine navigation in many different ways, for example:

How to Use a Boat Compass, Complete beginners guide - https://improvesailing.com/guides/boat-compass

Marine Navigation, How to Navigate the Oceans and Waterways - https://www.liveabout.com/about-marine-navigation-2747149

Magnetic Compass Calibration and Heading Calculation - https://www.youtube.com/watch?v=iIRLTyp8vhg


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...