Heading/Track/Compass/Navigation Discussion with Photos

[CaspersCruiser]

I don’t want to beat a dead horse, but for those interested in such, here are some photos to illustrate heading/track/compass concepts discussed in some recent threads.

To review the terms used:

Heading is the direction the bow is pointing

Track (or course made good) is the direction the boat has been moving. Track is often different from the boat’s heading because of wind or current.

Heading and track may be referenced to True North or Magnetic North. True north is located at the geographic North Pole. Magnetic North Pole is located in northern Canada and it is the location to which magnetic compasses point. Magnetic north is almost always used for vessel navigation.

Magnetic variation is the difference between true north and magnetic north. Magnetic variation will change based on geographic location. For example, here in the Louisville, Kentucky area, magnetic variation is 5° west. In Philadelphia, it is 12° west. In Seattle it is 15° east.

Compass deviation is the error in a magnetic compass (usually +/- a few degrees) caused by magnetic influences around where it is installed. Airplanes from the smallest to the largest have a compass deviation card posted on their magnetic compass detailing the error on various cardinal headings. This deviation is actually measured by aligning the aircraft centerline on a “compass rose” painted on the ramp or through the use of a master compass. I’ve never seen a compass deviation card on a boat, but those errors surely exist.

Don’t get wrapped around the axle worrying about magnetic variation or compass deviation. Just make sure your chartplotter and autopilot compass are set to reference magnetic north.

The only time you have to worry about magnetic variation or compass deviation is if you plot a course on a paper chart, which are referenced to true north, and you want to navigate that course plotted on the chart using your magnetic heading instruments. Then you will have to apply magnetic variation and compass deviation. That circumstance is way beyond the scope of this post.

This morning, I took my R27 out on the Ohio River 15 miles upstream from downtown Louisville to photographically document some of these concepts. The river is at normal pool and there haven’t been any major rain events lately so the river is stable with a typical current of about 1 mph. Winds were flat calm.

I started out by crossing the river roughly perpendicular to the current at a fairly slow speed so the current would have a greater influence on what direction the boat tracked. This photo shows the results. Note that the direction displays on the autopilot compass and chartplotter have an “M” next to them to denote they are in degrees magnetic. Unfortunately, due to image size limitations of the forum, some of the finer details in these photos will not be possible to read.



Autopilot compass display at the lower right shows heading is 320° (the direction the bow is pointing). Track is displayed on the chartplotter at the top of the far right column and is 315° (the direction the boat has been moving as influenced by the current). Track is shown on the Garmin 5212 under “GPS Heading”. That is a misnomer. It is a track!. Magnetic compass on top of the dash shows 317° (magnetic compass deviation in play here).

After my trip across the current, I turned parallel to the current (there was no wind) and this photo shows the results:



Heading and track are the same 224°. Magnetic compass 219° (again, magnetic compass deviation).

All of this may seem somewhat overwhelming, but if you’re completely unfamiliar with the concepts, I encourage you to get some training in them from someone who does understand them. It will make you a better captain and may come in handy if you have to navigate the old fashioned way or it may help you to detect nav equipment failure.

The iPad in the right side of the photos is displaying the Navionics app. I use it for its always up-to-date cartography and better bottom mapping.

 

[Wee Venture]

Thank you! I am rusty on these terms and your explanation is one of the clearest I have seen. Much appreciated! I want to have a working familiarity with them and not have to stop and remind myself which is which every time.
Now to round it out, how about explaining “bearing” (absolute and relative)? We see the term on our screens sometimes, maybe regarding a MARPA or AIS target or destination. If you add those bullet points, I will cut and paste all of it into a page for reference.
Thanks again,

John

 

[CaspersCruiser]

Here are the definitions of absolute and relative bearing as I understand them. Please, if someone has a better understanding or a better way to explain these concepts, don’t hesitate to make corrections or clarifications.

Absolute bearing is the direction to the object in question in degrees relative to true or magnetic north. In other words, the compass bearing to the object.

Relative bearing is the direction in degrees to the object relative to the bow of the boat with 0° being the bow.

For example, your boat is heading straight south on a magnetic heading of 180° with zero current and no wind. Exactly at 90° angle to your right is another vessel. The absolute bearing would be 270°, the compass direction to the vessel (180+90=270). The relative bearing would be 90°, the angle from the bow of the boat to the vessel.

To the “old head” pilots out there in Tugnut land, does this discussion of absolute and relative bearing remind you of trying to navigate with a fixed card ADF? Remember the old formula— MH+RB=MB to the station? Thank goodness just about all fixed card ADFs are in museums where they belong! And NDBs are headed that way.

 

[gwishon]

Great post! Just one point to add — wind will also affect the boat’s track, as I’m sure you know. So, the difference between heading and track will be the sum of the effects of current and wind.

Gordon
(Retired USAF Navigator/WSO)

 

[BB marine]

Great post Bill. I have had the issue of autopilot/compass disagreement. I have done a sea wizard a couple times and never really received green lights because the water conditions each time I planned to do it were not favorable. Many times it agrees many times it doesn't. I know my compass is correct. My compass/autopilot disagreement is anywhere from 0 degrees to 20 degrees off. The disagreement is confirmed by turning the radar on if I'm close to shore. We are planning on spending some time on KY lake before heading to Florida via the river system this fall ( If Covid19 is compliant) .I plan on doing a Sea Wizard then with the thought I would have better conditions.

 

[knotflying]

Great explanations Bill! And when all is said and done once you put in a destination point, no matter what the chartplotter or compass say, I assure you that we will all get there! :lol:

 

[Wee Venture]

Thank you, I am copying, pasting and saving!

John

 

[Submariner]

Great post and discussion. Thank you for illustrating and explaining all this!


Sent from my iPhone using Tapatalk

 

[CaspersCruiser]

.....Just one point to add — wind will also affect the boat’s track.... So, the difference between heading and track will be the sum of the effects of current and wind.

Gordon
(Retired USAF Navigator/WSO)

Gordon knows a thing or two about navigation. He used to find the way in the right seat of an F-111 at 480 or 540 knots (eight or nine miles a minute) below 500’ AGL (above ground level). At night. In the weather. And before GPS.

 

[gwishon]

Well, we did have INS. But it was analog.

Gordon