Our local Coast Guard Auxiliary Flotilla is in the middle of our annual course on navigation, which is open to the public. This year, over 20 local boaters have joined us to study the basics of coastal navigation. We are having a great time, learning about charts, dead reckoning, correcting for compass error, GPS, Chartplotters, Radar, and much more.
One of the challenges we all struggle with is mastering some of the terms that apply to navigation. In this article, I will attempt, with great brevity, to explain some of the terms you will encounter when learning navigational concepts. While many of these ideas are similar, understanding the subtle differences is very important. Let's explore terms that relate to direction, distance, speed, and time.
True Direction - Direction measured relative to the True (geographic) North Pole (shown on charts).
Magnetic Direction - Direction measured relative to the magnetic North Pole.
Compass Direction - The degrees shown on the compass.
Variation - The difference in degrees between True and Magnetic North. The amount is determined by your location on the planet.
Deviation - The difference in degrees between Magnetic and Compass. The amount is due to the direction your vessel is pointed and the various magnetic influences on your vessel.
Course - This is the direction of intended course, as laid out on a paper or electronic chart. This can be expressed as degrees True or degrees Magnetic. Degrees are measured from 000 to 360, and written with 3 digits. On a GPS, this would be your route line. 000 and 360 both refer to North; 090=East, 180=South, and 270=West.
Course - This is the direction of intended course, as laid out on a paper or electronic chart. This can be expressed as degrees True or degrees Magnetic. Degrees are measured from 000 to 360, and written with 3 digits. On a GPS, this would be your route line. 000 and 360 both refer to North; 090=East, 180=South, and 270=West.
Ship's Heading - This is the direction the bow of the boat is pointed. It can be expressed as degrees True, Magnetic, or Compass. In ideal conditions, a mariner could make his ship's heading the same as the intended course heading and arrive at his intended destination.
Track - This is the direction the ship is actually travelling. Because of wind and current, the track is usually not the same as the ship's heading or the intended course heading. (Note: boats must often "crab" through the water in a cross-current in order to follow a course; pointing the bow into the current.) The term "track" is often used on a GPS where it can either refer to your actual course, or a series of breadcrumb-like dots to show where you have been (track log).
COG - Course Over Ground is the instantaneous assessment of where you are actually going. A GPS derives this by taking two measurements of your position, slightly separated in time and calculates the direction of movement between the two points.
CMG - Course Made Good is the average course that you have followed.
Bearing - This is also a measure of direction, expressed with 3 digits, from 000 to 360. But, this direction is from some location to another. For example: from your current ship's position to the next waypoint, or to some charted object. Bearing can be expressed as relative to the ship's heading or it can be based on the chart. If it is measured as degrees clockwise from the bow of the boat, it is a relative bearing. If measured from your charted position to the charted object, it is an absolute bearing. To convert from relative to absolute, you simply add the ship's heading to the relative bearing to get the absolute bearing. These can be measured in degrees True or Magnetic.
XTE - Cross Track Error. Distance back to course line, when off course. With me so far? Good, let's add some more information to our databank. Speed is next.
SM & MPH - Statue Miles are used to measure distances on land or inland waters (the ICW) and speed is measured in (Statute) Miles Per Hour.
NM & Knots - Nautical Miles are used to measure distances over the ocean and speed is measured in Nautical Miles per Hour (called, "Knots"). Incidentally, one minute of latitude is equal to one NM, making it convenient to measure NM on charts. Not true of longitude.
SOG - Speed over Ground. Calculated on a GPS by recording the time travelled between two points and dividing it by the distance. SOG does account for the positive or negative effect of current.
STW - Speed through Water. Measured by the boat's speedometer. Does not account for the speed of the current. Finally, a couple of terms about time.
ETE - Estimated Time En route. The elapsed amount of time it will take to get from on point to another.
ETA - Estimated Time of Arrival. When will we get there, Daddy?
Hope that helps! Read. Study. Take a course. Be safe.