In
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation ...
, the heading of a vessel or aircraft is the
compass direction in which the craft's bow or nose is pointed. Note that the heading may not necessarily be the direction that the vehicle actually travels, which is known as its ''
course
Course may refer to:
Directions or navigation
* Course (navigation), the path of travel
* Course (orienteering), a series of control points visited by orienteers during a competition, marked with red/white flags in the terrain, and corresponding ...
'' or ''track''. Any difference between the heading and course is due to the motion of the underlying medium, the air or water, or other effects like
skidding or
slipping
Slipping is a technique used in boxing that is similar to bobbing. It is considered one of the four basic defensive strategies, along with blocking, holding, and clinching. It is performed by moving the head to either side so that the opponent ...
. The difference is known as the ''drift'', and can be determined by the ''
wind triangle
In air navigation, the wind triangle is a graphical representation of the relationship between aircraft motion and wind. It is used extensively in dead reckoning navigation.
The wind triangle is a vector diagram, with three vectors.
*The air ve ...
''. At least seven ways to measure the heading of a vehicle have been described.
Notation
Heading is typically based on
cardinal direction
The four cardinal directions, or cardinal points, are the four main compass directions: north, east, south, and west, commonly denoted by their initials N, E, S, and W respectively. Relative to north, the directions east, south, and west are ...
s, so 0° (or 360°) indicates a direction toward
true north
True north (also called geodetic north or geographic north) is the direction along Earth's surface towards the geographic North Pole or True North Pole.
Geodetic north differs from ''magnetic'' north (the direction a compass points toward t ...
, 90° true
east
East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth.
Etymology
As in other languages, the word is formed from the fac ...
, 180° true
south
South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both east and west.
Etymology
The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...
, and 270° true
west
West or Occident is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth.
Etymology
The word "west" is a Germanic word passed into some ...
.
TVMDC
TVMDC is a
mnemonic
A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding.
Mnemonics make use of elaborative encoding, retrieval cues, and image ...
for converting true, magnetic and compass headings. TVMDC is a mnemonic initialism for true heading, variation, magnetic heading, deviation, compass heading. The most common use of the TVMDC method is correcting courses during nautical navigation.
Background
The
Geographic North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is called the True North Pole to distinguish from the Mag ...
around which the Earth rotates is not in exactly the same position as the
Magnetic North Pole
The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...
. From any position on the globe, a direction can be determined to either the Geographic North Pole or to the Magnetic North Pole. These directions are expressed in degrees from 0–360°, and also fractions of a degree. The differences between these two directions at any point on the globe is
magnetic variation
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) and ...
(also known as
magnetic declination
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) an ...
, but for the purposes of the mnemonic, the term 'variation' is preferred). When a compass is installed in a vehicle or vessel, local anomalies of the vessel can introduce error into the direction that the compass points. The difference between the local
Magnetic North
The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the planet's magnetic field points vertically downward (in other words, if a magnetic compass needle is allowed t ...
and the direction that the compass indicates as north is known as
magnetic deviation
Magnetic deviation is the error induced in a compass by ''local'' magnetic fields, which must be allowed for, along with magnetic declination, if accurate bearings are to be calculated. (More loosely, "magnetic deviation" is used by some to mea ...
.
Determine variation in North America
Magnetic variation
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) and ...
(also known as
magnetic declination
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between magnetic north (the direction the north end of a magnetized compass needle points, corresponding to the direction of the Earth's magnetic field lines) an ...
) is different depending on the geographic position on the globe. The Magnetic North Pole is currently in Northern Canada and is moving generally south. A straight line can be drawn from the Geographic North Pole, down to the Magnetic North Pole and then continued straight down to the equator. This line is known as the
agonic line
A contour line (also isoline, isopleth, or isarithm) of a function of two variables is a curve along which the function has a constant value, so that the curve joins points of equal value. It is a plane section of the three-dimensional graph ...
, and the line is also moving. In the year 1900, the agonic line passed roughly through Detroit and then was east of Florida. It currently passes roughly west of Chicago, IL, and through New Orleans, LA. If a navigator is located on the agonic line, then variation is zero: the Magnetic North Pole and the Geographic North Pole appear to be directly in line with each other. If a navigator is east of the agonic line, then the variation is westward; magnetic north appears slightly west of the Geographic North Pole. If a navigator is west of the agonic line, then the variation is eastward; the Magnetic North Pole appears to the east of the Geographic North Pole. The farther the navigator is from the agonic line, the greater the variation. The local magnetic variation is indicated on
NOAA
The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
nautical charts at the center of the
compass rose
A compass rose, sometimes called a wind rose, rose of the winds or compass star, is a figure on a compass, map, nautical chart, or monument used to display the orientation of the cardinal directions (north, east, south, and west) and thei ...
. The magnetic variation is indicated along with the year of that variation. The annual increase or decrease of the variation is also usually indicated, so that the variation for the current year can be calculated.
Determine deviation
A compass installed in a vehicle or vessel has a certain amount of error caused by the magnetic properties of the vessel. This error is known as
compass deviation. The magnitude of the compass deviation varies greatly depending upon the local anomalies created by the vessel. A
fiberglass
Fiberglass ( American English) or fibreglass (Commonwealth English) is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cl ...
recreational vessel will generally have much less compass deviation than a steel-hulled vessel. Electrical wires carrying current have a small magnetic field around them and can cause deviation. Any type of magnet, such as found in a speaker can also cause large magnitudes of compass deviation. The error can be corrected using a deviation table. Deviation tables are very difficult to create. Once a deviation table is established, it is only good for that particular vessel, with that particular configuration. If electrical wires are moved or anything else magnetic (speakers, electric motors, etc.) are moved, the deviation table will change. All deviations in the deviation table are indicated west or east. If the compass is pointing west of the Magnetic North Pole, then the deviation is westward. If the compass is pointing east of the Magnetic North Pole, then the deviation is eastward.
Formula
Calculating TVMDC is done with simple arithmetic. First arrange the values vertically:
* True
* Variation
* Magnetic
* Deviation
* Compass
The formula is always added moving down, and subtracted when moving up. The most complicated part is determining if the values are positive or negative. The True, Magnetic, and Compass values are directions on the compass, they must always be a positive number between 0–360. Variation and Deviation can be positive or negative. If either Variation or Deviation is westward, then the values are entered into the equation as positive. If the Variation or Deviation is eastward, then the values are entered into the formula as a negative. Some use the
mnemonic
A mnemonic ( ) device, or memory device, is any learning technique that aids information retention or retrieval (remembering) in the human memory for better understanding.
Mnemonics make use of elaborative encoding, retrieval cues, and image ...
: True Virgins Make Dull Companions - Going downward Add Whiskey (or West). An alternative, working the opposite direction: Can Dead Men Vote Twice.
Examples
True course is 120°, the Variation is 5° West, and the Deviation is 1° West.
* T: 120°
* V: +5°
* M: 125°
* D: +1°
* C: 126°
Therefore, to achieve a true course of 120°, one should follow a compass heading of 126°.
True course is 120°, the Variation is 5° East and the Deviation is 1° East.
* T: 120°
* V: −5°
* M: 115°
* D: −1°
* C: 114°
True course is 035°, the Variation is 4° West and the Deviation is 1° East.
* T: 035°
* V: +4°
* M: 039°
* D: −1°
* C: 038°
True course is 306°, the Variation is 4° East and the Deviation is 11° West.
* T: 306°
* V: −4°
* M: 302°
* D: +11°
* C: 313°
Reverse
The formula can also be calculated in reverse. The formula is subtracted when moving up.
Compass course is 093°, the Deviation is 4° West and the Variation is 3° West.
* T: 086°
* V: -3°
* M: 089°
* D: -4°
* C: 093°
Thus, when following a compass course of 093°, the true course is 086°.
CDMVT
Aviation: CDMVT Can Dead Man Vote Twice: Mnemonic. Easy way to calculate compass magnetic or true north is maintaining the original signs for variation and deviation (+ for east and -for west):
C+(Var)= M+(Dev)= T /
T-(Dev)= M-(Var)= C
See also
*
Bearing (navigation)
In navigation, bearing or azimuth is the horizontal angle between the direction of an object and north or another object. The angle value can be specified in various angular units, such as degrees, mils, or grad. More specifically:
* Absolut ...
*
Heading indicator
The heading indicator (HI), also known as a directional gyro (DG) or direction indicator (DI), is a flight instrument used in an aircraft to inform the pilot of the aircraft's heading.
Use
The primary means of establishing the heading in ...
- Flight instrument
*
Ship motions
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
Reference axes
The '' vertical/Z axis'', or ''yaw axis'', is an imaginary line running vertically through the ship and through its ...
References
Bibliography
* Sea School (1977) True Virgins Make Dull Companions. Add Whiskey Going downward: Mnemonic
* {{cite book, title=The Weekend Navigator: Simple Boat Navigation With GPS and Electronics, first=Robert J., last=Sweet, edition=Illustrated, publisher=McGraw-Hill Professional, year=2004, isbn=978-0-07-143035-7, pag
66url=https://archive.org/details/weekendnavigator0000swee/page/66
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