NAVIGATION BASIC TERMS

Basic navigation terms explained

Latitude

Latitude if a place is defined as the arc of the meridian contained between the Equator and the parallel of the latitude through that place.The arcs running parallel to the equator is defined as the parallels of latitude.Latitude is measured between 0°-90° named either North or South depending on the direction from the equator.

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Longitude

The longitude of a place is defined as the arc on the Equator contained between the prime meridian and the meridian passing through that place, measured East or West depending on the direction from the prime meridian.Longitude is measured from 0°-180°

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Parallels of Latitude

Parallels of latitude are the small circles which run parallel to the equator.The plane of the parallels of latitude are of the same plane with that of the equator.

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D’lat

D’lat between two places is measured as the arc between the parallels of latitude through the two places.D’lat is measured North or South.Eg. Find the D’lat between Latitude 70° N & 20° NSolution:If the same Names, find the difference between the two, else add both.In this case Diff(70°-20°)=50° S.We are moving more south from 70°N to 20°N, hence S


D’long

D’long between two places is measured as the shortest arc on the equator contained between the meridians of the two places in the question.It is named either E or W depending on the location of the two meridians.

Eg:Find the D’long between 10° W & 130°W.Solution:Find the difference between both.Check the direction.In this case, the difference between both is :120°.Direction :We are moving more West.Hence the answer will be 120° W


Departure

Departure is defined as the East-West distance between any two points.It is always express in Nautical Miles.


Middle Latitude

It is the geographical mean between two latitudes.In the figure, M is the geographical latitude.

middle lat

 


Mean Latitude

Mean latitude is the arithmetic mean between two latitudes.Mean Latitude is found out by adding both latitudes and just by finding the mean.


Nautical Mile

A Nautical Mile is defined as the;

”Length of the arc of a meridian subtending an angle of 1′ at the centre of curvature at that place”.It means that the length of  Nautical Mile is not the same at all the places;It varies from place to place.It measures the most at the poles and least at the Equator.However for the purpose of standardization, the length of 1 Nautical mile is taken as 1852.3 m.


Statute Mile

Statute mile is an arbitrary measure of length which is equal to 1609.3 m.It is also termed as a land mile.


True Course

True course is the angle between the True North and the Ship’s Fore & Aft Line.

True Course


True Bearing

True Bearing is the angle between the True North & the Object, which is measured at the Observer.

True Bearing


Variation

Variation is defined as the angle between True North & Magnetic North.

Variation varies with places.It can be easily found out from the compass rose or can be obtained from the Variation Charts.The naming of variation can be made thorough by remembering the following;

-If Magnetic North lies to the right side of the Magnetic North, error is named as WEST (ERROR WEST COMPASS BEST)

-If Magnetic North lies to the left side of the Magnetic North, error is named as EAST(ERROR EAST, COMPASS LEAST)


Deviation

Deviation is the angle between the magnetic meridian and the Compass card.Deviation varies with the ships heading, type of ship, cargo, proximity to the steel structures etc.The naming can be made thorough by remembering the following;

-If Compass North lies to the right side of the Magnetic North, error is named as WEST

-If the Compass North lies to the left side of the Magnetic North, error is named as EAST


Compass Error

The compass error can be found by taking the difference between the Variation & Deviation, but by carefully applying the sign convention.

Eg,Given;

Variation=8° E

Deviation=10° W

Compass Error= 2°W

Take the difference between both and apply the sign of the Greatest.

Eg.2 Given Variation=4° W, Dev=2° W, Find Compass Error.

Sol;Add both the values and since both the signs are of same name, give the same name

Ans=6° W