Navigation Definitions Flashcards
Meridian
● These are semi great circles that run north to south and join at the earth’s True Poles.
● They are ‘semi’ circles because they run from pole to pole.
○ The two meridians making up a great circle by covering 180° each but one will be west and the other one will be east.
○ Ex. 30°W and 150°E are on the same great circle
Prime Meridian
● The Prime Meridian, 0° of longitude, passes through Greenwich, England (near London).
● The International Date Line is located at 180° of longitude, opposite side to the prime meridian.
➢ At this meridian the day officially changes from one to the next.
Meridian of Longitude
● Longitude is measured in
➢ degrees (°)
➢ minutes (‘)
➢ and seconds (“).
● There are 60 minutes to a degree and 60 seconds to a minute.
● Note that these are angular measures, NOT times.
➢ That said, one hour of time is also equal to 15° of Longitude.
➢ The earth rotates 15° in an hour, one time zone.
● Meridians of Longitude are measured east or west of the Prime Meridian
Latitude
● Parallels of Latitude are measured north or south of the Equator.
● Like Longitude, Latitude is expressed in degrees, minutes and seconds.
➢ 1 minute latitude equals 1 NM
➢ 60 minutes latitude equals 1°
➢ 60 seconds equals 1 minute of latitude.
● Parallels of Latitude run east and west, parallel to the Equator.
Note: A NM is 6 080 feet - in order to work well with map scale
Equator
● This is the line of Latitude that is equidistant from the poles.
➢ This is where we start out measurements of Latitude from.
➢ By this definition, the equator is 0° of Latitude.
● The Equator is both a Great Circle and Rhumb Line.
● The circumference of the Equator is 24000 miles
Great Circles
● This is a circle on the surface of a sphere whose plane will cut the sphere into two equal pieces.
● A great circle represents the shortest distance between two points on the surface of the earth.
● A great circle does not cross the meridians of longitude through which it passes at a constant angle, the heading of an aircraft following a great circle route must be changed frequently to maintain a track.
Rhumb Lines
● These are lines on the earth that cut all meridians through which it passes at the same angle.
● All parallels of latitude are rhumb lines, including the equator which is also a great circle.
● Rhumb lines offer the navigator the advantage of following a constant heading with the disadvantage of not being the shortest distance between two points on the surface of the globe.
● A straight line drawn on the Transverse Mercator Projection will be a rhumb line
Variation
● The magnetic poles do not occupy the same place as the geographic poles.
➢ The magnetic meridians do not coincide with the True meridians.
● The angle between the magnetic meridians and the true meridians is called magnetic variation or magnetic declination
Compensating for Variation
● To convert True to Magnetic (navigation log)
➢ Add Westerly and Subtract Easterly Variation
● To convert Magnetic to True (written exams)
➢ Subtract Westerly and Add Easterly Variation
● Remember whenever we convert TRUE headings to MAGNETIC headings
Variation east, magnetic least
Variation west, magnetic best
Isogonic Lines
● On a map dashed lines that represent the same amount of magnetic variation (variation lines).
● They are not always straight due to the effects of localized differences in the earth’s magnetic field so they bend and curve.
Agonic Line
● An agonic line is where the geographic pole and the magnetic pole line up giving zero magnetic variation.
● There is one agonic line in each hemisphere.
➢ In the western hemisphere (at the moment as it does change over time) it lies just west of Thunder Bay.
● If you are East of the agonic line then the variation is Westerly (compass points west but still east is least).
● If you are West of the agonic line then the variation is Easterly
Deviation
● Due to the magnetic fields associated with the metal and radio equipment in an aircraft the compass may not point directly to the North Magnetic Pole.
● The angle through which the compass needle is deflected from the magnetic meridian is called deviation.
● To reduce the deviation some compasses can be fitted with correction magnets that are adjusted to correct the deviation as much as possible.
● Then to record the remaining amount of deviation the aircraft is turned to a known compass heading and compared with what it shows.
● This difference is the deviation and documented on the compass correction card.
● This process is called “swinging the compass”.
● Remember that deviation East is least (subtract) and deviation West is best (add).
Ex. “For 000 steer 002” is westerly deviation and 2° must be added to the original course. Or “Deviation east, compass least”, “Deviation west, compass best”
Track
● The track is the direction an aircraft intends to travel over the ground.
➢ The intended track may be represented by a straight line drawn on a chart.
● Track is the angle between this line and a meridian, measured clockwise through 360 ̊.
● As in the case of headings, tracks are named true or magnetic or compass with reference to the meridian from which they are measured.
● The track made good is the actual path travelled by the aircraft over the ground.
➢ Like the intended track it may be represented by a line drawn on a chart and (provided it is a reasonably straight line) its direction measured from a true or magnetic or compass north
Heading
● This is the direction the aircraft’s nose is pointed in relation to true or magnetic north.
➢ When creating a Nav Log we will usually start out with the TRUE heading and then correct to MAGNETIC heading by correcting for Variation and then Deviation to get compass heading.
● Our heading may differ substantially from our track.
➢ It all depends on the wind strength and direction
Airspeed
● This is our speed through the air.
● Indicated airspeed is the airspeed that is read of the airspeed indicator.
● True airspeed is calculated based on positional error, altitude, and temperature.
