Geographic Coordination Systems

Precise position reporting relies on standardized methods for describing locations on the Earth’s surface. Understanding geographic coordination systems helps pilots recognize how latitude, longitude, and grid references support navigation, chart interpretation, and communication.


Geographic Coordination Systems

Introduction to Geographic Coordination Systems

  • Geographic coordinate systems identify every location on the Earth by a set of numbers, letters, or both
    • Most popular is the latitude and longitude system
  • They are necessary references for fixes or other locations used in navigation
  • Different formats may be required for different flight management systems
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Geographic Coordination Systems

Latitude and Longitude

  • Lines of latitude and longitude are expressed as either degrees (°) minus (') and seconds ("), also referred to as DMS, and even further truncated to provide decimals of minutes or decimals of degrees, for example:
    • Airnav includes three lat/long formats for an airport such as KDXR:
      1. 41-22-17.5240 N 073-28-55.8820 W
      2. 41-22.292067 N 073-28.931367 W
      3. 41.3715344 N,-73.4821894 W
    • If we take the latitude reading of number 3 above (41.3715344) and multiply the decimal by 60 we get (.3715344*60) 41° 22.29206'N, which is how we get number 2 above
      • This is now the airport's lat/long expressed in degrees and decimal minutes
      • The same for longitude would be 28.931367W
    • If we take the decimals of minutes (.29206) and multiply that by 60, we get (.29206*60) 17.5236, which when rounded is how we get number 1 above
      • This is now the airport's lat/long expressed in degrees, minutes, and seconds
      • The same for longitude would be 55.8820

  • Latitude:

    • The "latitude" of a point on the Earth's surface is the angle between the equatorial plane and the straight line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth
    • This line passes a few kilometers away from the center of the Earth except at the poles and the equator where it passes through Earth's center
    • Lines joining points of the same latitude trace circles on the surface of the Earth called parallels, as they are parallel to the equator and to each other
    • The north pole is 90°N; the south pole is 90°S
    • The 0° parallel of latitude is designated the equator, the fundamental plane of all geographic coordinate systems
    • The equator divides the globe into Northern and Southern Hemispheres
    • One nautical mile corresponds to one minute of latitude
      • Thus, degrees of latitude are approximately 60 Nautical Miles apart

  • Longitude:

    • The "longitude" of a point on the Earth's surface is the angle east or west from a reference meridian to another meridian that passes through that point
    • All meridians are halves of great ellipses (often improperly called great circles), which converge at the north and south poles
    • A line, which was intended to pass through the Royal Observatory, Greenwich (a suburb of London, UK), was chosen as the international zero-longitude reference line, the Prime Meridian
    • Places to the east are in the eastern hemisphere, and places to the west are in the western hemisphere
    • The antipodal meridian of Greenwich is both 180°W and 180°E
    • The zero/zero point is located in the Gulf of Guinea about 625 km south of Tema, Ghana
    • In contrast to lines of latitude, lines of longitude are not a consistant distance apart, as they converge at the poles
      • Still, at the equator they are 'roughly' 60 Nautical Miles apart
  • The combination of these two components specifies the position of any location on the planet, but does not consider altitude nor depth

Geographic Coordination Systems

Conclusion

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Geographic Coordination Systems

References