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1013.2 millibar (29.92 inches), and are separated by specific pressure intervals. (Flight levels are
expressed in three digits that represent hundreds of feet; for example, flight level 250 represents a
barometric altimeter indication of 25,000 feet, and flight level 260 is an indication of 26,000 feet.)
Flight Plan—Predetermined information for the conduct of a flight. That portion of a flight log that is
prepared before the mission.
GEOREF—A worldwide position reference system that may be applied to any map or chart graduated
in latitude and longitude, regardless of projection. It is a method of expressing latitude and longitude in a
form suitable for rapid reporting and plotting. (This term is derived from the words The World
Geographic Reference System.)
Geostrophic Wind—The mathematically calculated wind, which theoretically blows parallel to the
contour lines, in which only pressure-gradient force and Coriolis force are considered.
Gradient Wind—Generally accepted as the actual wind above the friction level, influenced by Coriolis
force, pressure gradient, and centrifugal force.
Graticule—In cartography, a network of lines representing the earth's parallels of latitude and meridians
of longitude.
Greenwich Meridian—The prime meridian that passes through Greenwich, England, and from which
longitude is measured east or west.
Grid Navigation—A method of navigation using a grid overlay for direction reference.
Grivation (GRIV)—Angular difference in direction between grid north and magnetic north. It is
measured east or west from grid north.
Ground Plot—A graphic representation of track and groundspeed.
Ground Range—The horizontal distance from the subpoint of the aircraft to an object on the ground.
Ground Return—The reflection from the terrain as displayed on a CRT.
382 AFPAM11-216 1 MARCH 2001
Groundspeed (GS)—The horizontal component of the speed of an aircraft relative to the earth's
surface.
Ground Wave—A radio wave that is propagated over the surface of the earth and tends to parallel the
earth's surface.
Heading—The direction in which the longitudinal axis of an aircraft is pointed, usually expressed in
degrees clockwise from north (true, magnetic, compass or grid).
Compass Heading (CH)—The reading taken directly from the compass.
Grid Heading (GH)—The heading of an aircraft with reference to grid north.
Magnetic Heading (MH)—The heading of an aircraft with reference to magnetic north.
True Heading (TH)—The heading of an aircraft with reference to true north.
Heat of Compression Error—The error caused by the increase in the indication of the free air
temperature gauge due to air compression and friction on the case around the sensitive element.
Hertz (Hz)—The standard unit notation for measure of frequency in cycles per second. Sixty cycles per
second is 60 Hz.
Homing—The technique whereby an aircraft is directed toward a specific point, keeping the aircraft
pointed toward the point by visual, radio, radar, or similar references.
Horizon—The apparent or visible junction of the earth and sky as seen from any specific position. Also
called the apparent, visible, or local horizon. A horizontal plane passing through a point of vision or
perspective center. The apparent or visible horizon approximates the true horizon only when the point of
vision is very close to sea level.
Bubble Horizon—An artificial horizon parallel to the celestial horizon, established by means of a
bubble level.
Celestial Horizon—The great circle on the celestial sphere whose plane passes through the center of
the earth and is parallel to the plane tangent to the earth at the observer's position.
Visible Horizon—The circle around the observer where earth and sky appear to meet. Also called
natural horizon or sea horizon.
Hour Angle:
Greenwich Hour Angle (GHA)—The angular distance measured from the upper branch of the
Greenwich meridian westward through 360° to the upper branch of the hour circle passing through a
body.
Local Hour Angle (LHA)—The angular distance measured from the upper branch of the observer's
meridian westward through 360° to the upper branch of the hour circle passing through a body.
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Sidereal Hour Angle (SHA)—Angular distance measured from the upper branch of the hour circle
of the first point of Aries westward through 360° to the upper branch of the hour circle passing
through a body.
Hour Circle—See Circles.
Index Error—An error caused by the misalignment of the measurement mechanism of an instrument.
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