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Inherent Distortion—The distortion of the display of a received radar signal caused by the design
characteristics of a particular set.
Initial Point (IP)—A well-defined point, easily distinguishable visually or electronically, used as a
starting point for the run to the target.
Intercept, Celestial—The difference in minutes of arc between computed and observed altitudes or
between precomputed and sextant altitudes. It is labeled T (toward) or A (away) as the observed (or
sextant) altitude.
International Date Line—The line coinciding approximately with the antimeridian of Greenwich,
modified to avoid certain habitable land. In crossing this line, there is a date change of 1 day (gain 1 day
heading west, lose 1 day heading east).
Isobar—A line joining points of equal pressure.
Isogonic Line (Isogonal)—A line drawn on a chart joining points of equal magnetic variation.
Isogriv—A line on a map or chart that joins points of equal angular difference between grid north and
magnetic north.
Isotach—A line drawn on a chart joining points of equal windspeed.
Isotherm—A line drawn on a chart joining points of equal temperature.
Knots (k)—Nautical miles per hour.
Lateral Axis—The straight line passing through the center of gravity of an aircraft perpendicular to the
fuselage.
Latitude (lat)—Angular distance measured north or south of the equator along a meridian, 0° through
90°.
Line of Constant Bearing—A line from a fixed or moving point to a moving object or fixed point that
retains a constant angular value with respect to a reference line.
Line of Position (LOP)—A line indicating a series of positions in which the observer is estimated to be
at the time of the observation.
384 AFPAM11-216 1 MARCH 2001
Log—A written record of computed or observed flight data generally applied to the written navigational
record of a flight.
Longitude (long)—The angular distance east or west of the Greenwich meridian, measured in the plane
of the equator or of a parallel from 0° to 180°.
Longitudinal Axis—A straight line through the center of gravity of an aircraft parallel to the fuselage.
Lubber Line—A reference mark representing the longitudinal axis of an aircraft.
Mach Number—The ratio of the velocity of a body to that of sound in the surrounding medium.
Magnetic Dip—The vertical displacement of the compass needle from the horizontal caused by the
earth's magnetic field.
Magnetic Direction—A direction measured clockwise from the magnetic meridian. MAP READING.
See Navigational Aids.
Map Symbols—Figures and designs used to represent topographical, cultural, and aeronautical features
on a map or chart.
Mean Sea Level (MSL)—The average height of the surface of the sea for all stages of the tide, used as
a reference for elevations.
Mean Sun—An imaginary sun traveling around the equinoctial at the average annual rate of the true
sun.
Meridional Part—A unit of measurement equal to 1 minute of longitude at the equator.
Most Probable Position (MPP)—The most accurate estimate of position where an element of doubt
exists as to the true position.
Nadir—That point on the celestial sphere directly beneath the observer and directly opposite the zenith.
Nautical Mile (NM)—A measure of distance equal to 1 minute of arc on the earth's surface. The United
States has adopted the international nautical mile equal to 1,852 meters or 6,076 feet.
Navigational Aids—Any means of obtaining a fix or line of position as an aid to dead reckoning.
Celestial—The determination of position by reference to celestial bodies.
Map Reading—The determination of position by identification of landmarks with their
representations on a map or chart.
Pressure Differential—The determination of the average drift or the crosswind component of the
wind effect on the aircraft for a given period by taking D readings and applying the formula:
AFPAM11-216 1 MARCH 2001 385
Radar—The determination of position by obtaining bearing and range information (or a
combination of each) from a radar scope.
Radio—The determination of position by use of radio facilities.
NAVSTAR Global Positioning System—A space-based navigation, 3-dimensional positioning and
time distribution system. It will provide precise, continuous, all-weather, common grid, worldwide
navigation, positioning, and timing information to land, sea, air, and space-based users. The joint
program, with the US Air Force the lead service, allows distances to be measured instantly rather than
measuring angles as in celestial navigation.
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