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Intercardlnal polnl—Any of the four
diractkms midway between the cardinal poims;
norfhaast. soufheasl, southwest, or northwest.
MIL-HDBK-850
Interferometer-An apparatus used to
pmducs and measura interference from hvo or
more coherent wave trains from the same
source. Used to measure wavelengths, to
measure an@sr width of sources, to determine
the angular positkm of sources (as in satellite
tracMng), snd for other purposes. See also
radio Interferometer.
Interim Terrain Data (ITD)-A DMA digital
producf consisting of oomiguous data eels
comprised of attributed and urraymbolizad
feature Information in six thematic files (Surlace
Configuration, Vegetation, Surface Materials,
Surface Drainage, Transportation, and
Obstaclea) and a sevemh file of Digital Terrain
Elevation Data (DTED) Level 1. The data content
is equivalent to the content of either Tactical
Terrain Analysis Data Bases (7TADBe) or
Planning Terrain Analyais Dala Bases (PTADBs).
An enhanced Iranspafat!on file consisting of
attributed roads (afl-waaf her fhrough Carl track),
bridges, and tunnels was added to the data set,
See Plannfng Tarraln Analysls Data
Base; Tactical Terreln Analysls Date
Baae.
Interior angle trrsverse-in surveying, a
closed lravarae wherein distances are maasured
and only Inlerlor angle measurements are used.
Intarlor orlantatlon-The determining
(anafyiicalfy or in a photogrammetric instrument)
of the intarior perspective of the photograph as
it was at the instant of exposure. Elements of
imerbr Orfemation are the calibrated focal
Iangth, bcatbn of the calibrated principal point,
and the calibrated lens distortion. Also called
Inner orlantation.
Intertor perapactlve canter—See
parapectlve center.
Interior to a curtre-That area lying toward
the Conceva sfde of a curva and incfuded within
the area of the circle of which the curve Is a part
of fhe circumterenca.
lnterlocklrtg angle-in till analysis of oblique
photographs, the angle betwesn the optical
axea of Ihe varflcal and oblkfue cameraa. Ttte
dihedral angle betwaan lhe planes of the
vert!cal and oblique photographs.
. .
intermediate contour Urse-A conlour line
drawn bahveen index contours. (@ending on
the wnfour interval, there are thrae or four
imerrndale contours between the irrfex
contours.
Intermediate orbft-A cemnsf force orbft that
Is tangent to the real orbii al some poim. A
fictitious aatellile traveling in the kdermadiefe
orbit would hava the sama position, but not the
same velocity, as tha real salellita af the poim of
tangency.
Internal data structure-The organization
within a ayslem of data and particularly, the
reference linkages among data rslarrwnts. Sae
also Data structure.
International Gravity Standardlzatlon
Net 1971 (IGSN 71)—An adiusted worldwide
network of gravity measurements consisting of
absolula, pendulum, and gravimeter
observations. The IGSN 71 was approved and
adopted by fhe International Union of Geodesy
and Geophysics in 197t, and replaces the
Potsdam datum as fhe international graviiy
standard.
International date llnS-(JCS) The line
coinciding approximately with the amemerfdian
Of Greenwich, modified to avoid certain
habitable land. In crossing fhis line there is a
date change 01 one day. Also called date Ifna.
International eliipaold—A refarence elffpsoid
defined by Hayiord in 1924 with samimajor
axis(a) as 6,378,388.0 reefers, and flattening of
li297.
Internatlonaf gravity formula—A
devalopmem of the formula for theoretical
gravity, based on the aasumptfone thal the
spheroid of raference is an exact ellipsoid of
revolution having the dimensions of the
international ellipsoid of reference (Madrid,
1924), rotating about its minor axia once in a
sidereal day; fhaf the surface of the ellipsoid is a
level surface: and that gravity al the Equator
equals 978.049 gals.
Intarnatlonal low watar (ILW)—A plane of
reference bafow maan sea Iavef; half the range
between mean Iowar low water and mean
higher high water multiplied by 1.5.
Internetfonal ayatam of Unlt9 (Sf)-The
practical international metric system of unlls